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Temperature dependence of birefringences and lattice strains in K 2 SeO 4 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.

Hydrostatic pressure effect on the dielectric constant e c or K 2 SeO 4 was studied. The incommensurate transition temperature T 2 lowers linearly with the slope of -7.3 ±0.2 K/kbar. The Curie temperature T 3 also decreases with pressure, but the slope denpends on pressure; the initial slope is -17.2 ±2.5 K/kbar and the slope above 0.6 kbar is -11.4 ±1.0 K/kbar. The dielectric behaviors under the pressure are qualitatively discussed based on the theoretical treatments already developed by other authors, and some problems are suggested.

Brillouin scattering study was made on the ferroelectric (NH 4 ) 2 BeF 4 over the temperature range –150°C∼20°C, and the temperature dependence of elastic constants c 44 , c 55 and c 66 was determined.