Preparation and characterization of uniform submicrometer metal niobate particles: Part II. Magnesium niobate and potass
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Amorphous spherical particles of magnesium and potassium niobates were produced by homogeneous precipitation in solutions of magnesium and potassium salts, respectively, in the presence of polymeric niobium oxo-hydroxide ions. At pH 9 solids of definitive compositions were achieved and converted to crystalline powders of MgNb 2 O 6 and KNbO 3 on calcination. Chemical mechanisms for the formation and transformation of such colloidal particles are offered.
I. INTRODUCTION Uniform submicrometer spherical particles of lead niobate were produced in the solution of Pb-NTA (nitrilotriacetate) and Nb-NTA complexes at pH 8.1 The composition of these solids differed with the conditions of the preparation of the complexes, pH, reaction temperature, and time. The precipitation reaction was envisioned as a combination of different kinds of cationic lead hydroxides and anionic polynuclear niobium oxo-hydroxides. At pH 8 two lead complexes, i.e., Pb(OH)+ and Pb 3 (OH) 4 2+ dominate, and they are believed to react readily with dinuclear, tetranuclear, and hexanuclear niobium oxo-hydroxide ions as the polymerization of niobium solute complexes proceeds. The precipitation of the same niobium complexes with other, not readily hydrolyzable metal ions, may yield metal niobate particles of stoichiometric compositions, such as LiNbO 3 , KNbO3, and Sro.6Bao.4Nb206, which are important ferroelectric materials for optical and electrical applications.2"4 This work describes the precipitation of uniform submicrometer size amorphous particles of magnesium and potassium niobates by interaction of the corresponding cations with niobium oxo-hydroxide complexes in aqueous solutions at pH 8-9 over the temperature range of 25-70 °C, and the crystallization of the so obtained powders by calcination.
II. EXPERIMENTAL A. Particle preparation Reagent-grade niobium pentachloride (SPEX), magnesium nitrate, potassium chloride, hydrochloric acid (all J. T. Baker), ammonium hydroxide (VWR), and
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>Part of a Ph.D. Thesis by M. J. Kim.
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http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 4, Apr 1992
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hydrogen peroxide (Mallinckrodt) were used asreceived. Powder of niobium pentachloride was slowly introduced into a 30% hydrogen peroxide solution in an ice-water bath. The resulting yellow peroxoniobium solution was acidified with hydrochloric acid (37%) and finally diluted with doubly distilled water. This solution, containing 0.1 moldm" 3 NbCl5, 0.1 moldnr 3 HC1, and 5.0 moldm" 3 H 2 O 2 , could be used for a week or so, when stored at room temperature. Stock solutions of magnesium nitrate, potassium chloride (both 0.1 moldm" 3 ), and ammonium hydroxide (0.5 mol dm"3) were filtered on 0.2 /j,m pore size Nuclepore polycarbonate membranes to remove any possible solid contaminants. The stock solutions of the peroxoniobium complex were mixed either with magnesium nitrate or with potassium chloride solutions, diluted with distilled water, and the pH was adjusted with ammonium hydroxide. The final concentrations of niobium and
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