STRUCTURAL PHASE-TRANSITIONS IN K3NA(SEO4)2 CRYSTALS

Thermal, dielectric, and elastic properties of K3Na(SeO4)2 crystals grown from aqueous solution are studied versus temperature. Two improper structural phase transitions arc observed, on cooling, in the temperature range from 300 to 400 K: the nonferroic continuous phase transition with no change of 3mBAR point group at 346 K, and the ferroelastic discontinuous phase transition from the trigonal point group 3mBAR 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.