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Thermal, dielectric, and elastic properties of K3Na(SeO4)2 crystals grown from aqueous solution are studied versus temperature. Two improper structural phase transitions are observed, on cooling, in the temperature range from 300 to 400 K: the nonferroic continuous phase transition with no change of 3m point group at 346 K, and the ferroelastic discontinuous phase transition from the trigonal point group 3m to the monoclinic 2/m one at 334 K. The first transition is manifested as anomalous temperature behaviour of heat capacity and thermal expansion of the crystal. The second one is detected by direct observations of domain structure in the polarized light below 334 K and by softening of elastic moduli in selected crystallographic directions.

The temperature dependence of dielectric permittivities, shear moduli and internal friction of LiCsSO, crystals were measured. Large anomalies were found in G, and G2 shear moduli and in the internal friction Q-' for the samples in u and 6 directions, near the Curie point T, = 202 K. The temperature dependence of stress-strain hysteresis loops was also studied. In the samples cut out along the u direction a single loop was found while in the 6 direction a double hysteresis loop was observed.