Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO3 has been 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 I 4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T-c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T-A similar to 282 K. 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 T-c, 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 = 240 K, 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 that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO3 properties.