Light-induced intrinsic defects in PLZT ceramics

This paper reports on an EPR study of a ferroelectric, 1.8/65/35, and an an tiferroelectric, 2/95/5, of optically transparent Pb1-yLayZr1-xTixO3 (PLZT) ceramics within a broad temperature range (20-300 K) after illumination at a wavelength of 365-725 nm. Illumination with ultraviolet light, whose pho ton energy corresponds to the band gap of these materials, at T < 50 K crea tes a number of photoinduced centers: Ti3+, Pb+, and Pb3+. It is shown that these centers are generated near a lanthanum impurity, which substitutes f or both the Pb2+ and, partially, Ti4+ ions through carrier trapping from th e conduction or valence band into lattice sites. The temperature ranges of the stability of these centers are measured, and the position of their loca l energy levels in the band gap is determined. The most shallow center is T i3+, with its energy level lying 47 meV below the conduction band bottom. T he Pb3+ and Pb+ centers produce deeper local levels and remain stable in th e 2/95/5 PLZT ceramics up to room temperature. The migration of localized c arriers 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 photoconduc tion. The part played by localized charges in the electrooptic phenomena oc curring in the PLZT ceramics is discussed. (C) 2000 MAIK "Nauka/Interperiod ica".