NONLINEAR-OPTICAL STUDY OF PHASE-TRANSITION IN LEAD MAGNESIUM NIOBATE

The second harmonic (SH) intensity generated in relaxer ferroelectric Pb(Mg1/3Nb2/3)O-3 (PMN) was measured using ceramic samples in field-co oling and field-heating after zero-field-cooling processes in the temp erature range from 100 to 300 K. Even in the zero-field-cooling proces s, the SH intensity was observed to increase steeply at about 240 K an d saturate near 150 K. This phenomenon can be interpreted by the incoh erent summation of the SH amplitude due to scattering by grain boundar ies. The behavior also confirms that a polar rhombohedral region devel ops in the cubic perovskite matrix with decreasing temperature. The no n-ergodic behavior was also observed in different cooling/heating path s under an electric field.