The influence of oxygen vacancies on the relaxors with perovskite structure was considered in the framework of Landau-Ginzburg-Devonshire phenomenological theory. We focused on the PZN-PLZT relaxor, where an earlier experimental investigation of the influence of oxygen vacancies on the polar properties was performed and evidence of oxygen vacancy induced ferroelectricity was obtained. Since the oxygen vacancies are known to be elastic dipoles, they affect the elastic and electric fields due to Vegard and flexoelectric couplings. We have shown that a negative Curie temperature ${T_C}^{*}$ of a relaxor is renormalized by the elastic dipoles due to the electrostriction coupling and could become positive at some large enough concentration of the vacancies. A positive renormalized temperature $T_C^R=T_C^{*}+\mathrm{\Delta }T$ is characteristic for the ferroelectric state. At $T<T_C^R$, all the polar properties could be calculated in the conventional way for ferroelectrics, but the obtained experimental data favor the coexistence of the ferroelectric phase with a relaxor state, i.e., the presence of a morphotropic region in PZN-PLZT relaxor. At $T>T_C^R$, the random field characteristic for relaxors is preserved, but since the mean square deviation of the polarization is nonzero, the coexistence with a dipole glass state is not excluded. For the case $T>T_C^R$, we calculated the local polarization and electric field induced by the flexochemical coupling with oxygen vacancies.

• Received 31 March 2018
• Revised 18 June 2018

DOI:https://doi.org/10.1103/PhysRevB.98.094102