Spin orientations of the spin-half Ir{sup 4+} ions in Sr{sub 3}NiIrO{sub 6}, Sr{sub 2}IrO{sub 4}, and Na{sub 2}IrO{sub 3}: Density functional, perturbation theory, and Madelung potential analyses

Journal Article · Mon Mar 21 04:00:00 EDT 2016 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4943889· OSTI ID:22660822

Gordon, Elijah E.; Whangbo, Myung-Hwan ^[1]; Xiang, Hongjun ^[2]; Köhler, Jürgen ^[3]

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1. Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (United States)
2. Key Laboratory of Computational Physical Sciences (Ministry of Education), State Key Laboratory of Surface Physics, Collaborative Innovation Center of Advanced Microstructures, and Department of Physics, Fudan University, Shanghai 200433 (China)
3. Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart (Germany)

The spins of the low-spin Ir{sup 4+} (S = 1/2, d{sup 5}) ions at the octahedral sites of the oxides Sr{sub 3}NiIrO{sub 6}, Sr{sub 2}IrO{sub 4}, and Na{sub 2}IrO{sub 3} exhibit preferred orientations with respect to their IrO{sub 6} octahedra. We evaluated the magnetic anisotropies of these S = 1/2 ions on the basis of density functional theory (DFT) calculations including spin-orbit coupling (SOC), and probed their origin by performing perturbation theory analyses with SOC as perturbation within the LS coupling scheme. The observed spin orientations of Sr{sub 3}NiIrO{sub 6} and Sr{sub 2}IrO{sub 4} are correctly predicted by DFT calculations, and are accounted for by the perturbation theory analysis. As for the spin orientation of Na{sub 2}IrO{sub 3}, both experimental studies and DFT calculations have not been unequivocal. Our analysis reveals that the Ir{sup 4+} spin orientation of Na{sub 2}IrO{sub 3} should have nonzero components along the c- and a-axis directions. The spin orientations determined by DFT calculations are sensitive to the accuracy of the crystal structures employed, which is explained by perturbation theory analyses when interactions between adjacent Ir{sup 4+} ions are taken into consideration. There are indications implying that the 5d electrons of Na{sub 2}IrO{sub 3} are less strongly localized compared with those of Sr{sub 3}NiIrO{sub 6} and Sr{sub 2}IrO{sub 4}. This implication was confirmed by showing that the Madelung potentials of the Ir{sup 4+} ions are less negative in Na{sub 2}IrO{sub 3} than in Sr{sub 3}NiIrO{sub 6} and Sr{sub 2}IrO{sub 4}. Most transition-metal S = 1/2 ions do have magnetic anisotropies because the SOC induces interactions among their crystal-field split d-states, and the associated mixing of the states modifies only the orbital parts of the states. This finding cannot be mimicked by a spin Hamiltonian because this model Hamiltonian lacks the orbital degree of freedom, thereby leading to the spin-half syndrome. The spin-orbital entanglement for the 5d spin-half ions Ir{sup 4+} is not as strong as has been assumed.

• OSTI ID: 22660822
• Journal Information: Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 11 Vol. 144; ISSN JCPSA6; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
CRYSTAL STRUCTURE
D STATES
DEGREES OF FREEDOM
DENSITY FUNCTIONAL METHOD
DISTURBANCES
EXPERIMENTAL DATA
IRIDIUM IONS
L-S COUPLING
OXIDES
PERTURBATION THEORY
SODIUM COMPOUNDS
SPIN ORIENTATION
STRONTIUM COMPOUNDS
TRANSITION ELEMENTS