X-ray, thermal and dielectric studies of chemically derived lanthanum-modified lead zirconium titanate (7:65:35) ceramics

Abstract

Lanthanum-modified lead zirconium titanate (PLZT) ceramics, a solid solution of PbTiO3 (ferroelectric) and PbZrO3 (anti-ferroelectrics), are considered to be attractive materials for many device applications such as non-volatile memories, transducers and photographic pick-ups. The formation of PLZT solid solution depends to a large extent on the physicochemical characteristics of the starting raw materials [1]. It is well known [2] that the conventional (high-temperature solid-state reaction) method of preparing PLZT powders causes compositional fluctuations and chemical inhomogeneity, which impair the electromechanical properties of the finished products. The powders prepared by chemical methods such as coprecipitation [3], spray drying [4], freeze drying and liquid drying [5] and sol-gel [6] methods have gained tremendous importance because of their better purity, better homogeneity and enhanced reactivity leading to the development of improved ceramics. A recent observations of diffuse phase transition [7] and relaxor behaviour [8, 9] in PLZT have attracted more attention for thorough studies of the compounds (with different La compositions) synthesised from different methods. In this work, PLZT (7:65:35 La:Zr:Ti) powders were prepared by the chemical method of evaporative decomposition of nitrate solutions. Detailed studies on the structure IX-ray diffraction (XRD)] and phase transition (by thermal and dielectric methods) of the fabricated samples were made to obtain a better understanding of this material for device applications. Reagent grade nitrate salts of lead,_ lanthanum and zirconium and metal-alkoxide (tetra-isopropyl titanate) were used to prepare an aqueous solution of the PLZT compound of composition 7:65:35. Lead nitrate, lanthanum nitrate and zirconyl nitrate were first dissolved in doubly distilled water. Tetraisopropyl titanate (a liquid) was then added directly to the nitrate solution while stirring. The titanium hydrolysed to a intermediate Ti(OH)4 phase and slowly redissolved. Concentrated nitric acid (HNOfl was then added to the solution, which increased the rate at which the solution cleared. Caution was exercised in adding HNO3 to prevent a strong exothermic reaction. PLZT powder was then obtained by heating the solution at 230 °C. The dried powder was subsequently calcined at various temperatures ranging from 230 to 920 °C for 16 h in an air atmosphere. The calcined and milled powder was uniaxially cold pressed at 6 x 107 kgm -2 into pellets of diameter 11.2 mm and thickness 1-2 mm using