On the role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

TL;DR

This study reveals a structural phase transition in EuTiO3 from cubic to tetragonal involving TiO6 tilting, with evidence of local disorder affecting its properties, challenging previous assumptions of a cubic structure at low temperatures.

Contribution

It demonstrates the existence of a second-order phase transition in EuTiO3 and highlights the role of intrinsic lattice disorder in its structural behavior.

Findings

Identification of a cubic to tetragonal phase transition at 235K

Evidence of local octahedral tilting above the transition temperature

Mismatch between local and average structures indicating intrinsic disorder

Abstract

Up to now the crystallographic structure of the magnetoelectric perovskite EuTiO3 was considered to remain cubic down to low temperature. Here we present high resolution synchrotron X-ray powder diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle indicates a second-order phase transition with an estimated Tc=235K. This critical temperature is well below the recent anomaly reported by specific heat measurement at TA\sim282K. By performing atomic pair distribution function analysis on diffraction data we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion but the former is characterized by a reduced value of the tilting angle compared to the latter. At T=240K data show the presence of local octahedra tilting identical to the low temperature one, while the average crystallographic structure remains cubic. On this basis, we propose intrinsic lattice disorder to be of fundamental importance in the understanding of EuTiO3 properties.