Titanium off-centre displacement in isotopically substituted SrTiO3

Bulk SrTiO3 undergoes a ferroelectric phase transition in ambient conditions only when oxygen atoms have been replaced with a heavier isotope. Depending on the experimental technique chosen, there are both indications of the transition being of the displacive type, or order-disorder type. It is possible that the Ti atoms in SrTiO3 are displaced off-centre even in the tetragonal phase, but due to the randomness of the displacement direction the average structure (as seen in diffraction experiments) remains tetragonal. We propose a pair distribution function measurement on two powder samples of SrTiO3, one isotopically normal, and the other containing 60% oxygen-18 isotope. By using Polaris we will gain access to values of Q large enough to detect displacement of the magnitude of 0.04-0.05 Angstrom, which is the range predicted by modelling and implied by gamma ray diffraction experiments. The experiment will verify if titanium atoms are randomly displaced in SrTiO3 already above the ferroelectric transition temperature, and if the magnitude of the displacement is affected by the change in oxygen mass.