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.
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