We predict a new non-steady-state optothermal electromotive force effect in relaxor ferroelectrics. The effect is observed and studied in a PbNi \(_{1/3}\) Ta \(_{2/3}\) O \(_3\) crystal at light wavelength \(\lambda =532\) nm and room temperature, which is significantly above the broad, frequency-dependent maximum of the permittivity ( \(T_m\simeq 89\) K), below which remanent polarization and the relaxor ferroelectric state arise. We focus on the pronounced mismatch in high-frequency behaviour of the non-steady-state EMF and conductivity response due to nonuniform laser heating and develop the ensuing theory of the effect.
