@article{whangbo_koo_bruecher_puphal_kremer_2022, title={Absence of Spin Frustration in the Kagome Layers of Cu2+ Ions in Volborthite Cu3V2O7(OH)(2)center dot 2H(2)O and Observation of the Suppression and Re-Entrance of Specific Heat Anomalies in Volborthite under an External Magnetic Field}, volume={7}, ISSN={["2410-3896"]}, DOI={10.3390/condmat7010024}, abstractNote={We determined the spin exchanges between the Cu2+ ions in the kagome layers of volborthite, Cu3V2O7(OH)2x2H2O, by performing the energy-mapping analysis based on DFT+U calculations, to find that the kagom\'e layers of Cu2+ ions are hardly spin-frustrated, and the magnetic properties of volborthite below ~75 K should be described by very weakly interacting antiferromagnetic uniform chains made up of effective S=1/2 pseudospin units. This conclusion was verified by synthesizing single crystals of not only Cu3V2O7(OH)2x2H2O but also its deuterated analogue Cu3V2O7(OD)2x2D2O and then by investigating their magnetic susceptibilities and specific heats. Each kagome layer consists of intertwined two-leg spin ladders with rungs of linear spin trimers. With the latter acting as S=1/2 pseudospin units, each two-leg spin ladder behaves as a chain of S=1/2 pseudospins. Adjacent two-leg spin ladders in each kagome layer interact very weakly, so it is required that all nearest-neighbor spin exchange paths of every two-leg spin ladder remain antiferromagnetically coupled in all spin ladder arrangements of a kagome layer. This constraint imposes three sets of entropy spectra with which each kagome layer can exchange energy with the surrounding on lowering the temperature below ~1.5 K and on raising the external magnetic field B. We discovered that the specific heat anomalies of volborthite observed below ~1.5 K at B = 0 are suppressed by raising the magnetic field B to ~4.2 T, that a new specific heat anomaly occurs when B is increased above ~5.5 T, and that the imposed three sets of entropy spectra are responsible for the field-dependence of the specific heat anomalies.}, number={1}, journal={CONDENSED MATTER}, author={Whangbo, Myung-Hwan and Koo, Hyun-Joo and Bruecher, Eva and Puphal, Pascal and Kremer, Reinhard K.}, year={2022}, month={Mar} }
 @misc{whangbo_koo_kremer_2021, title={Spin Exchanges between Transition Metal Ions Governed by the Ligand p-Orbitals in Their Magnetic Orbitals}, volume={26}, ISSN={["1420-3049"]}, DOI={10.3390/molecules26030531}, abstractNote={In this review on spin exchanges, written to provide guidelines useful for finding the spin lattice relevant for any given magnetic solid, we discuss how the values of spin exchanges in transition metal magnetic compounds are quantitatively determined from electronic structure calculations, which electronic factors control whether a spin exchange is antiferromagnetic or ferromagnetic, and how these factors are related to the geometrical parameters of the spin exchange path. In an extended solid containing transition metal magnetic ions, each metal ion M is surrounded with main-group ligands L to form an MLn polyhedron (typically, n = 3–6), and the unpaired spins of M are represented by the singly-occupied d-states (i.e., the magnetic orbitals) of MLn. Each magnetic orbital has the metal d-orbital combined out-of-phase with the ligand p-orbitals; therefore, the spin exchanges between adjacent metal ions M lead not only to the M–L–M-type exchanges, but also to the M–L…L–M-type exchanges in which the two metal ions do not share a common ligand. The latter can be further modified by d0 cations A such as V5+ and W6+ to bridge the L…L contact generating M–L…A…L–M-type exchanges. We describe several qualitative rules for predicting whether the M–L…L–M and M–L…A…L–M-type exchanges are antiferromagnetic or ferromagnetic by analyzing how the ligand p-orbitals in their magnetic orbitals (the ligand p-orbital tails, for short) are arranged in the exchange paths. Finally, we illustrate how these rules work by analyzing the crystal structures and magnetic properties of four cuprates of current interest: α-CuV2O6, LiCuVO4, (CuCl)LaNb2O7, and Cu3(CO3)2(OH)2.}, number={3}, journal={MOLECULES}, author={Whangbo, Myung-Hwan and Koo, Hyun-Joo and Kremer, Reinhard K.}, year={2021}, month={Feb} }
 @article{koo_shankar_orlandi_sundaresan_whangbo_2020, title={On Ferro- and Antiferro-Spin-Density Waves Describing the Incommensurate Magnetic Structure of NaYNiWO6}, volume={59}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.0c02651}, abstractNote={The incommensurate magnetic structure (0.47, 0, 0.49) of NaYNiWO6 exhibits unconventional spin-density waves (SDWs) along the [100] direction, in which up and down spins alternate in each half-wave. This is in contrast to conventional SDWs, in which only one type of spin is present in each half-wave. We probed the formation of these unconventional SDWs by evaluating the spin exchanges of NaYNiWO6 based on density functional theory calculations and analyzing the nature of the spin frustration in NaYNiWO6 and by noting that a SDW is a superposition of two cycloids of opposite chirality. The unconventional SDWs along the [100] direction originate from the spin-frustrated antiferromagnetic chains of Ni2+ ions along that direction, leading to conventional SDWs along the [101] direction and unconventional SDWs along the [001] direction.}, number={24}, journal={INORGANIC CHEMISTRY}, author={Koo, Hyun-Joo and Shankar, Ravi P. N. and Orlandi, Fabio and Sundaresan, Athinarayanan and Whangbo, Myung-Hwan}, year={2020}, month={Dec}, pages={17856–17859} }
 @article{koo_kremer_whangbo_2020, title={Orbital Magnetic Moments of the High-Spin Co2+ Ions at Axially-Elongated Octahedral Sites: Unquenched as Reported from Experiment or Quenched as Predicted by Theory?}, volume={59}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.0c02929}, abstractNote={Neutron diffraction studies on magnetic solids composed of axially elongated CoO4X2 (X = Cl, Br, S, Se) octahedra show that the ordered magnetic moments of their high-spin Co2+ (d7, S = 3/2) ions are greater than 3 μB, i.e., the spin moment expected for S = 3/2 ions, and increase almost linearly from 3.22 to 4.45 μB as the bond-length ratio rCo-X/rCo-O increases from 1.347 to 1.659 where rCo-X and rCo-O are the Co-X and Co-O bond lengths, respectively. These observations imply that the orbital moments of the Co2+ ions increase linearly from 0.22 to 1.45 μB with increasing the rCo-X/rCo-O ratio from 1.347 to 1.659. We probed this implication by examining the condition for unquenched orbital moment and also by evaluating the magnetic moments of the Co2+ ions based on DFT+U+SOC calculations for those systems of the CoO4X2 octahedra. Our work shows that the orbital moments of the Co2+ ions are essentially quenched and, hence, that the observations of the neutron diffraction studies are not explained by the current theory of magnetic moments. This discrepancy between experiment and theory urges one to check the foundations of the current theory of magnetic moments as well as the current method of neutron diffraction refinements for ordered magnetic structures.}, number={24}, journal={INORGANIC CHEMISTRY}, author={Koo, Hyun-Joo and Kremer, Reinhard K. and Whangbo, Myung-Hwan}, year={2020}, month={Dec}, pages={18319–18324} }
