Linear and nonlinear light scattering near the phase transition inKH2PO4

The angular and temperature dependence of the linear light scattering and the temperature dependence of the second-harmonic nonlinear light scattering were studied in two different samples of ${\mathrm{KH}}_2$ ${\mathrm{PO}}_4$ (KDP) crystal: one from a zone close to the seed and another from a zone far from the seed. The angular dependence of the intensity of the linearly scattered light from the sample from the zone close to the seed shows a strong peak for the near Z(YX)Y scattering. This scattering originates from growth bands parallel to the (01¯1) lattice planes in the pyramidal growth sector. The temperature dependence of the magnitude of this peak shows a considerable increase in the vicinity of the phase transition from the paraelectric to ferroelectric phase. Such an increase in the linear light scattering was not observed in the sample from the zone far from the seed. The anomalies related to the growth bands parallel to the (010) lattice planes in the prismatic growth sector were investigated also by second-harmonic light scattering. Here again a considerable increase in the intensity of the frequency-doubled scattered light was observed in the vicinity of the phase-transition temperature. The results of the light-scattering experiments are compared with an x-ray topographic study of the lattice defects.