Negative Thermal Expansion and Phase Transitions in Niobium Oxyfluoride

The rhenium trioxide (ReO3) structure is adopted by a number of materials, and is interesting because it can readily give rise to negative thermal expansion (NTE), i.e. the volume of the material unusually decreases on heating. This behaviour is not well understood, however, with only some ReO3-type materials showing NTE without an obvious distinction. Niobium oxyfluoride (NbO2-xF1+x) also adopts the ReO3-type structure, and we have recently discovered that its thermal expansion can be tuned directly with x, from positive thermal expansion (x = 0) to negative thermal expansion (x = 0.6). This experiment aims to perform variable temperature measurements in order to understand the origin of this change in behaviour, using the anion sensitivity of neutrons to analyse the position and thermal motion of O and F atoms. In addition to better understanding these niobium oxyfluorides, we hope that these results will lead to a more general understanding of why NTE occurs in only some ReO3-type materials.