 @article{vasilchikova_nalbandyan_shukaev_koo_whangbo_lozitskiy_bogaychuk_kuzmin_tagirov_vavilova_et al._2020, title={Peculiarities of magnetic ordering in the S=5/2 two-dimensional square-lattice antimonate NaMnSbO4}, volume={101}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.101.054435}, abstractNote={An orthorhombic compound, $\mathrm{NaMnSb}{\mathrm{O}}_{4}$, represents a square net of magnetic $\mathrm{M}{\mathrm{n}}^{2+}$ ions residing in vertex-shared oxygen octahedra. Its static and dynamic magnetic properties were studied using magnetic susceptibility, specific heat, magnetization, electron spin resonance (ESR), nuclear magnetic resonance (NMR), and density-functional calculations. Thermodynamic data indicate an establishment of the long-range magnetic order with ${T}_{\mathrm{N}}\ensuremath{\sim}44\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is preceded by a short-range one at about 55 K. In addition, a nontrivial wasp-waisted hysteresis loop of the magnetization was observed, indicating that the ground state is most probably canted antiferromagnetic. Temperature dependence of the magnetic susceptibility is described reasonably well in the framework of two-dimensional square-lattice model with the main exchange parameter $J=\ensuremath{-}5.3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is in good agreement with density-functional analysis, NMR, and ESR data.}, number={5}, journal={PHYSICAL REVIEW B}, author={Vasilchikova, Tatyana and Nalbandyan, Vladimir and Shukaev, Igor and Koo, Hyun-Joo and Whangbo, Myung-Hwan and Lozitskiy, Andrey and Bogaychuk, Alexander and Kuzmin, Vyacheslav and Tagirov, Murat and Vavilova, Evgeniya and et al.}, year={2020}, month={Feb} }
 @article{sorolla_wang_koo_whangbo_jacobson_2020, title={Synthesis of the Elusive S=1/2 Star Structure: A Possible Quantum Spin Liquid Candidate}, volume={142}, ISSN={["1520-5126"]}, DOI={10.1021/jacs.0c00901}, abstractNote={Materials with two-dimensional, geometrically frustrated, spin-1/2 lattices provide a fertile playground for the study of intriguing magnetic phenomena such as quantum spin liquid (QSL) behavior, but their preparation has been a challenge. In particular, the long-sought, exotic spin-1/2 star structure has not been experimentally realized to date. Here we report the synthesis of [(CH3)2(NH2)]3[CuII3(μ3-OH)(μ3-SO4)(μ3-SO4)3]·0.24H2O with an S = 1/2 star lattice. On the basis of the magnetic susceptibility and heat capacity measurements, the layered Cu-based compound exhibits antiferromagnetic interactions but no magnetic ordering or spin freezing down to 2 K. The spin-frustrated material appears to be a promising QSL candidate.}, number={11}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Sorolla, Maurice, II and Wang, Xiqu and Koo, Hyun-Joo and Whangbo, Myung-Hwan and Jacobson, Allan J.}, year={2020}, month={Mar}, pages={5013–5016} }
 @article{koo_whangbo_2019, title={Effect of Nonmagnetic Ion Deficiency on Magnetic Structure: Density Functional Study of Sr2MnO2Cu2-xTe2, Sr2MO2Cu2Te2 (M = Co, Mn), and the Oxide-Hydrides Sr2VO3H, Sr3V2O5H2, and SrVO2H}, volume={58}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.9b02456}, abstractNote={Two seemingly puzzling observations on two magnetic systems were analyzed. For the oxide-hydrides Sr2VO3H, Sr3V2O5H2, and SrVO2H, made up of VO4H2 octahedra, the spin orientations of the V3+ (d2, S = 1) ions were reported to be different, namely, perpendicular to the H-V-H bond in Sr2VO3H but parallel to the H-V-H bond in Sr3V2O5H2 and SrVO2H, despite that the d-state split patterns of the VO4H2 octahedra are similar in the three oxide-hydrides. Another puzzling observation is the contrasting magnetic structures of Sr2CoO2Cu2Te2 and Sr2MnO2Cu1.58Te2, consisting of the layers made up of corner-sharing MO4Te2 (M = Co, Mn) octahedra. The Co2+ spins in each CoO2Te2 layer are antiferromagnetically coupled with spins perpendicular to the Te-Co-Te bond, whereas the Mn3+/Mn2+ ions of each MnO2Te2 layer are ferromagnetically coupled with spins parallel to the Te-Mn-Te bonds. We investigated the cause for these observations by performing first-principles density functional theory (DFT) calculations for stoichiometric phases Sr2VO3H, Sr3V2O5H2, SrVO2H, Sr2CoO2Cu2Te2, and Sr2MnO2Cu2Te2, as well as nonstoichiometric phase Sr2MnO2Cu1.5Te2. Our study reveals that the V3+ ions in all three oxide-hydrides should have the spin orientation parallel to the H-V-H bond. The unusual magnetic structure of the MnO2Te2 layers of Sr2MnO2Cu1.52Te2 arises from the preference of a Mn3+ spin to be parallel to the Te-Mn-Te bond, the ferromagnetic spin exchange between adjacent Mn3+ and Mn2+ ions, and the nearly equal numbers of Mn3+ and Mn2+ ions in each MnO2Te2 layer. We show that the spin orientation of the magnetic ions in an antiferromagnetically coupled perovskite layer, expected in the absence of nonmagnetic ion vacancies, cannot be altered by the magnetic ions of higher oxidation that result from trace vacancies at the nonmagnetic ion sites.}, number={21}, journal={INORGANIC CHEMISTRY}, author={Koo, Hyun-Joo and Whangbo, Myung-Hwan}, year={2019}, month={Nov}, pages={14769–14776} }
 @article{whangbo_xiang_koo_gordon_whitten_2019, title={Electronic and Structural Factors Controlling the Spin Orientations of Magnetic Ions}, volume={58}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/acs.inorgchem.9b00687}, DOI={10.1021/acs.inorgchem.9b00687}, abstractNote={Magnetic ions M in discrete molecules and extended solids form MLn complexes with their first-coordinate ligand atoms L. The spin moment of M in a complex MLn prefers a certain direction in coordinate space because of spin-orbit coupling (SOC). In this minireview, we examine the structural and electronic factors governing the preferred spin orientations. Elaborate experimental measurements and/or sophisticated computational efforts are required to find the preferred spin orientations of magnetic ions, largely because the energy scale of SOC is very small. The latter is also the very reason why one can readily predict the preferred spin orientation of M by analyzing the SOC-induced highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) interactions of the MLn complexes in terms of qualitative perturbation theory. The strength of this HOMO-LUMO interaction depends on the spin orientation, which is governed by the selection rules based on the minimum |ΔLz| value (i.e., the minimum difference in the magnetic quantum numbers) between the HOMO and LUMO. With the local z axis of MLn chosen as its n-fold rotational axis, the preferred spin orientation is parallel to the z axis (∥z) when |ΔLz| = 0 but perpendicular to the z axis (⊥z) when |ΔLz| = 1.}, number={18}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Whangbo, Myung-Hwan and Xiang, Hongjun and Koo, Hyun-Joo and Gordon, Elijah E. and Whitten, Jerry L.}, year={2019}, month={Jun}, pages={11854–11874} }
 @article{uma_vasilchikova_sobolev_raganyan_sethi_koo_whangbo_presniakov_glazkova_vasiliev_et al._2019, title={Synthesis and Characterization of Sodium-Iron Antimonate Na2FeSbO5: One-Dimensional Antiferromagnetic Chain Compound with a Spin-Glass Ground State}, volume={58}, ISSN={["1520-510X"]}, DOI={10.1021/acs.inorgchem.9b00212}, abstractNote={A new oxide, sodium–iron antimonate, Na2FeSbO5, was synthesized and structurally characterized, and its static and dynamic magnetic properties were comprehensively studied both experimentally by dc and ac magnetic susceptibility, magnetization, specific heat, electron spin resonance (ESR) and Mössbauer measurements, and theoretically by density functional calculations. The resulting single-crystal structure (a = 15.6991(9) Å; b = 5.3323 (4) Å; c = 10.8875(6) Å; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which alternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The 57Fe Mössbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in a distorted tetrahedral oxygen coordination. The magnetic susceptibility and specific heat data show the absence of a long-range magnetic ordering in Na2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates spin-glass-type behavior with a unique two-step freezing at Tf1 ≈ 80 K and Tf2 ≈ 35 K. Magnetic hyperfine splitting of 57Fe Mössbauer spectra was observed below T* ≈ 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T < T*) exhibit a relaxation behavior caused by critical spin fluctuations, indicating the existence of short-range correlations. The stochastic model of ionic spin relaxation was used to account for the shape of the Mössbauer spectra below the freezing temperature. A complex slow dynamics is further supported by ESR data revealing two different absorption modes presumably related to ordered and disordered segments of spin chains. The data imply a spin-cluster ground state for Na2FeSbO5.}, number={17}, journal={INORGANIC CHEMISTRY}, author={Uma, Sitharaman and Vasilchikova, Tatyana and Sobolev, Alexey and Raganyan, Grigory and Sethi, Aanchal and Koo, Hyun-Joo and Whangbo, Myun-Hwan and Presniakov, Igor and Glazkova, Iana and Vasiliev, Alexander and et al.}, year={2019}, month={Sep}, pages={11333–11350} }
