Optical Frequency Dispersion Near Ferroelectric Relaxor Phase Transition in Lead Barium Niobate Crystal

Abstract

Temperature dependent birefringence and optical refractive indices of PBN (Pb 1−x Ba x Nb 2 O 6 ) crystal (1 − x = 0.57) were measured in several optical wavelengths (λ = 694 nm, 633 nm, 535 nm, and 450 nm) to understand the optical frequency dependency of ferroelectric phase transitions in relaxor ferroelectric crystals of tungsten bronze structure. Local polarization is verified to be dynamically activated by thermal process and probed effectively by suitable wavelength of light. An optical isotropic point, as function of temperature and light frequency, is reported at which the crystal's standing birefringence is fully compensated by polarization. A modified single oscillator model is used to calculate the index of refraction in the ferroelectric phase. The deviation temperature from the single oscillator model is reported to be significant marking the crossover from macroscopic to microscopic polarization. A new parameter, optical Curie temperature region, defined by the temperature difference between the well known Burn's temperature and the deviation temperature (from the single oscillator model for index of refraction) is explored for its significance in depolarization behavior of the micro- to nano-polar clusters of the ferroelectric relaxor.