6533b856fe1ef96bd12b2f23

RESEARCH PRODUCT

Interpretation of unexpected behavior of infrared absorption spectra ofScF3beyond the quasiharmonic approximation

subject

description

Scandium fluoride (${\mathrm{ScF}}_{3}$), having cubic ${\mathrm{ReO}}_{3}$-type structure, has attracted much scientific attention due to its rather strong negative thermal expansion (NTE) in the broad temperature range from 10 to 1100 K. Here we use the results of diffraction and extended x-ray absorption fine-structure (EXAFS) spectroscopy to interpret the influence of NTE on the temperature dependence of infrared absorption spectra of ${\mathrm{ScF}}_{3}$. Original infrared absorption and EXAFS experiments in a large temperature range are presented and interpreted using ab initio lattice dynamics simulations within and beyond quasiharmonic approximations. We demonstrate that ab initio electronic structure calculations, based on the linear combination of atomic orbitals method with hybrid functionals, are able to reproduce well the experimental values of lattice parameter ${a}_{0}$, band gap ${E}_{g}$, and lattice dynamics in ${\mathrm{ScF}}_{3}$. However, the simulations performed within quasiharmonic approximation fail to reproduce the temperature dependence of two infrared active bands due to the F--Sc--F bending (at 220 ${\mathrm{cm}}^{\ensuremath{-}1}$) and Sc--F stretching (at 520 ${\mathrm{cm}}^{\ensuremath{-}1}$) modes present in the infrared absorption spectra. To overcome this problem, an approach beyond the quasiharmonic approximation is proposed: It accounts for the negative thermal expansion of the lattice and for fluorine atom displacements due to strong F vibrational motion perpendicular to the cubic axes and allows us to explain qualitatively the temperature behavior of infrared spectra of ${\mathrm{ScF}}_{3}$.