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RESEARCH PRODUCT

Weak itinerant ferromagnetism and electronic and crystal structures of alkali-metal iron antimonides: NaFe4Sb12andKFe4Sb12

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The synthesis, chemical, structural, and magnetic properties of alkali-metal compounds with filled-skutterudite structure, $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$, are described. X-ray and neutron diffraction and elemental analysis established the crystal structure without defects and disorder on the cation site. The temperature and pressure dependence of the cubic unit cell of $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and the displacement parameter of Na are investigated. The electronic structure is calculated by density functional methods (LMTO, FPLO). Quantum chemical calculations (electron localization function) reveal the covalent character of both $\mathrm{Fe}\penalty1000-\hskip0pt\mathrm{Sb}$ and $\mathrm{Sb}\penalty1000-\hskip0pt\mathrm{Sb}$ interactions. Electronic structure calculations within the local density approximation exhibit a band ferromagnetic ground state and predict a half-metallic behavior. In contrast to isostructural alkaline-earth compounds ($\mathrm{Ca}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{Ba}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$), the alkali-metal skutterudites are itinerant electron ferromagnets with small magnetic moments ($\ensuremath{\approx}0.25{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Fe}$ atom) and ${T}_{\mathrm{C}}\ensuremath{\approx}85\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Yet the paramagnetic moments of all four compounds are between $1.5{\ensuremath{\mu}}_{\mathrm{B}}$ and $1.7{\ensuremath{\mu}}_{\mathrm{B}}$ per Fe atom, indicating similar Stoner factors. Temperature-dependent $^{57}\mathrm{Fe}$ and $^{121}\mathrm{Sb}$ M\"ossbauer spectroscopies confirm the ferromagnetic state in the sodium compound with very small hyperfine fields at the iron and antimony sites.