Time dependences of dielectric and acoustic properties in PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ and PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$–7PbTiO$_{3}$ single crystals

The time changes of permittivity, damping and velocity of sound are studied for PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ and PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$–7PbTiO$_{3}$ single crystals in the electric fields of $0<E<6$ kV/cm. The room-temperature local symmetry of these compounds is shown to be rather more monoclinic than rhombohedral, which is typical of other similar systems. No abrupt sound attenuation anomalies are observed upon the phase transition into the ferroelectric phase in the electric field. The effects seem to be far from the crystal symmetry changes and are attributed to the gradual transition of near-range monoclinic domains into long-range monoclinic domains, as well as to the emergence of the ferroelectric phase. The polarized phase induced in the electric field is only partially stable.