 @article{koo_kasapbasi_whitten_whangbo_2019, title={The Conceptual Dilemma of the One-Electron Picture in Describing the Uniaxial Magnetism at Linear Coordination Sites}, volume={2019}, ISSN={1434-1948}, url={http://dx.doi.org/10.1002/ejic.201900370}, DOI={10.1002/ejic.201900370}, number={21}, journal={European Journal of Inorganic Chemistry}, publisher={Wiley}, author={Koo, Hyun-Joo and Kasapbasi, Esra E. and Whitten, Jerry L. and Whangbo, Myung-Hwan}, year={2019}, month={May}, pages={2630–2634} }
 @article{dorolti_cario_corraze_janod_vaju_koo_kan_whangbo_2010, title={Half-Metallic Ferromagnetism and Large Negative Magnetoresistance in the New Lacunar Spinel GaTi3VS8}, volume={132}, ISSN={["0002-7863"]}, DOI={10.1021/ja908128b}, abstractNote={The lacunar spinel compounds GaTi(4-x)V(x)S(8) (0 < x < 4), consisting of Ti(4-x)V(x) tetrahedral clusters, were prepared and their structures were determined by powder X-ray diffraction. The electronic structures of GaTi(4-x)V(x)S(8) (x = 0, 1, 2, 3) were examined by density functional calculations, and the electrical resistivity and magnetic susceptibility of these compounds were measured. Our calculations predict that GaTi(3)VS(8) is a ferromagnetic half-metal, and this prediction was confirmed by magnetotransport experiments performed on polycrystalline samples of GaTi(3)VS(8). The latter reveal a large negative magnetoresistance (up to 22% at 2 K), which is consistent with the intergrain tunnelling magnetoresistance expected for powder samples of a ferromagnetic half-metal and indicates the presence of high spin polarization greater than 53% in GaTi(3)VS(8).}, number={16}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Dorolti, Eugen and Cario, Laurent and Corraze, Benoit and Janod, Etienne and Vaju, Cristian and Koo, Hyun-Joo and Kan, Erjun and Whangbo, Myung-Hwan}, year={2010}, month={Apr}, pages={5704–5710} }
 @article{koo_whangbo_2008, title={Analysis of the spin lattice model for the spin-gapped layered compounds Na3Cu2SbO6 and Na2Cu2TeO6 on the basis of electronic structure calculations}, volume={47}, ISSN={["1520-510X"]}, DOI={10.1021/ic701153z}, abstractNote={The spin lattice model for the spin-gapped layered magnetic solids Na3Cu2SbO6 and Na2Cu2TeO6 was examined by evaluating the three spin exchange interactions of their Cu2MO6 (M = Sb, Te) layers in terms of spin dimer analysis based on extended Hückel tight binding calculations and mapping analysis based on first principles density functional theory electronic band structure calculations. For both compounds, our calculations show that the two strongest spin exchange interactions, that is, the Cu-O...O-Cu super-superexchange (J2) and the Cu-O-Cu superexchange (J1) interactions, form alternating chains that interact weakly through the Cu-O-Cu superexchange (J3) interactions. The dominant one of the three spin exchange interactions is J2, and it is antiferromagnetic in agreement with the fact that both of the compounds are spin gapped. For Na3Cu2SbO6 and Na2Cu2TeO6, the superexchange J1 is calculated to be ferromagnetic, hence, leading to the alternating chain model in which antiferromagnetic and ferromagnetic spin exchange interactions alternate. This picture does not agree with the recent experimental analysis, which showed that the temperature-dependent magnetic susceptibilities of both compounds should be described by the alternating chain model in which two antiferromagnetic spin exchange interactions of different strengths alternate.}, number={1}, journal={INORGANIC CHEMISTRY}, author={Koo, Hyun-Joo and Whangbo, Myung-Hwan}, year={2008}, month={Jan}, pages={128–133} }
 @article{koo_lee_wilson-short_dai_whangbo_2007, title={On the relevance of an antiferromagnetic dimer model for the spin-gapped magnetic solids Cu(terpy)Mo2O7 and Cu(OH)(p-pyc)H2O}, volume={46}, ISSN={["1520-510X"]}, DOI={10.1021/ic061897p}, abstractNote={The magnetic solids Cu(terpy)Mo2O7 (terpy = terpyridine) and Cu(OH)(p-pyc)H2O (p-pyc = p-pyridinecarboxylate) have a spin gap and possess chains of Cu2+ ions in which two different Cu···Cu distances alternate. On the basis of their reported crystal structures, the spin-exchange interactions of these compounds were examined by performing spin dimer analysis to determine whether an antiferromagnetic dimer or an alternating antiferromagnetic chain model is appropriate for their magnetic properties. Our analysis shows that an antiferromagnetic dimer model is correct for both compounds because of the anisotropic overlap between the magnetic orbitals of their Cu2+ sites.}, number={7}, journal={INORGANIC CHEMISTRY}, author={Koo, Hyun-Joo and Lee, Changhoon and Wilson-Short, Gareth B. and Dai, Dadi and Whangbo, Myung-Hwan}, year={2007}, month={Apr}, pages={2498–2502} }
 @article{koo_whangbo_2007, title={Spin dimer analysis of the magnetic structures of A(2)V(3)O(9) (A = Ba, Sr) and eta-Na9V14O35: Importance of the V4+-O center dot center dot center dot O-V4+ super- superexchange interactions mediated by the O-V5+-O bridges}, volume={9}, ISSN={["1293-2558"]}, DOI={10.1016/j.solidstatesciences.2007.06.013}, abstractNote={Abstract Spin dimer analysis was carried out for the magnetic oxides of V4+ (d1) ions, A2V3O9 (A = Ba, Sr) and η-Na9V14O35, to account for their magnetic structures. After identifying the V(Oeq)4 square planes containing the magnetic orbitals of the V4+ (d1) ions, we analyzed the relative strengths of the various V–Oeq–V and V–Oeq⋯Oeq–V spin exchange interactions. The V–Oeq⋯Oeq–V spin exchange interactions mediated by Oeq–V5+–Oeq bridges are found to be crucial in determining the magnetic structures of A2V3O9 (A = Ba, Sr) and η-Na9V14O35. Our analysis suggests that η-Na9V14O35 should have two different spin gaps.}, number={9}, journal={SOLID STATE SCIENCES}, author={Koo, Hyun-Joo and Whangbo, Myung-Hwan}, year={2007}, month={Sep}, pages={824–832} }
 @article{koo_lee_whangbo_2006, title={Spin dimer analysis of the magnetic structures of Ba3Cr2O8, Ba3Mn2O8, Na4FeO4, and Ba2CoO4 with a three-dimensional network of isolated MO4 (M = Cr, Mn, Fe, Co) tetrahedra}, volume={45}, ISSN={["1520-510X"]}, DOI={10.1021/ic061773c}, abstractNote={The spin exchange interactions of the magnetic oxides Ba3Cr2O8, Ba3Mn2O8, Na4FeO4, and Ba2CoO4 with a three-dimensional network of isolated MO4 (M = Cr, Mn, Fe, Co) tetrahedra were examined by performing spin dimer analysis on the basis of tight-binding electronic structure calculations. Although the shortest O...O distances between adjacent MO4 tetrahedra are longer than the van der Waals distance, our analysis shows that the super-superexchange interactions between adjacent MO4 tetrahedra are substantial and determine the magnetic structures of these oxides. In agreement with experiment, our analysis predicts a weakly interacting isolated AFM dimer model for both Ba3Cr2O8 and Ba3Mn2O8, the (0.0, 0.5, 0.0) magnetic superstructure for Na4FeO4, the (0.5, 0.0, 0.5) magnetic superstructure for Ba2CoO4, and the presence of magnetic frustration in Ba2CoO4. The comparison of the intra- and interdimer spin exchange interactions of Ba3Cr2O8 and Ba3Mn2O8 indicates that orbital ordering should be present in Ba3Cr2O8.}, number={26}, journal={INORGANIC CHEMISTRY}, author={Koo, Hyun-Joo and Lee, Kwang-Soon and Whangbo, Myung-Hwan}, year={2006}, month={Dec}, pages={10743–10749} }
