Local structure of perovskites ReO₃ and ScF₃ with negative thermal expansion: Interpretation beyond the quasiharmonic approximation

We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF₃ were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF₃ is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF₃ allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.