Synthesis and Properties of the Helium Clathrate and Defect Perovskite [He2-x □ x ][CaNb]F6.

The defect double perovskite [He2-x □ x ][CaNb]F6, with helium on its A-site, can be prepared by the insertion of helium into ReO3-type CaNbF6 at high pressure. Upon cooling from 300 to 100 K under 0.4 GPa helium, ∼60% of the A-sites become occupied. Helium uptake was quantified by both neutron powder diffraction and gas insertion and release measurements. After the conversion of gauge pressure to fugacity, the uptake of helium by CaNbF6 can be described by a Langmuir isotherm. The enthalpy of absorption for helium in [He2-x □ x ][CaNb]F6 is estimated to be ∼+3(1) kJ mol-1, implying that its formation is entropically favored. Helium is able to diffuse through the material on a time scale of minutes at temperatures down to ∼150 K but is trapped at 100 K and below. The insertion of helium into CaNbF6 reduces the magnitude of its negative thermal expansion, increases the bulk modulus, and modifies its phase behavior. On compressing pristine CaNbF6, at 50 and 100 K, a cubic (Fm3̅m) to rhombohedral (R3̅) phase transition was observed at <0.20 GPa. However, a helium-containing sample remained cubic at 0.4 GPa and 50 K. CaNbF6, compressed in helium at room temperature, remained cubic to >3.7 GPa, the limit of our X-ray diffraction measurements, in contrast to prior reports that upon compression in a nonpenetrating medium, a phase transition is detected at ∼0.4 GPa.

Thermal Expansion and Response to Pressure of Double-ReO3-Type Fluorides NaMVF6 (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.