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

Abstract

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, AIIBIVF6, providing for a detailed exploration of the chemical and structural factors controlling NTE and many opportunities for the design of controlled thermal expansion materials.

Negative Thermal Expansion Near the Precipice of Structural Stability in Open Perovskites

“…Inset focuses on available data for ReO 3 on 4 different samples, one (ReO 3 -I) over an extended temperature range (Chatterji et al, 2009b) and the other 3 (ReO 3 -II a-c) over a smaller temperature range, investigating the effects of compositional disorder (Rodriguez et al, 2009) (see section 5). Data taken from: ZrW 2 O 8 (Mary et al, 1996), CaZrF 6 (Hancock et al, 2015), ScF 3 -I (Handunkanda et al, 2015), ScF 3 -II (Greve et al, 2010), ReO 3 -I (Chatterji et al, 2009b), and ReO 3 -II (Rodriguez et al, 2009). …”

“…To address this question will open avenues to discovery of new NTE materials and advancing technology born from its unique properties. While the precise mechanisms behind the dramatic SNTE in ZrW 2 O 8 are still under contention, a variety of other simpler systems with equally impressive SNTE have been discovered in recent years (Rodriguez et al, 2009; Greve et al, 2010; Hancock et al, 2015). In moving toward the goal of a deeper understanding of SNTE mechanisms, we sharpen our focus on the growing class of perovskite materials exhibiting NTE, including ScF 3 , ReO 3 and related structural family members.…”

Two-dimensional nanoscale correlations in the strong negative thermal expansion material ScF3

“…This type of NTE phenomenon refers to the unusual tendency for materials to shrink when heated as a property of bond network topology and the associated fluctuations [1][2][3] , which is distinct from NTE arising from electronic or magnetic instabilities observed in InVar 4 and valence transitions 5 . Recently, nonmagnetic ionic insulator ScF 3 has drawn particularly intense attention in the chemistry community 6 as the first instance of a perovskitestructured material with strong, isotropic, and thermally persistent NTE, a distinction which no longer appears to be an isolated case 7 . ScF 3 displays strong NTE (-15 ppm/K<α L <0) over a temperature window of 1000 K, but is also unusual in its lack of any type of phase transition whatsoever within the solid state: the system retains a small, four-atom (formula) unit cell at all temperatures below the solid-liquid phase boundary T <1800K.…”

High Pressure Behavior of Chromium and Yttrium Molybdate (Cr2Mo3O12, Y2Mo3O12)

“…Most materials get stiffer at high pressures due to reduction of empty space upon compression. However, pressure induced softening has been observed in a number of NTE material (Pantea et al, 2006; Fang and Dove, 2013, 2014; Fang et al, 2013; Morelock et al, 2013; Gallington et al, 2014; Hancock et al, 2015; Alabarse et al, 2017; Hester et al, 2017b; Ticknor et al, 2018), and has been linked to the facile polyhedral rotations that give rise to their expansion behavior. In many cases, amorphization is preceded by pressure induced softening as well.…”