 @article{woodward_backov_abboud_dai_koo_whangbo_meisel_talham_2005, title={Dramatic variation of magnetic exchange through double end-on azide bridges in a series of ladder-like copper(II) coordination polymers}, volume={44}, ISSN={["1520-510X"]}, DOI={10.1021/ic049175q}, abstractNote={Three ladder-like coordination polymers, [Cu2(phprpy)2-mu-(N3)2(N3)2], 1; [Cu2(terpy)2-mu-(N3)4Cu2-mu-(N3)2(N3)2], 2; and[Cu2(terpy)2-mu-(N3)2(N3)2Cu3-mu-(N3)4(N3)2], 3, consisting of Cu2+ ions with double end-on azide bridges were synthesized, their crystal structures and magnetic properties were determined, and spin dimer analysis was performed to explain the signs and strengths of their strong spin exchange interactions [phprpy is 4-(3-phenylpropyl)pyridine and terpy is 2,2':6,2''-terpyridine]. Although these compounds have ladder-like arrangements of Cu2+ ions, their magnetic structures are described as isolated dimers for 1 and 2 and as isolated trimers for 3. The predominant spin exchange paths in 1-3 have double end-on azide bridges linking adjacent Cu2+ ions, and the geometrical parameters of these bridging structures are similar. However, the spin dimer of 1 exhibits a strong ferromagnetic coupling; that of 2, a strong antiferromagnetic coupling; and that of 3, a weak ferromagnetic coupling. These findings are well explained by the present spin dimer analysis and show that the nature and geometry of the nonbridging ligands can have a strong influence on the sign and strength of the spin exchange interaction between Cu2+ ions connected by double end-on azide bridges.}, number={3}, journal={INORGANIC CHEMISTRY}, author={Woodward, JD and Backov, RV and Abboud, KA and Dai, D and Koo, HJ and Whangbo, MH and Meisel, MW and Talham, DR}, year={2005}, month={Feb}, pages={638–648} }
 @article{soulard_rocquefelte_petit_evain_jobic_itie_munsch_koo_whangbo_2004, title={Experimental and theoretical investigation on the relative stability of the PdS2- and pyrite-type structures of PdSe2}, volume={43}, DOI={10.1021/ic035396}, number={6}, journal={Inorganic Chemistry}, author={Soulard, C. and Rocquefelte, X. and Petit, P. E. and Evain, M. and Jobic, S. and Itie, J. P. and Munsch, P. and Koo, H. J. and Whangbo, M. H.}, year={2004}, pages={1943–1949} }
 @article{dai_koo_whangbo_2004, title={Investigation of the incommensurate and commensurate magnetic superstructures of LiCuVO4 and CuO on the basis of the isotropic spin exchange and classical spin approximations}, volume={43}, ISSN={["1520-510X"]}, DOI={10.1021/ic035314c}, abstractNote={The spin lattices of magnetic oxides LiCuVO(4) and CuO are made up of CuO(2) ribbon chains. The incommensurate and commensurate magnetic superstructures of these oxides were examined by calculating the total spin exchange interaction energies of their long-range order spin arrangements on the basis of the isotropic spin exchange and classical spin approximations. The incommensurate superstructure (0, 0.532, 0) of LiCuVO(4) was analyzed to find that the next-nearest-neighbor spin exchange interaction J(nnn) is more strongly antiferromagnetic than the nearest-neighbor spin exchange interaction J(nn) in the CuO(2) chains. With this finding, we reassessed the relative strengths of the spin exchange interactions of LiCuVO(4) and CuO and then analyzed the relative energies of their long-range order spin arrangements. The incommensurate superstructure (0, 0.532, 0) of LiCuVO(4) is explained when the J(nn)/J(nnn) ratio is -0.40. Both the incommensurate superstructure (0.506, 0, -0.483) and the commensurate superstructure (0.5, 0, -0.5) of CuO, which occur at 231 and 212.5 K, respectively, are well explained in terms of the calculated total spin exchange interaction energies. The incommensurate superstructure of CuO becomes commensurate by a slight change in one interchain spin exchange interaction, which is due probably to a slight structure change brought about by the temperature lowering.}, number={13}, journal={INORGANIC CHEMISTRY}, author={Dai, D and Koo, HJ and Whangbo, MH}, year={2004}, month={Jun}, pages={4026–4035} }
 @article{manson_schlueter_koo_whangbo_2004, title={Reexamination of the magnetic properties of Cu-2(dca)(4)(2,5-me(2)pyz) {dca = dicyanamide; me(2)pyz = dimethylpyrazine}: isolated spin-1/2 dimers versus long-range magnetic ordering}, volume={43}, ISSN={["0020-1669"]}, DOI={10.1021/ic0354769}, abstractNote={The magnetic properties of Cu2(dca)4(2,5-me2pyz) have been reexamined. The extended structure of Cu2(dca)4(2,5-me2pyz) can be viewed in terms of Cu2(2,5-me2pyz)4+ dimer units interconnected via μ1,5-dca ligands. The bulk magnetic susceptibility χ(T) and the isothermal M(H) of Cu2(dca)4(2,5-me2pyz) are shown to be well described by an isolated dimer model. This finding was confirmed by carrying out a spin dimer analysis based on tight-binding calculations, which shows that the 2,5-me2pyz ligand provides a substantial spin exchange interaction between the Cu2+ ions while the dca ligands do not.}, number={13}, journal={INORGANIC CHEMISTRY}, author={Manson, JL and Schlueter, JA and Koo, HJ and Whangbo, MH}, year={2004}, month={Jun}, pages={4007–4011} }
 @article{shikano_kremer_ahrens_koo_whangbo_darriet_2004, title={Synthesis and characterization of a magnetic semiconductor Na2RuO4 containing one-dimensional chains of RU6+}, volume={43}, number={1}, journal={Inorganic Chemistry}, author={Shikano, M. and Kremer, R. K. and Ahrens, M. and Koo, H. J. and Whangbo, M. H. and Darriet, J.}, year={2004}, pages={07-} }
 @article{manson_lecher_gu_geiser_schlueter_henning_wang_schultz_koo_whangbo_2003, title={Cu(HCO2)(2)L {L = pyrazine, 4,4 '-bipyridine}: employing the formate anion as a building block in three-dimensional coordination polymers}, ISSN={["1477-9234"]}, DOI={10.1039/b302631k}, abstractNote={The formate anion, HCO2−, has been used infrequently as a building block in molecular magnetic materials. We have synthesized and structurally and magnetically characterized two new Cu(HCO2)2L compounds, where L represents pyrazine (pyz) and 4,4′-bipyridine (bipy). The single crystal structure of Cu(HCO2)2(pyz), 1, has been established by both X-ray (295 K) and neutron diffraction (20 K). The compound consists of tetragonally-elongated CuN2O4 octahedra made up of four bridging formate anions and two neutral pyz ligands. The 3D polymeric network is comprised of 2D Cu(HCO2)2 layers that are fused together by the linear pyz spacers, which form Cu–pyz-Cu chains. Cu(HCO2)2(bipy), 2, is chiral and has a more complex framework than 1. The CuO4N2 octahedra align in two unique orientations relative to one another, owing to the 41 and 21 screw axes that lie along the a and b-axes. The octahedra are connected via four bridging HCO2− anions and two bridging bipy ligands, resulting in a unique 3D scaffold structure. The magnetic behavior of 1 and 2 indicates antiferro- and ferromagnetic interactions, respectively, and the exchange couplings in both 1 and 2 are well reproduced by a 1D spin Hamiltonian. Spin dimer analysis was carried out to evaluate the relative strengths of the various spin exchange paths. It is found that the interaction through HCO2− is comparatively weak, and the strong Cu2+ spin exchange interactions are mediated by the pyz and bipy ligands via the σ-pathway. A possible reason as to why 2 displays ferromagnetic coupling is proposed.}, number={14}, journal={DALTON TRANSACTIONS}, author={Manson, JL and Lecher, JG and Gu, JY and Geiser, U and Schlueter, JA and Henning, R and Wang, XP and Schultz, AJ and Koo, HJ and Whangbo, MH}, year={2003}, pages={2905–2911} }
 @article{whangbo_koo_dai_jung_2003, title={Interpretation of the magnetic structures of Cu2Te2O5X2 (X = Cl, Br) and Ca3.1Cu0.9RuO6 on the basis of electronic structure considerations: Cases for strong super-superexchange interactions involving Cu2+ ions}, volume={42}, DOI={10.1021/ic020551}, number={12}, journal={Inorganic Chemistry}, author={Whangbo, M. H. and Koo, H. J. and Dai, D. and Jung, D.}, year={2003}, pages={3898–3906} }
