Birefringence and Lattice Strain in K₂Seo₄

Temperature dependence of birefringences and lattice strains in K₂SeO₄ has been investigated. They exhibit anomalies at the upper transition temperature (129.5 K), whereas no anomalies are found at the Curie temperature (93 K). Experimental results are analyzed by a phenomenological theory based on the incommensurate order parameter. Spontaneous birefringences are ascribed to the coupling with the order parameter and to the photoelastic effect through the spontaneous lattice strains.