  The ability to control the structure of a crystalline solid on ultrafast
timescales bears enormous potential for information storage and manipulation or
generating new functional states of matter [1]. In many materials where the
ultrafast control of crystalline structures has been explored, optical
excitation pushes materials towards their less ordered high temperature phase
[2{9] as electronically driven ordered phases melt and possible concomitant
structural modifications relax. Nonetheless, for a few select materials it has
been shown that photoexcitation can slightly enhance the amplitude of an
electronic ordering phenomenon (i.e. its electronic order parameter) [9{13].
Here we show via femtosecond hard X-ray diffraction that photodoping of the
perovskite EuTiO3 transiently increases the order parameter associated with a
purely structural [14] phase transition represented by the antiferrodistortive
rotation of the oxygen octahedra. This can be understood from an ultrafast
charge-transfer induced reduction of the Goldschmidt tolerance factor [15],
which is a fundamental control parameter for the properties of perovskites
