Ferroelastic transition and the relaxor state in SrTiO$_3$–PbMg$_{1/3}$Nb$_{2/3}$O$_3$

Dielectric and acoustic properties of the $(1-x)$SrTiO$_3$–PbMg$_{1/3}$Nb$_{2/3}$O$_3$ solid solutions (0 $\le x\le$ 1) have been studied at temperatures ranging from 4.2 to 350 K. It has been found that the improper ferroelastic transition exists up to a concentration $x$ = 0.4 and that the phase transition temperature grows with $x$ increasing from 0 to 0.4. As the concentration of the second component is further increased, the transition becomes suppressed by the relaxor phase forming at $x_c\approx$ 0.22. The results obtained are discussed in terms of the current concepts of relaxor ferroelectrics.