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) (<i>α</i>_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 (<i>K</i>_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