  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.
