Large Negative Thermal Expansion and Anomalous Behavior on Compression in Cubic ReO₃-Type A^IIB^IVF₆: CaZrF₆ and CaHfF₆

CaZrF₆ and CaHfF₆ display much stronger negative thermal expansion (NTE) (α_{L100 K} ∼ -18 and -22 ppm·K^-1, respectively) than ZrW₂O₈ and other corner-shared framework structures. Their NTE is comparable to that reported for framework solids containing multiatom bridges, such as metal cyanides and metal-organic frameworks. However, they are formable as ceramics, transparent over a wide wavelength range and can be handled in air; these characteristics can be beneficial for applications. The NTE of CaZrF₆ is strongly temperature-dependent, and first-principles calculations show that it is largely driven by vibrational modes below ∼150 cm^-1. CaZrF₆ is elastically soft with a bulk modulus (K_300K) of 37 GPa and, upon compression, starts to disorder at ∼400 MPa. The strong NTE of CaZrF₆, which remains cubic to <10 K, contrasts with cubic CoZrF₆, which only displays modest NTE above its rhombohedral to cubic phase transition at ∼270 K. CaZrF₆ and CaHfF₆ belong to a large and compositionally diverse family of materials, A^IIB^IVF₆, providing for a detailed exploration of the chemical and structural factors controlling NTE and many opportunities for the design of controlled thermal expansion materials.