 @article{koo_whangbo_lee_2003, title={Investigation of the spin exchange interactions and magnetic structures of the CrVO4-type transition metal phosphates, sulfates, and vanadates by spin dimer analysis}, volume={42}, ISSN={["0020-1669"]}, DOI={10.1021/ic030115e}, abstractNote={The CrVO(4)-type magnetic oxides MM'O(4) consist of edge-sharing MO(4) octahedral chains condensed with M'O(4) tetrahedra and exhibit a wide variety of magnetic structures. The magnetic properties of these oxides were examined by studying their spin exchange interactions on the basis of spin dimer analysis. The nature and magnitudes of the intra- and interchain spin exchange interactions depend on the square-to-rectangle distortion in the basal planes of the MO(4) chain and on the difference between the M 3d and O 2p orbital energies. The spiral magnetic structures of beta-CrPO(4) and MnSO(4) originate from the pseudohexagonal arrangement of the MO(4) chains and the frustrated interchain antiferromagnetic interactions.}, number={19}, journal={INORGANIC CHEMISTRY}, author={Koo, HJ and Whangbo, MH and Lee, K}, year={2003}, month={Sep}, pages={5932–5937} }
 @article{whangbo_dai_koo_jobic_2003, title={Investigations of the oxidation states and spin distributions in Ca3Co2O6 and Ca3CoRhO6 by spin-polarized electronic band structure calculations}, volume={125}, ISSN={["1879-2766"]}, DOI={10.1016/S0038-1098(02)00872-4}, abstractNote={Spin-polarized electronic band structure calculations were carried out for the magnetic solids Ca 3 CoMO 6 (M=Co, Rh), and the local electronic structures of their transition metal atoms at the octahedral and trigonal prism sites were examined. Our calculations show that Ca 3 CoMO 6 (M=Co, Rh) has four unpaired spins per formula unit, the magnetic moment comes predominantly from the trigonal prism site, and the metal atoms of both the octahedral and trigonal prism sites have the oxidation state +3.}, number={7-8}, journal={SOLID STATE COMMUNICATIONS}, author={Whangbo, MH and Dai, D and Koo, HJ and Jobic, S}, year={2003}, month={Feb}, pages={413–417} }
 @article{koo_whangbo_2003, title={Magnetic superstructures of cupric oxide CuO as ordered arrangements of one-dimensional antiferromagnetic chains}, volume={42}, ISSN={["0020-1669"]}, DOI={10.1021/ic020576k}, abstractNote={In cupric oxide CuO, each Cu(2+) ion has 12 nearest-neighbor Cu(2+) ions grouped into six pairs related by inversion symmetry. The relative strengths of the Cu-O-Cu superexchange interactions in cupric oxide CuO were estimated by spin dimer analysis to confirm that the strongest superexchange interactions form one-dimensional antiferromagnetic chains along the [101] direction, and the remaining interactions are weak. We analyzed ordered arrangements of these one-dimensional antiferromagnetic chains to examine why the antiferromagnetic phase transition of CuO below 212.5 K adopts a (2a, b, 2c) superstructure. The local spin arrangement around each Cu(2+) ion is more balanced in the ordered spin structures leading to a (2a, b, 2c) supercell than in any other ordered spin structures.}, number={4}, journal={INORGANIC CHEMISTRY}, author={Koo, HJ and Whangbo, MH}, year={2003}, month={Feb}, pages={1187–1192} }
 @misc{whangbo_koo_dai_2003, title={Spin exchange interactions and magnetic structures of extended magnetic solids with localized spins: theoretical descriptions on formal, quantitative and qualitative levels}, volume={176}, ISSN={["1095-726X"]}, DOI={10.1016/S0022-4596(03)00273-1}, abstractNote={Low-energy excitation energies of a magnetic solid with localized spins are probed by magnetic susceptibility, neutron scattering and Raman scattering measurements, and are analyzed using a spin Hamiltonian with a set of spin exchange parameters. The nature and values of the spin exchange parameters deduced from this analysis depend on what spin exchange paths one includes in the spin Hamiltonian. In this article, we review how spin exchange interactions of magnetic solids with localized spins are described on formal, quantitative and qualitative theoretical levels, investigate antisymmetric and anisotropic interactions for general spin dimers, and discuss the spin exchange interactions and magnetic structures of various extended magnetic solids on the basis of spin dimer analysis. Strongly interacting spin exchange paths of a magnetic solid are determined by the overlap between its magnetic orbitals, so that the strongly interacting spin unit of a magnetic solid does not necessarily have the same geometrical feature as does the arrangement of its magnetic ions or spin-carrying molecules. Therefore, in interpreting results of magnetic susceptibility, inelastic neutron scattering or Raman scattering measurements, it is essential to employ a set of spin exchange parameters chosen on the basis of proper electronic structure considerations. Spin dimer analyses based on extended Hückel tight binding calculations provide a reliable and expedient means to study the relative strengths of superexchange and super-superexchange spin exchange interactions.}, number={2}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Whangbo, MH and Koo, HJ and Dai, D}, year={2003}, month={Dec}, pages={417–481} }
 @article{gauthier_jobic_evain_koo_whangbo_fouassier_brec_2003, title={Syntheses, structures, and optical properties of yellow Ce2SiS5, Ce6Si4S17, and Ce4Si3S12 materials}, volume={15}, ISSN={["0897-4756"]}, DOI={10.1021/cm0211711}, abstractNote={The Ce−S−Si system has been explored in search of new, yellow nontoxic pigments. We prepared a new Ce−S−Si phase (Ce6Si4S17), determined the crystal structures of Ce2SiS5, Ce4Si3S12, and Ce6Si4S17, and measured the chromatic properties of these Ce−S−Si phases. The differences in the yellow hue of these compounds were probed by analyzing their Ce3+ ion environments and calculating their electronic band structures. Ce6Si4S17 is distinguished from Ce2SiS5 and Ce4Si3S12 in terms of the Ce3+ structural environment and the Ce3+ 4f1→5d0 transition gap. This gap is wider for Ce6Si4S17 than for Ce2SiS5 and Ce4Si3S12 (i.e., 2.51 vs. ∼2.36 eV), and Ce6Si4S17 exhibits room-temperature luminescence whereas Ce2SiS5 and Ce4Si3S12 do not. Ce4Si3S12 and Ce6Si4S17 possess chromatic properties similar to those found for industrial pigments such as PbCrO4, BiVO4, and CdS, and show thermal and chemical stabilities.}, number={4}, journal={CHEMISTRY OF MATERIALS}, author={Gauthier, G and Jobic, S and Evain, M and Koo, HJ and Whangbo, MH and Fouassier, C and Brec, R}, year={2003}, month={Feb}, pages={828–837} }
 @article{montgomery_starkey_engel_bollinger_tittelbach_schweitzer_heinen_koo_whangbo_2003, title={Synthesis of the highly ID conductor (THTTF)(4)I-19}, volume={133}, ISSN={["0379-6779"]}, DOI={10.1016/S0379-6779(02)00409-5}, abstractNote={Abstract The suitability of tetrahydrotetrathiafulvalene (THTTF) as an organic metal precursor was investigated. In general, the radical-cation salts of THTTF grew with difficulty, were of poor quality, and had low electrical conductivities. The most interesting salt that we characterized to date is (THTTF) 4 (I 5 ) 2 (I 3 )(I 2 ) 3 , which possesses a unique 1D conducting network. The X-ray structure, resistivity measurements, ESR spectra, and band calculations for (THTTF) 4 I 19 are discussed.}, journal={SYNTHETIC METALS}, author={Montgomery, LK and Starkey, KP and Engel, DB and Bollinger, J and Tittelbach, M and Schweitzer, D and Heinen, I and Koo, HJ and Whangbo, MH}, year={2003}, month={Mar}, pages={381–383} }
