Phase transitions in leucite: Order parameter behaviour and the Landau potential deduced from Raman spectroscopy and birefringence studies

Abstract

Hard mode Raman Spectroscopy and optical birefringence studies have been used to evaluate the temperature behaviour of order parameters for the cubictetragonal phase transition in leucite. There is a strong temperature-dependence of Raman line frequencies near 498 cm⁻¹ and 529 cm⁻¹, and the optical birefringence decreases towards zero on heating. Above the transition temperature leucite still shows slight birefringence, which correlates with the persistence of the 200 X-ray reflexion at these temperatures, and is not compatible with the ideal cubic symmetry. Quantitative observations of the temperature evolution of Raman line-frequencies and the birefringence are consistent with an order parameter showing T _1g symmetry, and a nearly-tricritical phase transition behaviour. The results of earlier X-ray and DSC studies imply the operation of an additional order parameter, with E _g symmetry; this is observed, but its influence on the transition behaviour is significant only at temperatures close to T _c. The two order parameters are related via linear-quadratic or biquadratic coupling. We develop the form of the Landau Potential for leucite — including both order parameters and their couplings — to discuss the present observations and previous structural data.