We present femtosecond hard x-ray diffraction experiments that study the effect of photodoping on the perovskite EuTiO_{3}. We observe an ultrafast transient increase of reflection intensities that are directly related to the antiferrodistortive rotation of the oxygen octahedra. This increase relates directly to an increase of the order parameter associated with a purely structural phase transition that is opposite to the behavior of increasing the materials temperature. First-principles calculations show that the creation of electron-hole pairs leads to changes in the vibrational potential that is consistent with an increase of the order parameter. The order parameter increase can be understood from an ultrafast charge-transfer-induced reduction of the Goldschmidt tolerance factor, which is a fundamental control parameter for the properties of perovskites.
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