$KNbO_3$ single crystals possess ferroelectric properties and very high second order nonlinear optical effects in the orthorhombic phase at room temperature.
Single crystals of a size of about 14 × 14 × 15 ㎣ and of an excellent quality were grown by the high temperature solution growth technique. As temperature decreases from the growth temperature (~1060℃) to room temperature, the crystal experiences two structural phae transitions, one at 434.5℃ (cubic to tetragonal) and the other at 222.8℃ (tetragonal to orthorhombic). The structural transitions are accompanied by the spontaneous polarizations in the tetragonal and orthorhombic phases and these lead to the multi-domain structures in the ferroelectric state. However the single domain was fabricated by the electric poling technique at about 200℃ with the applied field of 4kv/cm.
The dielectric measurements were carried out on the electric field induced multi-domain specimens and on the thermally stabilized multi-domain specimens in the polar directions. It was observed that electric field induced multi-domain sample shows the maximum dielectric responses at around 290℃. And the higher the driving frequencies, the higher the temperatures at which the dielectric responses exhibit maximum values. The thermally stabilized multi-domain sample clearly demonstrates the existence of the two phase transitions and the results favor that the transitions are of the first order. The existence of the two phase transitions was also supported by the differential scanning calorimetry (DSC) data. Both cooling and heating runs of dielectric measurements show the thermal hysteresis of about 5.4℃ (cubic to tetragonal) and 14.6℃ (tetragonal to orthorhombic) respectively. Curie temperature was estimated to be about 355℃ by using the Curie-Weiss law fit.
