LAUSR

A method for determining the temperature dependences of the lattice parameters of ferroelastics with two successive $$\bar {3}m$$–2/m–2 phase transitions (if the lattice parameters at two arbitrary temperatures in each phase are known) is proposed, which is based on the condition of macrosymmetry conservation in the paraelastic phase of multidomain crystal. Two analytical solutions are obtained for the second-order phase transition (with small spontaneous rotations in ferroelastic phases disregarded). The solutions are verified by examples of Pb3(PO4)2 and Pb3(VO4)2. It is shown that Pb3(PO4)2 and Pb3(VO4)2 undergo two phase transitions. With allowance for spontaneous rotations, it is established that the spontaneous strain tensor in both phases contains four nonzero components.