A Unified View of the Structural Phase Transition in SrTiO3

Strontium titanate, or SrTiO3, is the most widely studied perovskite oxide, a family of materials of vital importance in basic science and energy technologies. It was discovered over 50 years ago that SrTiO3 undergoes a phase transition at 105 K, where the Ti-O octahedra that make up its structure rotate in an alternating pattern. This is called an “antiferrodistortive” transition. What is so important about this structural change in SrTiO3 is that it has become a model for similar transitions in a large number of compounds, occurring via a mechanism known as mode softening. Although this mechanism has been understood for some time, some aspects of the transition remain mysterious. One example is that the transition temperature shifts from 105 K when different elements are substituted into SrTiO3. This shift can be downwards (to 0 Kelvin) or upwards, to above room temperature, the trends being very difficult to understand. In this work these trends have been understood for the first time, providing substantially deepened understanding. The key concept is the strain introduced by the foreign ion, more specifically the “charge strain” or “valence mismatch”. The authors combined new experimental data with a model based on bond valence sum calculations to establish a new parameter that controls, and can be used to predict, the transition temperature, providing new insight into an over 50 year old problem.