The cubic ReO3-type framework is simple but has all of the key required features for negative thermal expansion (NTE) due to the transverse thermal motion of bridging atoms. ReO3 itself does not display NTE near ambient conditions, but there is a potentially large family of fluorides and oxyfluorides with the same structure that could display NTE. To illustrate this point, ScF3 contracts upon heating from at least 10 to around 1100 K, with quite pronounced NTE below ambient temperature (~-12 ppm K-1 at ~100 K). We have examined the role of both cation and anion site disorder in the thermal expansion of the ReO3-type structure. In an effort to tailor its thermal expansion, ScF3 has been doped with trivalent metal cations of varying sizes (Y3+, Ti3+, and Al3+), and the thermal expansion and compressibility of the resulting solid solutions have been studied with variable temperature/pressure powder X-ray diffraction. In this presentation, we discuss the effects of cation substitution on coefficient of thermal expansion (CTE) and compressibility, as well as a cubic-to-rhombohedral phase transition that occurs upon the incorporation of relatively small amounts of dopant into the ScF3 framework. Another ReO3-type material, TaO2F, displays near-zero thermal expansion, and we have examined the local structure of this material with variable temperature total X-ray scattering in an effort to understand the role of O/F site disorder in its quite low thermal expansion. In this presentation, we also discuss a supercell model featuring different Ta�O and Ta�F distances and O/F off-axis displacements that better describes the local structure of TaO2F than does the simple cubic ReO3-type model with random O and F site distribution.

Role and effects of disorder in the thermal expansion of ReO<sub>3</sub>-type fluorides and oxyfluorides

247th National Meeting (2014)

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Role and effects of disorder in the thermal expansion of ReO3-type fluorides and oxyfluorides

Role and effects of disorder in the thermal expansion of ReO₃-type fluorides and oxyfluorides