Phonon Softening and Dispersion in EuTiO3

TL;DR

This study investigates phonon dispersion and softening in EuTiO3, revealing a structural transition at 287 K, and finds that its phonon behavior closely resembles that of SrTiO3, indicating universal scaling.

Contribution

It provides detailed phonon dispersion measurements in EuTiO3, identifies a displacive structural transition, and compares phonon behavior with SrTiO3, highlighting universal features.

Findings

Structural transition at T0=287 K confirmed by x-ray diffraction.

Phonon softening at the R-point indicates a displacive transition.

Phonon dispersion in EuTiO3 closely resembles SrTiO3, scaled by mass.

Abstract

We measured phonon dispersion in single crystal EuTiO$_3$ using inelastic x-ray scattering. A structural transition to an antiferrodistortive phase was found at a critical temperature $T_0$=287$\pm$1 K using powder and single-crystal x-ray diffraction. Clear softening of the zone boundary \emph{R}-point \textbf{q}=(0.5 0.5 0.5) acoustic phonon shows this to be a displacive transition. The mode energy plotted against reduced temperature could be seen to nearly overlap that of $\rm SrTiO_3$, suggesting a universal scaling relation. Phonon dispersion was measured along $\Gamma$-$X$ (0 0 0)$\rightarrow$(0.5 0 0). Mode eigenvectors were obtained from a shell model consistent with the \textbf{q}-dependence of intensity and energy, which also showed that the dispersion is nominally the same as in $\rm SrTiO_3$ at room temperature, but corrected for mass. The lowest energy optical mode, determined to be of Slater character, softens approximately linearly with temperature until the 70-100 K range where the softening stops, and at low temperature, the mode disperses linearly near the zone center.