Thermal Expansion And Response To Pressure Of Double-Reo3-Type Fluorides (Namf6)-F-V (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 similar 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, K-0, of 14.6(8) GPa and first derivative of the bulk modulus with respect to pressure, K-0', of -18(3) for cubic NaNbF6, and K-0 = 14.47(3) GPa and K-0' = -21.56(7) for cubic NaTaF6. When subject to similar to 0.3 GPa at 300 K, both compounds exhibit a phase transition from Fm (3) over barm to R (3) over bar. The R (3) over bar phases exhibit negative linear compressibility over a limited pressure range. A further transition with phase coexistence occurs at similar to 2.5-3.0 GPa for NaNbF6 and similar to 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.