Antiferrodisorsion transition and dielectric relaxation in SrTiO$_3$SrMg$_{1/3}$Nb$_{2/3}$O$_3$

Dielectric and acoustic studies of solid solutions were carried out (1-x)SrTiO$_{3-x}$SrMg$_{1/3}$Nb$_{2/3}$O$_3$. The dependence of the temperature of the antiferrodisorsion phase transition on the concentration x of the second component is obtained. The suppression of dielectricrelaxation at $x\le$ 0.03 and temperatures below the temperature of the antiferrodisorsion transition, as well as the coexistence of dielectric relaxation and antiferrodisorsion instability at 0.03 $<x\le$ 0.15. The reasons for the blurring of the dependence of the relative speed of sound in the transition region and the features of the interaction of antiferrodisorsion instability and the dielectric subsystem are discussed.