 @article{shultz_vostrikova_bodnar_koo_whangbo_kirk_depperman_kampf_2003, title={Trends in metal-biradical exchange interaction for first-row M-II(nitronyl nitroxide-semiquinone) complexes}, volume={125}, ISSN={["0002-7863"]}, DOI={10.1021/ja020715x}, abstractNote={We report molecular structures and temperature-dependent magnetic susceptibility data for several new metal complexes of heterospin triplet ground-state biradical ligands. The ligands are comprised of both nitronyl-nitroxide (NN) and semiquinone (SQ) spin carriers. Five compounds are five-coordinate M(II) complexes (M = Mn, Co, Ni, Cu, and Zn), and one is a six-coordinate Ni(II) complex. Five compounds were structurally characterized. During copper complex formation a reaction with methanol occurs to form a unique methoxy-substituted SQ ring. Variable-temperature magnetic susceptibility studies are consistent with strong intraligand (NN-SQ and NN-PhSQ) ferromagnetic exchange coupling. For the five-coordinate Mn, Co, and Ni complexes, the S = 1 ligand is antiferromagnetically coupled to the metal. For both the five-coordinate Cu complex and the six-coordinate Ni complex, the ligand is ferromagnetically coupled to the metal spins in accordance with orbital symmetry arguments. Despite the low molecular symmetries, the predicted trend in metal-ligand exchange interactions is supported by spin dimer analysis based on extended Hückel calculations. For (NN-SQ)NiTp(Cum,Me)() (Tp(Cum,Me)() = hydro-tris(3-cumenyl-5-methylpyrazolyl)borate), an antisymmetric exchange term was required for the best fit of the magnetic susceptibility data. Antisymmetric exchange was less important for the other complexes due to inherently smaller Deltag. Finally, it is shown that intraligand exchange coupling is of paramount importance in stabilizing high-spin states of mixed metal-biradical complexes.}, number={6}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Shultz, DA and Vostrikova, KE and Bodnar, SH and Koo, HJ and Whangbo, MH and Kirk, ML and Depperman, EC and Kampf, JW}, year={2003}, month={Feb}, pages={1607–1617} }
 @article{dai_koo_whangbo_soulard_rocquefelte_jobic_2003, title={Trends in the structure and bonding in the layered platinum dioxide and dichalcogenides PtQ(2) (Q = O, S, Se, Te)}, volume={173}, ISSN={["0022-4596"]}, DOI={10.1016/s0022-4596(03)00100-2}, abstractNote={Abstract The structure and bonding of the layered platinum dioxide and dichalcogenides PtQ2 (Q=O, S, Se, Te) were analyzed on the basis of electronic band structure calculations using the full potential linearized augmented plane wave method. We examined why the c/a ratio in PtQ2 is considerably small compared with the value expected from the consideration of closely packed Q atoms (i.e., ∼1.40 vs. 1.67), and identified the electronic factor that causes the semiconducting properties in PtO2 and PtS2, the semimetallic property in PtSe2, and the metallic property in PtTe2. To a first approximation, the oxidation states of oxygen and platinum in PtO2 can be regarded as –2 and +4, respectively, but this picture is not applicable to PtTe2. As the ligand Q is changed from O to S to Se to Te, the energy gap between the Pt 5d and the ligand p levels gradually decreases, so that the ionic character of the Pt–Q bonding in PtQ2 is gradually diminished.}, number={1}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Dai, D and Koo, HJ and Whangbo, MH and Soulard, C and Rocquefelte, X and Jobic, S}, year={2003}, month={Jun}, pages={114–121} }
 @article{deplano_leoni_mercuri_schlueter_geiser_wang_kini_manson_gomez-garcia_coronado_et al._2002, title={A two-dimensional radical salt based upon BEDT-TTF and the dimeric, magnetic anion [Fe(tdas)(2)](2)(2-): (BEDT-TTF)(2)[Fe(tdas)(2)] (tdas=1,2,5-thiadiazole-3,4-dithiolate)}, volume={12}, ISSN={["1364-5501"]}, DOI={10.1039/b204435h}, abstractNote={In an attempt to synthesize new synthetic metals which couple magnetic properties to conductivity, we prepared the novel salt (BEDT-TTF)2[Fe(tdas)2] [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, hereafter referred to as ET; tdas = 1,2,5-thiadiazole-3,4-dithiolate] by the electrocrystallization technique. The crystal structure of this compound, as determined by single crystal X-ray diffraction, contains conducting, organic ET layers separated by dimerized, magnetic [Fe(tdas)2]22− anions. Due to the magnetic insulating ground electronic state of the ET layer, thermally activated conductivity is observed, with a room temperature value of about 1 S cm−1. This salt was also characterized by EPR spectroscopy, dc magnetization measurements and Raman spectroscopy. The electronic structure of the ET layers was investigated by extended Hückel tight-binding calculations, and the spin exchange interactions of the ET and [Fe(tdas)2]− layers were examined by spin-dimer analysis.}, number={12}, journal={JOURNAL OF MATERIALS CHEMISTRY}, author={Deplano, P and Leoni, L and Mercuri, ML and Schlueter, JA and Geiser, U and Wang, HH and Kini, AM and Manson, JL and Gomez-Garcia, CJ and Coronado, E and et al.}, year={2002}, pages={3570–3577} }
 @article{whangbo_koo_dai_villesuzanne_2002, title={Analysis of the electron localization, the anisotropy of electrical conductivity, the orbital ordering, and spin-exchange interactions in BaVS3 on the basis of first principles and semi-empirical electronic structure calculations}, volume={165}, ISSN={["1095-726X"]}, DOI={10.1006/jssc.2002.9550}, abstractNote={The electrical transport and magnetic properties of BaVS3, made up of individual VS3 octahedral chains, were examined on the basis of first principles and tight-binding electronic structure calculations. The electrical conductivity of BaVS3 is nearly isotropic despite its one-dimensional structural feature, because of the orbital interactions associated with the short S···S contacts within each VS3 chain and between adjacent VS3 chains. The probable cause for the metal–insulator transition at ∼70 K was examined in terms of first principles electronic structure calculations, which indicate that the metallic and magnetic insulating states of BaVS3 are nearly the same in energy. This is consistent with the observation that the metal–insulator transition at ∼70 K is caused by electron localization. The observed magnetic properties of BaVS3 below ∼70 K are readily explained under the assumption that the symmetry-broken t2g-orbitals act as the magnetic orbitals in the magnetic insulating state of BaVS3. The probable cause for the latter was discussed.}, number={2}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Whangbo, MH and Koo, HJ and Dai, D and Villesuzanne, A}, year={2002}, month={May}, pages={345–358} }
 @article{zhu_musfeldt_koo_whangbo_teweldemedhin_greenblatt_2002, title={Dimensionality effects on the optical properties of (PO2)(4)(WO3)(2m) (m=2, 4, 6, 7)}, volume={14}, ISSN={["0897-4756"]}, DOI={10.1021/cm011675j}, abstractNote={We report the 300 K polarized reflectance spectra and calculated electronic band structures of a series monophosphate tungsten bronzes, (PO2)4(WO3)2m (m = 2, 4, 6, 7). These materials have several layers of corner-sharing WO6 octahedra separated by one PO4 layer, leading to an octahedral layer thickness that is “tunable” with m. In the optical regime, the spectra of the m = 2, 4, 6, and 7 materials display an anisotropic electronic excitation, originating from the W intra-t2g d → d transition. The intensity and frequency of the intra-t2g d → d excitation vary with the octahedral layer thickness. Several vibrational modes along the interlayer direction of the m = 4, 6, and 7 compounds change with m as well. These results are consistent with the lattice becoming softer with increasing m. The low-frequency electrodynamics of the monophosphate tungsten bronzes shows a gap or pseudogap feature in the infrared region, demonstrating a ubiquitous bound-carrier response in these tungsten bronzes.}, number={6}, journal={CHEMISTRY OF MATERIALS}, author={Zhu, ZT and Musfeldt, JL and Koo, HJ and Whangbo, MH and Teweldemedhin, ZS and Greenblatt, M}, year={2002}, month={Jun}, pages={2607–2615} }
