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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.

Thermal expansion of Rb4LiH3(SO4)4 and K4LiH3(SO4)4 crystals is studied in the temperatures ranging from 80 to 300 K. At room temperature both these crystals are isomorphic. In Rb4LiH3(SO4)4 a ferroelastic phase transition is confirmed to occur at TC = 131.2 K. In the vicinity of TC anomalous temperature changes of the thermal expansion coefficients α1, α2, and α3 as well as the components of the strain tensor (e1 + e2) and e3 are found. No anomalous changes of K4LiH3(SO4)4 thermal expansion are observed in the temperature range studied. The results of K4LiH3(SO4)4 thermal expansion investigations suggest that anomalous changes in spontaneous polarization of this crystal, observed in pyroelectric studies at about 114 K, seem to be not a consequence of structural changes in the lattice parameters of this crystal.