@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{whangbo_koo_lee_gourdon_evain_jobic_brec_2001, title={On the structural and electronic factors governing the magnetic properties of the hexagonal perovskite-type oxides A(x)BO(3) (A = Ca, Sr, Ba; B = Co, Ni)}, volume={160}, ISSN={["0022-4596"]}, DOI={10.1006/jssc.2001.9230}, abstractNote={Abstract The hexagonal perovskite-type oxides A x B O 3 ( A =Ca, Sr, Ba; B =Co, Ni) consist of ( B O 3 ) ∞ chains made up of face-sharing B O 6 octahedra and B O 6 trigonal prisms. On the basis of molecular orbital calculations for their structural building blocks we examined why the transition metal atoms in the trigonal prisms of Sr 9/7 NiO 3 and Sr 14/11 CoO 3 occupy the positions away from their trigonal prism centers and why Ba 6/5 NiO 3 and Ca 3/2 CoO 3 exhibit apparently puzzling magnetic properties. Our analysis indicates that the structural building units of the ( B O 3 ) ∞ chains in these oxides adopt high-spin states to reduce the metal–metal σ-antibonding interactions between adjacent metal atoms as well as the on-site repulsion. This finding led us to predict the number of unpaired spins that the oxides Sr 9/7 NiO 3 , Sr 6/5 CoO 3 , and Sr 14/11 CoO 3 are expected to have.}, number={1}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Whangbo, MH and Koo, HJ and Lee, KS and Gourdon, O and Evain, M and Jobic, S and Brec, R}, year={2001}, month={Aug}, pages={239–246} }
 @article{whangbo_koo_lee_2000, title={Analysis of nearest-neighbor spin exchange interactions by molecular orbital calculations: anisotropic spin exchange interactions in MV3O7 (M = Cd, Ca, Sr) and alpha '-NaV2O5}, volume={114}, DOI={10.1016/s0038-1098(99)00571-2}, abstractNote={Spin-dimer analysis was carried out for the room temperature structures of MV3O7 (M=Cd, Ca, Sr) and the 15 K structure of α′-NaV2O5 reported by Lüdecke et al. to examine the relative magnitudes of the nearest-neighbor spin exchange interactions between their unpaired spins. Our analysis explains why the magnetic susceptibility of the stripe spin structure of MV3O7 (M=Cd, Ca, Sr) exhibits a one-dimensional spin-1/2 Heisenberg chain-like behavior. Our study indicates that the 15 K crystal structure of α′-NaV2O5 is likely to have a spin gap and exhibit spin frustration. The predicted spin frustration is not consistent with the singlet ground state expected for a spin-Peierls system.}, number={1}, journal={Solid State Communications}, author={Whangbo, M. H. and Koo, H. J. and Lee, K. S.}, year={2000}, pages={27–32} }
 @article{lee_koo_ren_whangbo_1999, title={Calculations and analysis of the electronic structures of transition-metal phosphosilicides Cu4SiP8, IrSi3P3, CoSi3P3, and FeSi4P4}, volume={147}, ISSN={["0022-4596"]}, DOI={10.1006/jssc.1999.8142}, abstractNote={Abstract Electronic structures of phosphosilicides Cu4SiP8, IrSi3P3, CoSi3P3, and FeSi4P4 were calculated using the extended Huckel tight-binding method and were analyzed in terms of a modified electron counting scheme. The d-block levels of these compounds appear well below the Fermi level, thereby indicating that the transition-metal atoms exist as d10 ions. Except for the case of IrSi3P3, the d-block energy levels of these compounds occur in a narrow energy window less than 1.5 eV wide. The Si 3p orbitals act as acceptor orbitals to the transition-metal d orbitals in IrSi3P3, CoSi3P3, and FeSi4P4.}, number={1}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Lee, KS and Koo, HJ and Ren, J and Whangbo, MH}, year={1999}, month={Oct}, pages={11–18} }
 @article{lee_koo_dai_ren_whangbo_1999, title={Electron counting scheme relevant for late transition metal compounds with weakly electronegative ligands. Electronic band structure study of phosphosilicides PtSi3P2 and NiSi2P3}, volume={38}, ISSN={["0020-1669"]}, DOI={10.1021/ic980761r}, abstractNote={The conventional electron counting scheme often leads to erroneous predictions for late transition metal compounds with weakly electronegative main group ligand atoms. The cause for this deficiency was analyzed, and a modified electron counting scheme was proposed for predicting the frontier orbitals of such compounds. The modified scheme predicts that the transition metal atoms of such compounds have a d10 electron count, and their frontier orbitals are represented by the highest-lying lone pair levels of the main group ligand atoms. These predictions were tested by carrying out electronic band structure calculations for phosphosilicides PtSi3P2 and NiSi2P3. The relevance of nonstoichiometric compositions of these phases was also discussed.}, number={2}, journal={INORGANIC CHEMISTRY}, author={Lee, KS and Koo, HJ and Dai, D and Ren, J and Whangbo, MH}, year={1999}, month={Jan}, pages={340–345} }
