Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO${}_3$ 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 $Pm$-3$m$ to tetragonal $I4\text{/}mcm$, involving TiO${}_6$ octahedra tilting, in analogy to the case of SrTiO${}_3$. 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 $T_A$ ∼ 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 EuTiO${}_3$ properties.

• Received 2 November 2011

DOI:https://doi.org/10.1103/PhysRevB.85.184107