 @article{whangbo_koo_dai_jung_2002, title={Effect of metal-ligand bond lengths on superexchange interactions in Jahn-Teller d(4) ion systems: Spin dimer analysis of the magnetic structure of marokite CaMn2O4}, volume={41}, ISSN={["1520-510X"]}, DOI={10.1021/ic0202458}, abstractNote={In marokite CaMn(2)O(4), all six Mn-O bonds of each MnO(6) octahedron are different because of the Jahn-Teller distortion so that every Mn(3+) (d(4)) ion has six different superexchange interactions with its neighboring Mn(3+) ions. The spin exchange interactions of CaMn(2)O(4) were examined on the basis of spin dimer analysis to find what geometrical parameters of the Mn-O-Mn superexchange paths control the signs and strengths of their spin exchange interactions. Our work correctly describes the magnetic structure of CaMn(2)O(4) observed from neutron powder diffraction measurements and shows that the antiferromagnetic interactions of the Mn-O-Mn paths depend primarily on the asymmetry and the Mn-O bond length of the Mn-O-Mn bridge, but not on the 90 degree angle Mn-O-Mn bond angle.}, number={21}, journal={INORGANIC CHEMISTRY}, author={Whangbo, MH and Koo, HJ and Dai, D and Jung, D}, year={2002}, month={Oct}, pages={5575–5581} }
 @article{whangbo_koo_villesuzanne_pouchard_2002, title={Effect of metal-oxygen covalent bonding on the competition between Jahn-Teller distortion and charge disproportionation in the perovskites of high-spin d(4) metal ions LaMnO3 and CaFeO3}, volume={41}, DOI={10.1021/ic0110427}, abstractNote={The perovskites LaMnO(3) and CaFeO(3) consisting of high-spin d(4) transition metal ions undergo different types of distortions, i.e., a Jahn-Teller distortion in LaMnO(3) and a charge disproportionation in CaFeO(3). We investigated the electronic factor causing this difference on the basis of first principles spin-polarized electronic band structure calculations for their ideal cubic structures and also tight-binding electronic band structure calculations for their ideal cubic and distorted structures. Our study shows that a charge disproportionation is favored over a Jahn-Teller distortion in CaFeO(3) because the covalent character is strong in the Fe-O bond, while the opposite is true for LaMnO(3) because the covalent character is weak in the Mn-O bond. In spin-polarized electronic band structure calculations, the covalency of the M-O (M = Fe, Mn) bond is enhanced in the up-spin bands but is reduced in the down-spin bands. Our analysis shows that electron-electron repulsion causes the energy gap between the metal 3d and the oxygen 2p orbitals to become larger for the down-spin than for the up-spin-orbital interactions. Thus in the d-block e(g) bands of both LaMnO(3) and CaFeO(3) the metal 3d orbital contribution is larger in the down-spin than in the up-spin bands.}, number={7}, journal={Inorganic Chemistry}, author={Whangbo, M. H. and Koo, H. J. and Villesuzanne, A. and Pouchard, M.}, year={2002}, pages={1920–1929} }
 @article{rozier_galy_chelkowska_koo_whangbo_2002, title={Electrical resistivity, magnetic susceptibility, X-ray photoelectron spectroscopy, and electronic band structure studies of Cu-2.33-xV4O11}, volume={166}, ISSN={["0022-4596"]}, DOI={10.1006/jssc.2002.9609}, abstractNote={The electronic and physical properties of Cu2.33V4O11 were characterized by electrical resistivity, magnetic susceptibility and X-ray photoelectron spectroscopy (XPS) measurements and by tight-binding electronic band structure calculations. Attempts to prepare Cu2.33−xV4O11 outside its narrow homogeneity range led to a mixture of Cu2.33V4O11, CuVO3 and β-CuxV2O5. The magnetic susceptibility data show no evidence for a magnetic/structural transition around 300 K. The XPS spectra of Cu2.33V4O11 reveal the presence of mixed valence in both Cu and V. The [Cu+]/[Cu2+] ratio is estimated to be 1.11 from the Cu 2p3/2 peak areas, so [V4+]/[V5+]=0.56 by the charge balance. Our electronic structure calculations suggest that the oxidation state of the Cu ions is +2 in the channels of CuO4 tetrahedra, and +1 in the channels of linear CuO2 and trigonal planar CuO3 units. This predicts that [Cu+]/[Cu2+]=1.33 and [V4+]/[V5+]=0.50, in good agreement with those deduced from the XPS study.}, number={2}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Rozier, P and Galy, J and Chelkowska, G and Koo, HJ and Whangbo, MH}, year={2002}, month={Jul}, pages={382–388} }
 @article{koo_whangbo_vernooy_torardi_marshall_2002, title={Flux growth of vanadyl pyrophosphate, (VO)(2)P2O7, and spin dimer analysis of the spin exchange interactions of (VO)(2)P2O7 and vanadyl hydrogen phosphate, VO(HPO4)center dot 0.5H(2)O}, volume={41}, ISSN={["1520-510X"]}, DOI={10.1021/ic020249c}, abstractNote={Large transparent blue crystals of vanadyl pyrophosphate, (VO)(2)P(2)O(7), were grown from a phosphorus pentoxide flux, and the single-crystal X-ray structure of (VO)(2)P(2)O(7) was determined with high precision. On the basis of spin dimer analysis, we examined the spin exchange interactions of (VO)(2)P(2)O(7) and its precursor VO(HPO(4)).0.5H(2)O. Our analysis of (VO)(2)P(2)O(7) using two high-precision crystal structures shows unambiguously that the V3-V4 chain has a larger spin gap than does the V1-V2 chain and that the super-superexchange (V-O...O-V) interaction is stronger than the superexchange (V-O-V) interaction in the V3-V4 chain while the opposite is true in the V1-V2 chain. Our analysis of VO(HPO(4)).0.5H(2)O reveals that the superexchange interaction must dominate over the super-superexchange interaction, in disagreement with the conclusion from a powder neutron scattering study of VO(DPO(4)).0.5D(2)O.}, number={18}, journal={INORGANIC CHEMISTRY}, author={Koo, HJ and Whangbo, MH and VerNooy, PD and Torardi, CC and Marshall, WJ}, year={2002}, month={Sep}, pages={4664–4672} }
 @article{clayhold_ulutagay-kartin_hwu_koo_whangbo_voigt_eaiprasertsak_2002, title={Magnetic properties of the low-dimensional cuprate Na5RbCu4(AsO4)(4)Cl-2}, volume={66}, number={5}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Clayhold, J. A. and Ulutagay-Kartin, M. and Hwu, S. J. and Koo, H. J. and Whangbo, M. H. and Voigt, A. and Eaiprasertsak, K.}, year={2002}, pages={052403–1} }
 @article{choi_musfeldt_wang_koo_whangbo_galy_millet_2002, title={Optical investigation of Na2V3O7 nanotubes}, volume={14}, ISSN={["1520-5002"]}, DOI={10.1021/cm010965x}, abstractNote={We report the electronic and vibrational properties of Na2V3O7 nanotubes and compare the response with other layered and nonlayered vanadates. The electronic structure of Na2V3O7 displays a strong similarity to that of nontubular vanadates. We assign the 1.2-eV band as a V d → d excitation and the 3.3- and 3.9-eV bands as O 2p → V 3d charge-transfer structures. Although band structure calculations predict additional fine structure in the density of states because of the tubular morphology, such features are not observed in the absorption spectrum. The vibrational spectrum displays triplet mode splitting as a result of reduced site symmetry, consistent with the three sightly different vanadium atoms that form the basic structural unit. A low-frequency rattling mode is observed in Na2V3O7 at 88 cm-1. This unique characteristic of the Na+-ion intercalated tubes might be connected with the ionic conductivity.}, number={2}, journal={CHEMISTRY OF MATERIALS}, author={Choi, J and Musfeldt, JL and Wang, YJ and Koo, HJ and Whangbo, MH and Galy, J and Millet, P}, year={2002}, month={Feb}, pages={924–930} }