 @article{lee_koo_whangbo_1999, title={Spin-spin interactions in the oxides A(3)M ' MO6 (M = Rh, Ir; A = Ca, Sr; M ' = alkaline earth, Zn, Cd, Na) of the K4CdCl6 structure type examined by electronic structure calculations}, volume={38}, ISSN={["0020-1669"]}, DOI={10.1021/ic9813819}, abstractNote={The oxides A(3)M'MO(6) (M = Rh, Ir; A = Ca, Sr; M' = alkaline earth, Zn, Cd) of the K(4)CdCl(6) structure type consist of isolated (MO(6))(8)(-) octahedral anions and exhibit an antiferromagnetic ordering at low temperatures. The spin-spin interactions in these oxides, Ca(3)NaMO(6) (M = Ir, Ru), and Sr(3)NaRuO(6) were examined by calculating how strongly the t(2g)-block levels of adjacent (MO(6))((6+)(n)()())(-) (n = 1, 2) anions interact in the presence and absence of the intervening cations A(2+) and M' (n)()(+) (n = 1, 2). Our calculations show that the spin-spin interactions in these oxides are three-dimensional, and the superexchange interactions occur mainly through the short intrachain and interchain M-O.O-M linkages. When the M(n)()(+) cation is very small compared with the A(2+) cation, the intrachain interaction is substantially stronger than the interchain interaction. The opposite is found when the sizes of the M(n)()(+) and A(2+) cations become similar.}, number={9}, journal={INORGANIC CHEMISTRY}, author={Lee, KS and Koo, HJ and Whangbo, MH}, year={1999}, month={May}, pages={2199–2203} }
 @article{lee_seo_whangbo_1997, title={Electronic band structure study of the anomalous electrical and superconducting properties of hexagonal alkali tungsten bronzes A(x)WO(3) (A=K, Rb, Cs)}, volume={119}, ISSN={["0002-7863"]}, DOI={10.1021/ja964455t}, abstractNote={The electrical and superconducting properties of hexagonal alkali tungsten bronzes AxWO3 (A = K, Rb, Cs) were examined by calculating the electronic band structure of a representative hexagonal tungsten bronze and analyzing reported crystal structures of AxWO3 (A = K, Rb, Cs). These bronzes possess one-dimensional (1D) and three-dimensional Fermi surfaces. The metal-to-semiconductor-to-metal transitions and superlattice reflections in KxWO3 and RbxWO3 are explained by a charge density wave (CDW) associated with the 1D Fermi surface. There occurs a maximum in the plots of the CDW onset temperature TB versus x for KxWO3 and RbxWO3. The presence of this maximum and the absence of a CDW in CsxWO3 reflect the balance of two opposing energy factors, the electronic instability and lattice stiffness, in forming a CDW. The dependence of the superconducting transition temperature TC on x suggests that a CDW transition removes lattice phonons conducive for superconductivity.}, number={17}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Lee, KS and Seo, DK and Whangbo, MH}, year={1997}, month={Apr}, pages={4043–4049} }
 @misc{lee_seo_whangbo_1997, title={Structural and electronic factors governing the metallic and nonmetallic properties of the pyrochlores A(2)Ru(2)O(7-y)}, volume={131}, ISSN={["0022-4596"]}, DOI={10.1006/jssc.1997.7497}, abstractNote={Abstract The metal-versus-semiconductor behavior of ruthenium pyrochlores A 2 Ru 2 O 7− y was examined by calculating their electronic band structures. This behavior is explained in terms of the Mott–Hubbard mechanism of electron localization. The width of the t 2 g -block bands of A 2 Ru 2 O 7− y increases with increasing Ru–O–Ru bond angle, and the Ru–O–Ru angle increases with increasing size of the A cation. There is a good linear relationship between the ionic radius of the A cation and the Ru–O–Ru bond angle. This relationship makes it possible to calculate the amount of the O′ atom vacancy based on the observed Ru–O–Ru angle in Tl 2 Ru 2 O 7− y .}, number={2}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Lee, KS and Seo, DK and Whangbo, MH}, year={1997}, month={Jul}, pages={405–408} }
 @article{lee_seo_whangbo_li_mackay_hwu_1997, title={Vacancy ordering as the cause for the electrical resistivity anomalies and superlattice modulations in ACu(7-x)S(4) (A = Tl, K, Rb)}, volume={134}, ISSN={["0022-4596"]}, DOI={10.1006/jssc.1997.7506}, abstractNote={Abstract The electronic structure of KCu 7 S 4 was calculated using the extended Huckel tight binding method, and the origin of the resistivity anomalies and the superlattice reflections of A Cu 7− x S 4 ( A =Tl, K, Rb) was probed. The present work shows that A Cu 7− x S 4 ( A =Tl, K, Rb) can be metallic only for x >0 and that the superlattice modulations of A Cu 7− x S 4 ( A =Tl, K, Rb) are caused by the vacancy ordering in the Cu(2) atom sites. The resistivity anomalies of A Cu 7− x S 4 are explained in terms of the vacancy ordering in the Cu(2)–Cu(2) chains.}, number={1}, journal={JOURNAL OF SOLID STATE CHEMISTRY}, author={Lee, KS and Seo, DK and Whangbo, MH and Li, H and Mackay, R and Hwu, SJ}, year={1997}, month={Nov}, pages={5–9} }