Optical detection of phase transitions in potassium niobate

Abstract

Luminescence data from potassium niobate crystals are reported which show intensity and/or wavelength variations on heating, or cooling, through the phase transition temperatures. The luminescence signals can clearly identify the structural changes between the various crystalline phases. The transition temperatures differ between heating and cooling. They were recorded near 247 and 491 K (heating) or 220 and 478 K (cooling). The hysteresis indicates the presence of metastable material, (e.g. a supercooled structure). The data resolve the previously cited differences in transition temperatures for KNbO3 from different laboratories. The luminescence signals show further details in the variations of spectra, intensity and transition discontinuities of the luminescence which are related to material quality between samples, even from a single supplier. The luminescence data underline the sensitivity of these crystals to structural damage from electron, X-ray or thermal treatments and offer the opportunity to assess crystalline quality prior to device fabrication.