Magnetic interactions and the puzzling absence of any Raman modes in EuTiO3.

Polycrystalline ceramics of EuxSr1-xTiO3 (x=0.03-1) and a single crystal of EuTiO3 characterized by synchrotron XRD have been investigated by Raman spectroscopy in order to detect the structural phase transition and the effect of an external magnetic field or hydrostatic pressures on the materials properties. The XRD measurements verified the cubic to tetragonal structural transition to the I4/mcm space group with transition temperature TS=282 K. Although in the low temperature tetragonal phase Raman active modes are allowed by symmetry, a unique absence of any Raman active mode has been observed down to 80 K. By applying an external magnetic field or alternatively hydrostatic pressure modes are activated in the Raman spectra. The temperature dependence of the main mode that is activated shows remarkable agreement with theoretical predictions. We attribute the puzzling absence of the Raman modes to a mechanism related to strong spin-lattice interaction that drives the cubic to tetragonal structural phase transition and makes the Raman tensor antisymmetric. On the contrary, the external perturbations induce a symmetric Raman tensor allowing even symmetry modes to be present in the spectra.