Simultaneous pyroelectric and dilatometric studies of phase transitions intriglycine sulphate and glycine phosphite crystals

Temperature dependences of the linear dimension L(T) and spontaneous polari zation P-s(T) for triglycine sulphate and glycine phosphite single crystals have been measured in the same run of temperature. A decrease in the spont aneous electrostriction coefficient Q(L) of both crystals studied is displa yed when approaching the corresponding phase transition point T-c. A temper ature dependence of the coefficients Q(L)(T) shows that the rates of temper ature changes of displacement of the atomic complexes of corresponding crys tal structure forming the square of spontaneous polarization P-s(2) (primar y order parameter) and the displacements of all atomic complexes forming th e spontaneous linear thermal expansion deltaL(s)/L (secondary order paramet er) are different. The applying external electric field E = 3x10(5) V/m to triglycine sulphate crystal does not influence essentially the temperature dependence of Q(L)(T) in the temperature range of phase transition below T- c.