X-ray absorption spectroscopy study of local dynamics and thermal expansion in ReO3

The thermal expansion of polycrystalline ReO3 is studied in vacuum by x-ray diffraction from 300 to 600 K and by extended x-ray absorption fine structure (EXAFS) at the Re L3 edge from 30 to 600 K. A detailed EXAFS analysis is presented up to the sixth coordination shell around Re. The crystal thermal expansion is weak throughout the explored range, negative below 100 K, positive from 150 to 500 K, and negative again above 500 K. The analysis of mean square relative displacements obtained by EXAFS and of mean square displacements available from x-ray and neutron diffraction measurements confirms the presence of significant static disorder in the investigated samples. EXAFS results suggest that below 500 K in ReO3 at least some of the octahedra are slightly and disorderly rotated by an average angle (θ), whose value decreases when the temperature increases. This explains why ReO3, in spite of the possibility of supporting rigid unit modes, shows a weak positive expansion at increasing temperatures, from 150 up to 500 K. Results are discussed and compared with recent experimental and theoretical work on materials characterized by negative thermal expansion.