Ultrafast Transient Increase of Oxygen Octahedral Rotations in a Perovskite

Via femtosecond hard x-ray diffraction we identified a perovskite system that complements the rare selection of materials in which a symmetry broken phase can be transiently stabilized by photoexcitation. EuTiO₃ exhibits a second order purely structural phase transition (T_C = 290 K) which is characterized by an antiferrodistortive rotation of the oxygen octahedra (see figure, left unit cell) with the rotation angle being the order parameter. We directly monitor this angle upon photoexcitation across the band gap (data points) via x-ray pulses obtained from a FEL. Within the first few-hundred femtoseconds after excitation we observe a transient increase of rotation angle. This stands in contrast to the situation of an increased temperature for which the order parameter decreases. We ascribe the surprising increase of the order parameter to a transient effective change of ionic sizes that transforms directly into an ultrafast change of the Goldschmidt tolerance factor. The ability to modify the Goldschmidt tolerance factor provides another tuning parameter to control electronic and magnetic properties of perovskites on ultrafast timescales.