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.

Low or negative thermal expansion (NTE) has been previously observed in members of the ZrP 2 O 7 family at temperatures higher than their order-disorder phase transitions. The thermoelastic properties and phase behavior of the low temperature superstructure and high temperature negative thermal expansion phases of ZrV 2 O 7 and HfV 2 O 7 were explored via in situ variable temperature/pressure powder x-ray diffraction measurements. The phase transition temperatures of ZrV 2 O 7 and HfV 2 O 7 exhibited a very strong dependence on pressure (∼ 700 K•GPa), with moderate compression suppressing the formation of their NTE phases below 513 K. Compression also reduced the magnitude of the coefficients of thermal expansion in both the positive and negative thermal expansion phases. Additionally, the high temperature NTE phase of ZrV 2 O 7 was found to be twice as stiff as the low temperature positive thermal expansion superstructure (24 and 12 GPa respectively).

CaZrF6 (I), CaHfF6 (II), and CoZrF 6 (III) are prepared by solid state reaction of CaF2 (CoF2) and ZrF4 (HfF4) in equimolar amounts (Cu or Ni tube in evacuated quartz tubes, 750 C (I) or 850 C (II, III), 24 h). (I) and (II) display a negative thermal expansion (NTE) of L 100 K  -18 and -22 ppm/K, resp., which is larger than that of ZrW 2O8 and other corner-shared framework structures. The NTE of (I) is strongly temperature-dependent, and first-principles calculations show that it is largely driven by vibrational modes below 150 cm -1 . (I) is a soft elastic material with a bulk modulus of 37 GPa and, upon compression, starts to disorder at 400 MPa. In contrast to the strong NTE of (I) which remains cubic down to <10 K, (III) only displays modest NTE above its rhombohedral to cubic phase transition at 270 K. -(HANCOCK, J. C.; CHAPMAN, K. W.; HALDER, G. J.; MORELOCK, C. R.; KAPLAN, B. S.; GALLINGTON, L. C.; BONGIORNO, A.; HAN, C.; ZHOU, S.; WILKINSON*, A. P.; Chem. Mater. 27 (2015) 11, 3912-3918, http://dx.doi.org/10.1021/acs.chemmater.5b00662 ; Ga. Inst. Technol., Sch. Chem. Biochem., Atlanta, GA 30332, USA; Eng.) -J. Schramke 31-006