Light-induced intrinsic defects in PLZT ceramics

Abstract

This paper reports on an EPR study of a ferroelectric, 1.8/65/35, and an antiferroelectric, 2/95/5, of optically transparent Pb_1−yLa_yZr_1−xTi_xO₃ (PLZT) ceramics within a broad temperature range (20–300 K) after illumination at a wavelength of 365–725 nm. Illumination with ultraviolet light, whose photon energy corresponds to the band gap of these materials, at T3+, Pb⁺, and Pb³⁺. It is shown that these centers are generated near a lanthanum impurity, which substitutes for both the Pb²⁺ and, partially, Ti⁴⁺ ions through carrier trapping from the conduction or valence band into lattice sites. The temperature ranges of the stability of these centers are measured, and the position of their local energy levels in the band gap is determined. The most shallow center is Ti³⁺, with its energy level lying 47 meV below the conduction band bottom. The Pb³⁺ and Pb⁺ centers produce deeper local levels and remain stable in the 2/95/5 PLZT ceramics up to room temperature. The migration of localized carriers is studied for both ceramic compositions. It is demonstrated that, under exposure to increased temperature or red light, the electrons ionized into the conduction band from Ti³⁺ are retrapped by the deeper Pb⁺ centers, thus hampering the carrier drift in the band and the onset of photoconduction. The part played by localized charges in the electrooptic phenomena occurring in the PLZT ceramics is discussed.