Temperature dependence of lattice parameters and spontaneous strain in Pb3(PO4)2

Precise measurements of the lattice parameters of Pb3(PO4)2 as a function of temperature are presented, from which the spontaneous strain and its tensor components are calculated. The ferroelastic transition is shown to be of first order. Mean-field theory is shown to describe the experimental behaviour of the material, with a crossover, at about 160°C, from a critical to a first-order regime. This result is interpreted on the basis of the temperature dependence of elastic constant c22.