Preparation Of Single-Phase Knbo 3 , Using Bimetallc Alkoxides
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PREPARATION OF SINGLE-PHASE KNbO, USING BIMETALUC ALKOXIDES MOSTAFA M. AMINI AND MICHAEL D. SACKS Ceramics Division, Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 ABSTRACT Single-phase KNbO 3 was prepared using bimetallic alkoxides. Potassium-niobIum ethoxide, KNb(OCH 5)6, and potassium-niobium propoxide, KNb(OC 3 H,),, were synthesized and subsequently hydrolyzed using several water concentrations. Potassium-deficient particles were rapidly precipitated when higher water concentrations were used and this resulted in the formation of a multiphase material after calcination. In contrast, single-phase KNbO 3 powders could be prepared by two methods: (1) hydrolysis of KNb(OC 3 H7),/propanol solutions using 1 mole water (per mole of propoxide) added as a water/propanol solution and (2) hydrolysis of KNb(OCHj,/ethanol solutions using 1 mole of water (per mole of ethoxide) added as a water/methanol solution. The latter method was also used to form thin films of KNbO,. INTRODUCTION Due to Its high electro-optic and nonlinear optical coefficients and good photorefractive properties, potassium niobate (KNbO3) is of great interest for a variety of applications, Including optical waveguides, frequency doublers, holographic storage systems, etc. [1-3] The conventional process for preparing potassium niobate involves mixing potassium carbonate and niobium oxide powders and subsequently reacting the mixture at high temperatures. [46] However, due to the relatively coarse scale of mixing of the reactants, it is difficult to obtain a chemically homogeneous, single-phase product. In addition, the high temperatures required for the chemical reaction also result in difficulties in maintaining the desired stoichiometry due to preferential volatilization of K2O. Furthermore, the conventional process is not applicable for the formation of thin films. In recent years, processing of ferroelectric thin films by sol-gel methods has been actively investigated. [7-12] The capability of achieving homogenous mixing of components on an extremely fine scale (e.g., molecular- or nanometer-level) offers the possibility of carrying out reactions at temperatures low enough to be compatible with semiconductor device technology. Only a few investigations of sol-gel processing of potassium niobate have been reported. Wu et al. [13] used potassium and niobium ethoxide precursors to synthesize potassium niobate, but were unable to form single-phase KNbO 3 as precipitates were reported to be deficient in potassium. Swartz et al. [14] used potassium and niobium methoxyethoxides to prepare potassium niobate. Bulk samples were reportedly single phase, but thin films had unidentified second phase(s) according to XRD results. Tuttle and coworkers [15] used potassium hydroxide and niobium ethoxide to prepare single-phase KNbO 3 powders and thin films. They also observed differences In phase development during heat treatment for bulk gels vs. thin films. A potassium deficient phase, K4NbO, 7, and a "low temperature,
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