@article{matar_eyert_villesuzanne_whangbo_2007, title={First-principles study of the electronic and magnetic structures of the tetragonal and orthorhombic phases of Ca3Mn2O7}, volume={76}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.76.054403}, abstractNote={On the basis of density functional theory electronic band structure calculations using the augmented spherical wave method, the electronic and magnetic properties of the orthorhombic and tetragonal phases of ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ were investigated and the spin exchange interactions of the orthorhombic phase were analyzed. Our calculations show that the magnetic insulating states are more stable than the nonmagnetic metallic state for both polymorphs of ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, the orthorhombic phase is more stable than the tetragonal phase, and the ground state of the orthorhombic phase is antiferromagnetic. The total energies calculated for the three spin states of the orthorhombic phase of ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ led to estimates of the spin exchange interactions ${J}_{\mathit{nn}}=\ensuremath{-}3.36\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and ${J}_{\mathit{nnn}}=\ensuremath{-}0.06\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. The accuracy of these estimates was tested by calculating the Curie-Weiss temperature within the mean-field approximation.}, number={5}, journal={PHYSICAL REVIEW B}, author={Matar, S. F. and Eyert, V. and Villesuzanne, A. and Whangbo, M.-H.}, year={2007}, month={Aug} }
 @article{villesuzanne_whangbo_2005, title={Comparative electronic band structure study of the intrachain ferromagnetic versus antiferromagnetic coupling in the magnetic oxides Ca3CO2O6 and Ca3FeRhO6}, volume={44}, ISSN={["1520-510X"]}, DOI={10.1021/ic0508775}, abstractNote={In the (MM'O6)infinity chains of the transition-metal magnetic oxides Ca3MM'O6 the MO6 trigonal prisms alternate with the M'O6 octahedra by sharing their triangular faces. In the (Co(2O6)infinity chains of Ca3Co2O6 (M = M' = Co) the spins are coupled ferromagnetically, but in the (FeRhO6)infinity chains of Ca3FeRhO6 (M = Fe, M' = Rh) they are coupled antiferromagnetically. The origin of this difference was probed by carrying out spin-polarized density functional theory electronic band structure calculations for ordered spin states of Ca3Co2O6 and Ca3FeRhO6. The spin state of a (MM'O6)infinity chain determines the occurrence of direct metal-metal bonding between the adjacent trigonal prism and octahedral site transition-metal atoms. The extent of direct metal-metal bonding in the (Co2O6)infinity chains of Ca3Co2O6 is stronger in the intrachain ferromagnetic state than in the intrachain antiferromagnetic state, so that the intrachain ferromagnetic state becomes more stable than the intrachain antiferromagnetic state. Such a metal-metal-bonding-induced ferromagnetism is expected to occur in magnetic insulators and magnetic metals of transition-metal elements in which direct metal-metal bonding can be enhanced by ferromagnetic ordering. In the (FeRhO6)infinity chains of Ca3FeRhO6 the ferromagnetic coupling does not lead to a strong metal-metal bonding and the adjacent spins interact by the Fe-O...O-Fe super-superexchange, hence leading to an antiferromagnetic coupling.}, number={18}, journal={INORGANIC CHEMISTRY}, author={Villesuzanne, A and Whangbo, MH}, year={2005}, month={Sep}, pages={6339–6345} }
 @article{kim_seo_kremer_kohler_villesuzanne_whangbo_2005, title={Large negative magnetoresistance of the rare-earth transition-metal intermetallic compound PrMnSi2}, volume={17}, ISSN={["0897-4756"]}, DOI={10.1021/cm051669d}, abstractNote={The rare-earth intermetallic compound PrMnSi2 consists of two magnetic sublattices, i.e., layers of Mn atoms and layers of Pr atoms, which repeat in the (−Mn−Pr−Pr−)∞ sequence. Electrical resistivity measurements were carried out for PrMnSi2 between 2 and 350 K under various magnetic fields. PrMnSi2 is a metal with two metal-to-metal transitions at ∼20 and ∼50 K, and its resistivity vs temperature curves show a significant hysteresis in the absence and presence of an external magnetic field. The resistivity is reduced by an external magnetic field, and this negative magnetoresistance reaches up to 47% at 9 T around 17 K where PrMnSi2 is antiferromagnetic at zero field. The magnetoresistance vs temperature curves of PrMnSi2 reveal three peaks at fields up to ∼3 T, which correspond to the three magnetic phase transitions of PrMnSi2 observed from magnetic susceptibility and heat capacity measurements. The negative magnetoresistance and the resistivity hysteresis of PrMnSi2 were accounted for by considering h...}, number={25}, journal={CHEMISTRY OF MATERIALS}, author={Kim, SH and Seo, DK and Kremer, RK and Kohler, M and Villesuzanne, A and Whangbo, MH}, year={2005}, month={Dec}, pages={6338–6341} }
