The phase transition of strontium titanate

The structural phase transition of strontium titanate has played a crucial role in the development of our understanding of structural phase transitions. It was the first system for which experiments showed a soft mode, non-classical exponents, two time scales and two length scales for the fluctuations above T-C. These experiments are reviewed. The first two of these results are now well understood but the last two are in contradiction with the accepted scaling theory of phase transitions. Various suggestions have been put forward and these are reviewed for accounting for these results in terms of defects and imperfections but they are not completely convincing. Further work is needed either to confirm that these suggestions are correct or to develop new explanations.