 @article{whangbo_koo_2002, title={Orbital interaction analysis of cooperative Jahn-Teller distortion, orbital ordering, spin ordering, and spin exchange interactions in magnetic solids}, volume={4}, ISSN={["1873-3085"]}, DOI={10.1016/S1293-2558(01)01261-4}, abstractNote={The phenomena of cooperative Jahn–Teller distortion, orbital ordering and spin exchange in magnetic solids were examined in terms of orbital interactions between adjacent spin sites. Cooperative Jahn–Teller distortion and orbital ordering of a magnetic solid occur to minimize the two-electron two-orbital destabilizing interactions between adjacent spin sites. The magnetic structures of cubic perovskites KCuF3, LaMnO3 and YVO3 are well accounted for in terms of the spin exchange interactions associated with their ordered magnetic orbitals.}, number={3}, journal={SOLID STATE SCIENCES}, author={Whangbo, MH and Koo, HJ}, year={2002}, month={Mar}, pages={335–346} }
 @article{whangbo_koo_2002, title={Spin dimer analysis of the spin exchange interactions in paramelaconite Cu4O3 and its analogue Ag2Cu2O3 and the spin ordering of the Cu2O3 spin lattice leading to their magnetic phase transitions}, volume={41}, ISSN={["1520-510X"]}, DOI={10.1021/ic020141x}, abstractNote={The magnetic structures of the Cu(2)O(3) spin lattices present in Cu(4)O(3) and Ag(2)Cu(2)O(3) were analyzed by studying their spin exchange interactions on the basis of spin dimer analysis. Calculations of spin exchange parameters were calibrated by studying LiCuVO(4) whose intrachain and interchain antiferromagnetic spin exchange parameters are known experimentally. The magnetic phase transition of Cu(4)O(3) at 42.3 K doubles the unit cell along each crystallographic direction. The spin arrangements of the Cu(2)O(3) lattice consistent with this experimental observation are different from conventional antiferromagnetic ordering. Our analysis indicates that spin fluctuation should occur in Cu(4)O(3), low-dimensional magnetism should be more important than magnetic frustration in Cu(4)O(3), and Ag(2)Cu(2)O(3) and Cu(4)O(3) should have similar structural and magnetic properties.}, number={13}, journal={INORGANIC CHEMISTRY}, author={Whangbo, MH and Koo, HJ}, year={2002}, month={Jul}, pages={3570–3577} }
 @misc{koo_whangbo_lee_2002, title={Spin dimer analysis of the three-dimensional antiferromagnetic ordering in the quaternary manganese sulfides BaLn(2)MnS(5) (Ln = La, Ce, Pr)}, volume={169}, ISSN={["1095-726X"]}, DOI={10.1016/S0022-4596(02)00021-X}, abstractNote={The quaternary manganese sulfides BaLn2MnS5 (Ln=La, Ce, Pr) consist of (MnS4)6− anions separated with short S⋯S distances slightly longer than the van der Waals distance. Nevertheless, these sulfides are known to undergo a three-dimensional (3D) antiferromagnetic ordering at a reasonably high temperature (i.e., TN=58.5, 62.0 and 64.5 K for Ln=La, Ce and Pr, respectively). The origin of this observation was probed by studying the Mn–S⋯S–Mn super–superexchange interactions of BaLn2MnS5 on the basis of spin dimer analysis. The non-bonding S⋯S contacts in the vicinity of the van der Waals distance are found essential in determining the strengths of the Mn–S⋯S–Mn super–superexchange interactions. The antiferromagnetic spin exchange between adjacent (MnS4)6− anions along the c-direction (J2) is calculated to be stronger than that in the ab-plane (J1) by a factor of ∼10, so that the strongly interacting spin units of BaLn2MnS5 (Ln=La, Ce, Pr) are 1D chains made up of the exchange paths J2. The relative strengths of the spin exchange interactions for the J1 and J2 paths are consistent with the finding that the Néel temperatures of BaLn2MnS5 are reasonably high, and they increase in the order BaLa2MnS5<BaCe2MnS5< BaPr2MnS5.}, number={2}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Koo, HJ and Whangbo, MH and Lee, KS}, year={2002}, month={Dec}, pages={143–148} }
 @article{whangbo_koo_dumas_continentino_2002, title={Theoretical investigation of the spin exchange interactions and magnetic properties of the homometallic ludwigite Fe3O2BO3}, volume={41}, ISSN={["1520-510X"]}, DOI={10.1021/ic010956q}, abstractNote={The homometallic ludwigite Fe(3)O(2)BO(3) has a complex structure made up of corner- and edge-sharing FeO(6) octahedra and exhibits a number of apparently puzzling magnetic properties. The reasons for these properties were probed by examining the trends in the spin exchange interactions of Fe(3)O(2)BO(3). To analyze the relative strengths of spin exchange interactions in such a complex magnetic solid, we first generalized the method of spin dimer analysis and then employed the resulting formulation to investigate how the magnetic properties of Fe(3)O(2)BO(3) are related to its reported crystal structures. The spin-orbital interaction energies calculated for various spin dimers of Fe(3)O(2)BO(3) provide estimates for the relative strengths of the associated spin exchange interactions, which in turn account for the observed magnetic properties of Fe(3)O(2)BO(3).}, number={8}, journal={INORGANIC CHEMISTRY}, author={Whangbo, MH and Koo, HJ and Dumas, J and Continentino, MA}, year={2002}, month={Apr}, pages={2193–2201} }
 @article{schlueter_geiser_wang_kini_ward_parakka_daugherty_kelly_nixon_winter_et al._2002, title={Trifluoromethylsulfonyl-based salts of BEDT-TTF: Crystal and electronic structures and physical properties}, volume={168}, ISSN={["1095-726X"]}, DOI={10.1006/jssc.2002.9750}, abstractNote={Abstract Three 2 : 1 salts of the organic donor molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) with trifluoromethylsulfonyl-based anions N(SO2CF3)2−, CH(SO2CF3)2− and C(SO2CF3)3− were prepared by electrocrystallization. These salts were characterized by single-crystal X-ray diffraction, electron spin resonance (ESR) spectroscopy, electrical resistivity measurements and electronic band structure calculations. (ET)2N(SO2CF3)2 is a two-dimensional (2D) metal, but its ESR spin susceptibility above 150 K shows a weakly semiconducting behavior, presumably because during ESR measurements the sample cooling rate is slow hence allowing the disordered anions to readjust their positions. (ET)2CH (SO2CF3)2 is a 2D metal and undergoes a metal-to-insulator (MI) transition at 110 K due probably to a geometry change of the donor molecule layers. (ET)2C(SO2CF3)3 is a one-dimensional (1D) metal and undergoes an MI between 180 and 240 K, which is expected to be of charge density wave type.}, number={2}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Schlueter, JA and Geiser, U and Wang, HH and Kini, AM and Ward, BH and Parakka, JP and Daugherty, RG and Kelly, ME and Nixon, PG and Winter, RW and et al.}, year={2002}, month={Nov}, pages={524–534} }
 @article{gourdon_jeanneau_evain_jobic_brec_2001, title={Influence of the metal-metal sigma bonding on the structures and physical properties of the hexagonal perovskite-type sulfides Sr9/8TiS3,Sr8/7TiS3, and Sr-8/7[Ti6/7Fe1/7]S-3}, volume={162}, ISSN={["0022-4596"]}, DOI={10.1006/jssc.2001.9361}, abstractNote={The structural and electronic consequences of the metal–metal sigma bonding in the hexagonal perovskite sulfides were examined by calculating the electronic band structures of Sr9/8TiS3 and Sr8/7TiS3, by synthesizing Sr8/7[Ti6/7Fe1/7]S3 and characterizing its crystal structure, and by measuring the magnetic susceptibilities of Sr9/8TiS3, Sr8/7TiS3, and Sr8/7[Ti6/7Fe1/7]S3. The (TiS3)∞ chains of Sr9/8TiS3 and Sr8/7TiS3 contain oligomer units (Oh)n (n=4, 5) made up of face-sharing TiS6 octahedra (Oh). Given the (TiS3)∞ chains aligned along the z direction, the only occupied d-block levels of each (TiS3)∞ chain are the most sigma bonding level of each (Oh)n oligomer, which is constructed from its nz2 orbitals. This “n-orbital two-electron” metal–metal sigma bonding controls the d-orbital density distribution and the Ti–Ti distances of the (Oh)n oligomers and is ultimately responsible for the semiconducting property of Sr9/8TiS3, Sr8/7TiS3, and Sr8/7[Ti6/7Fe1/7]S3. The Sr8/7[Ti6/7 Fe1/7]S3 structure is described by the trigonal symmetry, R̄3m (00γ)0s superspace group with the following parameters: as=11.4935(11) Å, cs=2.9986(7) Å, q=0.57161(15) c*, and Vs=343.05(15) Å3.}, number={1}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Gourdon, O and Jeanneau, E and Evain, M and Jobic, S and Brec, R}, year={2001}, month={Nov}, pages={103–112} }