Complex symmetry of phases and magnetic properties of EuTiO3

Some puzzling and unexplained observations regarding the cubic to "tetragonal" phase transition at TS = 282 K in the almost-multiferroic perovskite EuTiO3 have been reanalyzed to obtain deeper insight into the true structure and magnetic activity evolving below TS. For this purpose, birefringence and high-quality synchrotron x-ray-diffraction (XRD) data have been used where zero magnetic field data are compared to data under the influence of a direction-dependent magnetic field. The birefringence data have been analyzed with respect to the angle dependence of the domain orientation from which the dominance of the monoclinic symmetry is unambiguously proven. The XRD structural data have been taken as input to derive the magnetic exchange constants, the related Neel temperatures TN, and the energies of the possible magnetic ground states. Taking both results together we conclude that below TS the symmetry cannot be tetragonal, but only monoclinic followed by another symmetry lowering transition around T * approximate to 210 K. In addition, the involved magnetic exchange interactions have been calculated based on the experimental lattice constants, and it is shown that these support the conclusion that any kind of magnetism stems from a competition of the various ground-state energies and consequently induces a highly frustrated magnetically active state near room temperature.