 @article{kim_seo_kremer_kohler_villesuzanne_whangbo_2005, title={Observation of unusual hysteretic magnetic properties of the rare earth intermetallic compound PrMnSi2: Magnetic susceptibility, magnetization, heat capacity, and electronic band structure studies}, volume={17}, ISSN={["0897-4756"]}, DOI={10.1021/cm050622p}, abstractNote={The magnetic properties of the rare earth intermetallic compound PrMnSi2 were characterized by carrying out magnetic susceptibility, magnetization, and heat capacity measurements and performing APW+lo/LDA+U electronic band structure calculations for several ordered spin states of PrMnSi2. The temperature dependence of the magnetic susceptibilities shows strong hysteresis at low magnetic fields and exhibits hysteresis under magnetic fields up to ∼5 T. The magnetizations present unusual hysteresis loops as a function of temperature. The magnetic susceptibility and heat capacity measurements reveal that PrMnSi2 undergoes three magnetic phase transitions, one below ∼50 K and two below ∼20 K. The probable causes for the observed anomalous magnetic properties were discussed on the basis of nearly degenerate ordered spin states found from electronic structure calculations.}, number={14}, journal={CHEMISTRY OF MATERIALS}, author={Kim, SH and Seo, DK and Kremer, RK and Kohler, J and Villesuzanne, A and Whangbo, MH}, year={2005}, month={Jul}, pages={3711–3716} }
 @article{villesuzanne_whangbo_subramanian_matar_2005, title={Spin dimer and electronic band structure analyses of the ferromagnetism versus antiferromagnetism in SeCuO3 and TeCuO3}, volume={17}, ISSN={["0897-4756"]}, DOI={10.1021/cm050885j}, abstractNote={On the basis of spin dimer analysis and density functional theory electronic band structure calculations, we examined why the magnetic ground state of SeCuO3 is ferromagnetic while that of its isostructural analogue TeCuO3 is antiferromagnetic and estimated their spin exchange parameters. The essential difference between the magnetic properties of these oxides arises from their Cu−O(1)−Cu superexchange, but not their Cu−O(2)−Cu superexchange. Spin exchange paths relevant for understanding magnetic properties are those that contain magnetic orbitals.}, number={17}, journal={CHEMISTRY OF MATERIALS}, author={Villesuzanne, A and Whangbo, MH and Subramanian, MA and Matar, SF}, year={2005}, month={Aug}, pages={4350–4355} }
 @article{kohler_friedrich_whangbo_villesuzanne_2005, title={Synthesis and characterization of [PtIn6](GeO4)(2)O and its solid solution [PtIn6](GaO4)(2-x)(GeO4)(x)O-x/2 (0 <= x <= 2): Gradual color change of the solid solution from black (x=0) to yellow (x=2) as a consequence of quantum dot effect}, volume={127}, ISSN={["1520-5126"]}, DOI={10.1021/ja053280x}, abstractNote={A new phase [PtIn6](GeO4)2O, a filled variant of [PtIn6](GaO4)2, and the solid solution [PtIn6](GaO4)2-x(GeO4)xOx/2 (0 ≤ x ≤ 2) were prepared and characterized. Single-crystal structure refinements show that [PtIn6](GeO4)2O is isotypic with the mineral, sulfohalite Na6FCl(SO4)2, and crystallizes in the space group Fm3̄m (Z = 4) with a = 1006.0(1) pm. The building units of [PtIn6](GeO4)2O are isolated [PtIn6]10+ octahedra and (GeO4)4- tetrahedra, and the isolated O2- ions occupy the centers of the In6 octahedra made up of six adjacent PtIn6 octahedra. The lattice parameter of the solid solution [PtIn6](GaO4)2-x(GeO4)xOx/2 (0 ≤ x ≤ 2) varies gradually from a = 1001.3(1) pm at x = 0 to a = 1006.0(1) pm at x = 2, and the color of the solid solution changes gradually from black (x = 0) to red (x = 1) to yellow (x = 2). The cause for the gradual color change was examined by performing density functional theory electronic structure calculations for the end members [PtIn6](GaO4)2 and [PtIn6](GeO4)2O. Our analysis indicates that an oxygen atom at the center of a In6 octahedron cuts the In 5p/In 5p bonding interactions between adjacent [PtIn6]10+ octahedra thereby raising the bottom of the conduction bands, and the resulting quantum dot effect is responsible for the color change.}, number={37}, journal={JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}, author={Kohler, J and Friedrich, H and Whangbo, MH and Villesuzanne, A}, year={2005}, month={Sep}, pages={12990–12996} }
 @article{villesuzanne_whangbo_koo_2005, title={Unpaired spin populations and spin-pairing tendencies of the nonequivalent vanadium sites of the magnetic metal NaV6O11 investigated by electronic band structure calculations and spin dimer analysis}, volume={17}, ISSN={["1520-5002"]}, DOI={10.1021/cm050666y}, abstractNote={The unpaired spin populations and spin-pairing tendencies of the three different vanadium atoms of NaV6O11 above and below its trimerization temperature Tt = 245 K were examined by determining, as a function of the number of unpaired spins per formula unit, N, the total energies of NaV6O11 as well as the 3d orbital populations of the V(1), V(2), and V(3) atoms on the basis of first-principles spin-polarized electronic band structure calculations. Spin dimer analysis was also carried out to estimate the spin-pairing tendencies of the V(1) and V(2) sites above and below Tt. Our work does not support the heuristic assumptions employed to interpret the magnetic properties of NaV6O11 as well as its analogues SrV6O11 and PbV6O11. States with a wide range of N values (i.e., ∼4 < N < ∼9) are expected to contribute to the magnetic properties of NaV6O11 above and below Tt. The V(1), V(2), and V(3) atoms differ mainly in the extents of their spin polarizations but not in their oxidation states. The unpaired spin pop...}, number={17}, journal={CHEMISTRY OF MATERIALS}, author={Villesuzanne, A and Whangbo, MH and Koo, HJ}, year={2005}, month={Aug}, pages={4344–4349} }