Increase of X-ray reflection intensities and profile widths at the low- to high-V₃O₅ phase transition state

Abstract

An unexpected increase in peak intensity and profile half width has been observed in an X-ray diffraction experiment for many reflections from a crystal of V₃O₅ kept at its phase transition temperature. One explanation of the phenomena is that the ‘mosaic structure’ of the crystal is altered in the phase transition state, resulting in decreased extinction and increased profile widths. The results presented here are interesting spin off products of crystallographic studies of the mixed-valency oxide V₃O₅. The semiconductor low-V₃O₅ transforms in a first order transition at 155 °C into high-V₃O₅, which is a ‘poor metal’¹. The transformation is instantaneous and reversible and implies that an ordered distribution of V(III) and V(IV) atoms is replaced by an only partially ordered distribution, the changes in the atomic positions being fairly small (≲0.1). These facts are based on the results of accurate X-ray crystal structure determinations of the two modifications at 25 °C (ref. 2) and 185 °C (ref. 3) respectively. We started diffraction measurements of V₃O₅ at or near the phase transition temperature t_T, using the good temperature-control facility of the available non-ambient-temperature equipment⁴.