High-Pressure Raman Study of Non-Linear Optical Crystals

In this presentation, we report on the high-pressure Raman spectroscopic studies of some non-linear optical crystals, e. g. KNbO3 (KN), KIO3 (KI) and KTiOAsO4 (KTA), which are important for both their practical applications and theoretical values. The effect of pressure on KN is extremely large and there exist a rich variety of pressure-induced changes. Mode softening is found for some of the characteristic Raman bands. Intensity transfer and frequency repelling have been observed in the 200 cm-1 region as a result of strong coupling between two B1 (TO) modes, whose frequencies have been brought closer under pressure. There are three new crystalline and an amorphous phases, and two of the phase transitions are of displace type. Three first-order pressure-induced phase transitions are inferred for KTA. The first takes place at 3.2 GPa, indicated by drastic spectral changes related to TiO6 octahedral. The second occurs at 7.5 GPa involving both TiO6 and AsO4 groups. Several modes belonging to TiO6 also soften between 0～7.5 GPa. The third phase transition occurs near 10 GPa and the high-pressure phase should be of higher symmetry. Drastic spectral changes and phase transitions are observed for KI up to 32 GPa. The first transition occurs at 5.0 GPa characterized by major spectral changes of the 1, 2, and 4 modes. The other transitions are found at 8.7, 15 and 22 GPa. The phase that is stable between 15～22 GPa has a higher symmetry than the phases in the lower pressures. The IO3 pyramidal ions may have lost their identity and changed to IO6 octahedral above 22 GPa.