Perovskites are of great technological and geological importance, in large part, due to their considerable compositional and structural flexibility. However, the formation of perovskites with neutral species on their A-sites is very unusual. The formation, phase transitions, and properties of [He 2 ][CaZr]F 6 , which is the first helium-containing perovskite to be made, are reported. It is likely that a large family of related materials can also be prepared. On compression in neon, the negative thermal expansion (NTE) material CaZrF 6 amorphizes at ∼0.5 GPa. However, on compression in helium at room temperature, the gas is inserted into the structure to form a perovskite with helium on the A-site. This suppresses the amorphization until >3 GPa. The volume versus pressure and Raman measurements suggest that filling of the A-site, to give [He 2 ][CaZr]F 6 , is complete at >1 GPa. The presence of helium on the A-site in this perovskite leads to a reduction in the magnitude of NTE when compared to the parent phase CaZrF 6 , likely due to steric impediment of the transverse vibrational motion of fluoride. Helium also leads to considerable stiffening of the structure. At room temperature and ∼2.5 GPa, the helium-containing hybrid perovskite has a bulk modulus of ∼47 GPa, whereas CaZrF 6 has a bulk modulus of ∼40 GPa under ambient conditions. Cubic perovskite [He 2 ][CaZr]F 6 undergoes a structural phase transition at 15 K on compression, which may involve a cooperative tilting of framework octahedra to give a lower-symmetry phase, which is tentatively assigned as tetragonal.
Density measurements suggest that the incorporation of ZrF4 into the cubic ReO3-type structure of Sc1– x Zr x F3+x is associated with the creation of anion interstitials. X-ray total scattering measurements are consistent with the conversion of corner-sharing octahedra to edge-sharing polyhedra as the solid solutions become richer in ZrF4. The cubic (Pm3̅m) to rhombohedral (R3̅c) cooperative octahedral tilting transition seen for ScF3 moves to a higher pressure as increasing amounts of zirconium are added, and it is eventually suppressed completely (x = 0.4 and 0.5) so that the cubic phase persists to high pressure until an amorphization occurs. All the samples studied (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) display pressure-induced softening, and increasing the zirconium content leads to a higher zero-pressure bulk modulus. The incorporation of “excess fluoride” into ReO3-type fluorides is a powerful tool for suppressing the generally unwanted phase transitions seen when subjecting these materials to stress.
Several II−IV double-ReO 3 -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. CaZrF 6 , CaNbF 6 , CaTiF 6 , and MgZrF 6 , embody these properties. In contrast to the behavior of these II−IV materials, the I− V DROT material, NaSbF 6 , 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, NaNbF 6 and NaTaF 6 are shown to undergo first-order cubic-torhombohedral transitions on cooling to ∼130 K. Above this transition, NaNbF 6 shows modest NTE between 160 and 250 K, whereas NaTaF 6 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 NaNbF 6 , and K 0 = 14.47(3) GPa and K 0 ′= −21.56(7) for cubic NaTaF 6 . When subject to ∼0.3 GPa at 300 K, both compounds exhibit a phase transition from Fm3̅ m 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 NaNbF 6 and ∼4.5 GPa for NaTaF 6 . Compression of NaNbF 6 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 CaZrF 6 .
A variety of ReO3-type and double-ReO3-type metal fluorides have been examined for their potential as negative and low-thermal-expansion materials. However, they are susceptible to unwanted phase transitions on cooling and modest compression. The incorporation of excess fluoride into the ReO3 structure, to form materials such as [Mg1–x Zr x ]ZrF6+2x and cubic LuZrF7, can be used to eliminate unwanted phase transitions and control thermal expansion. Variable-temperature powder diffraction measurements on cubic LuZrF7 show close to zero thermal expansion between 100 and 200 K. On compression, cubic LuZrF7 does not undergo a long-range cooperative polyhedral titling phase transition typical of ReO3-type fluorides. Instead, it begins to amorphize on compression at >0.7 GPa. This amorphization is reversible at ambient temperature over the course of 24 h. Cubic LuZrF7 has a bulk modulus, K 0, of 44.8(8) GPa and displays pronounced pressure-induced softening (K 0′ = 29(1)), prior to the amorphization. In situ high-pressure X-ray total scattering data indicate that the local structure of cubic LuZrF7 is largely preserved out to ∼7 Å on amorphization, suggesting that there are no major changes in bonding associated with the amorphization and providing an explanation for why the amorphization is reversible. The amorphization is likely associated with the reorientation and distortion of coordination polyhedra in an uncorrelated manner.
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