Order and disorder in structural phase transitions

Fundamental aspects of structural phase transitions are discussed in view of XAFS measurements of the local structure below and above T_c. The experimental results show that the local structure and the order parameter have very different temperature dependencies. In particular, while the order parameter is zero above T_c the local structure remains distorted hundreds of degrees above the transition. This means that the local crystal distortions are disordered above T_c and partially disordered below it. This behavior has been observed in pure and mixed crystals and in ferroelectric as well as antiferrodistortive type crystals suggesting that it may be quite general.