The excess optical birefringence and phase transition in sodium nitrate

The temperature evolution of the birefringence in sodium nitrate shows a phase transition at 552 K which is continuous within experimental resolution (0.1 K). The excess birefringence is found to relate to the spontaneous strain via a temperature-independent elasto-optic coefficient. This behaviour is discussed microscopically in terms of structural and electronic contributions and is related to the order parameter by symmetry arguments. The critical exponent beta is then found to take the non-classical value of 0.22+or-0.01. This together with recent specific heat results indicate that the three-dimensional three-state Potts model provides a possible description of this phase transition provided the effects of defects are taken into account. Residual excess birefringence is observed above T c . A model involving 'ordered' and 'anti-ordered' configurations is proposed and orientational fluctuations in the anti-ordered configuration as well as sodium sublattice melting are proposed to account for this phenomenon.