Temperature evolution of the crystal structure of multiferroic solid solutions $(1-x)$Pb(Fe$_{2/3}$W$_{1/3}$O$_{3}$)–$(x)$PbTiO$_{3}$

Neutron diffraction was used to study the temperature evolution of the structure of $(1-x)$Pb(Fe$_{2/3}$W$_{1/3}$O$_{3}$)–$(x)$PbTiO$_{3}$ solid solutions of two compositions $x$ = 0.2 and 0.3, in which the existence of the morphotropic phase boundary (MPB) is observed, in the temperature range 90–400 K. It is shown that the system is in the two-phase state, in which the cubic and the tetragonal phases coexist, even at temperatures higher than MPB. The temperature dependences of the phase percentages were obtained. The static displacements of lead ions from the crystallographic position (000) are estimated in terms of a multiwell potential.