Tuning the structural instability of SrTiO3 by Eu doping: the phase diagram of Sr1-xEuxTiO3

Recently we have shown that the almost multiferroic system EuTiO3 undergoes a structural phase transition at TS=282K from cubic to tetragonal [1]. This transition is analogous to the one of SrTiO3 at TS=105K, where the oxygen octahedral rotations freeze out due to the softening of a transverse acoustic zone boundary mode. Besides of this similarity between SrTiO3 and EuTiO3, also others exist, namely a strong zone-center optic mode softening reminiscent of a ferroelectric instability, however, suppressed by quantum fluctuations [2, 3]. Both compounds have almost the same lattice constants since the ionic radii of Sr and Eu are the same. As such it is rather amazing that TS in the Eu compound is more than doubled as compared to the transition temperature of SrTiO3.