OPTICAL INSTABILITY IN INCOMMENSURATE BARIUM SODIUM NIOBATE CRYSTAL. S. V. Ivanova, Lebedev Physical Institute, Russian Academy of Sciences, Leninsky pr. 53, Moscow 117924, Russia
The distribution of the intensity of the radiation which had passed through the crystal Ba2NaNb5O15 (BSN) was investigated in a range of 20-500oC. The oscillations of the light scattering intensity near the temperatures of 200oC and 500oC was discovered if the laser beam was propagated in the crystal perpendicular to the ferroelectric c axis and had polarisation direction parallel to this one. Period of oscillations depends on the rate of the temperature change. The second order phase transitions were discovered at 200oC and 500oC using elastic light scattering method /1/.
The experiment consist in direction of the light beam of Ar ion laser (( =514.5nm , P=30-300mW) on the crystal and registration the scattering intensity passed through its on a screen behind the crystal. Single crystals were grown in Moscow University by the Czochralski method. The samples 2 x 3 x 5 mm were oriented in the [001], [100] and [010] directions.
Ferroelectric crystal BSN with tetragonal tungsten bronze type of structure has been noted for its excellent non-linear optical and electrooptical properties and interesting sequence of the phase transition on cooling, 4/mmm-4mm-mm2. The highest one, at 585oC, is a ferroelectric transition, the other, at about 300oC, is a ferroelastic one. The existence of incommensurate phase in BSN has been postulated and confirmed of different methods /2/.
The dynamics of beam formation and changes in the dielectric material on the microscopic level near the phase transition temperatures of about 200oC and 500oC are not well understood.
1. J.Schneck, J. C. Toledano, C. Joffrin, A. Aubree, B. Joukoff and A. Gabelotaud. Phys. Rev. B, 25, 1766 (1982)
2. S. V. Ivanova. Ferroelectrics, 170, 203 (1995)