Thermal Expansion and Response to Pressure of Double-ReO₃-Type Fluorides NaM^VF₆ (M = Nb, Ta)

Several II-IV double-ReO₃-type (DROT) fluorides are known to exhibit strong negative thermal expansion (NTE) over a wide temperature range while retaining a cubic structure down to 120 K or lower. CaZrF₆, CaNbF₆, CaTiF₆, and MgZrF₆, embody these properties. In contrast to the behavior of these II-IV materials, the I-V DROT material, NaSbF₆, has been reported to display a phase transition from rhombohedral to cubic above 300 K and positive thermal expansion both above and below the transition. In this work, NaNbF₆ and NaTaF₆ are shown to undergo first-order cubic-to-rhombohedral transitions on cooling to ∼130 K. Above this transition, NaNbF₆ shows modest NTE between 160 and 250 K, whereas NaTaF₆ exhibits near-zero thermal expansion over the range 210-270 K. These I-V systems are elastically softer than their II-IV counterparts, with a zero pressure bulk modulus, K₀, of 14.6(8) GPa and first derivative of the bulk modulus with respect to pressure, K₀', of -18(3) for cubic NaNbF₆, and K₀ = 14.47(3) GPa and K₀'= -21.56(7) for cubic NaTaF₆. When subject to ∼0.3 GPa at 300 K, both compounds exhibit a phase transition from Fm3̅m to R3̅. The R3̅ phases exhibit negative linear compressibility over a limited pressure range. A further transition with phase coexistence occurs at ∼2.5-3.0 GPa for NaNbF₆ and ∼4.5 GPa for NaTaF₆. Compression of NaNbF₆ in helium at room temperature and below provides no evidence for helium penetration into the structure to form a perovskite with helium on the A-site, as was previously reported for CaZrF₆.