Large Positive Thermal Expansion and Small Band Gap in Double-ReO<sub>3</sub>‑Type Compound NaSbF<sub>6</sub>

Double-ReO<sub>3</sub>-type structure compound NaSbF<sub>6</sub> undergoes a low-temperature rhombohedral to high-temperature cubic phase between 303 and 323 K, as revealed by temperature-dependent X-ray diffractions. Although many double-ReO<sub>3</sub>-type fluorides exhibit either low thermal expansion or negative thermal expansion (NTE), NaSbF<sub>6</sub> exhibits positive thermal expansion (PTE) with a large volumetric coefficient of thermal expansion, α<sub>v</sub> = 62 ppm/K, in its cubic phase. Raman spectroscopy reveals that the low-frequency transverse vibration of fluorine atoms is stiffened in NaSbF<sub>6</sub>, compared with the typical NTE compound CaZrF<sub>6</sub> with the same structure. The related weak contraction associated with the polyhedral rocking would be overcome by the notable elongation of the Na–F bond length on heating, thus leading to the large volumetric PTE. Unlike ScF<sub>3</sub> and CaZrF<sub>6</sub> which are insulators with a wide band gap, a relative small band gap of 3.76 eV was observed in NaSbF<sub>6</sub>. The small band gap can be attributed to the hybridization between the Sb 5s and F 2p orbitals.