Hydrothermal synthesis of perovskite and pyrochlore powders of potassium tantalate
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Sossina M. Haile Materials Science Department 138-78, California Institute of Technology, 1200 California Boulevard, Pasadena, California 91125
Carlos G. Levi and Fred F. Lange Materials Department and Materials Research Laboratory, University of California—Santa Barbara, Santa Barbara, California 93106 (Received 16 May 2002; accepted 17 September 2002)
Potassium tantalate powders were hydrothermally synthesized at 100 to 200 °C in 4 to 15 M aqueous KOH solutions. A defect pyrochlore, KTa2O5(OH) ⭈ nH2O (n ≈ 1.4), was obtained at 4 M KOH, but at 7–12 M KOH, this pyrochlore was gradually replaced by a defect perovskite as the stable phase. At 15 M KOH, there was no intermediate pyrochlore, only a defect perovskite, K0.85Ta0.92O2.43(OH)0.57 ⭈ 0.15H2O. Synthesis at higher KOH concentrations led to greater incorporation of protons in the perovskite structures. The potassium vacancies required for charge compensation of incorporated protons could accommodate water molecules in the perovskite structure.
I. INTRODUCTION
The perovskite KTaO3 is an incipient ferroelectric that exhibits a dielectric nonlinearity at low temperatures, on the order of the transition temperature of high-Tc superconductors. As such, a potential application is as a tunable element in microwave circuits.1 In addition, its close lattice match with the ferroelectric KNbO3 system makes it an ideal buffer layer for the growth of KNbO3 films.2 The growth of KTaO3 films by the hydrothermal method was carried out and detailed in a separate study.3 To gain a better understanding of the growth processes, potassium tantalate powders that precipitated along with the films were examined. Details of the defect and structural chemistries of these powders are the subject of the present study. Traditionally, hydrothermal synthesis is used to produce inorganic phases at moderate temperatures (T ⳱ 400 to 700 °C) and very high pressures. Recently, it has been used for low-temperature (艋200 °C) synthesis of powders and films of several oxides, including BaTiO3 and Pb(Zr,Ti)O3.4,5 In contrast, current
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J. Mater. Res., Vol. 17, No. 12, Dec 2002 Downloaded: 16 Mar 2015
techniques for synthesizing KTaO3 powders require relatively high temperatures. Conventional solid-state synthesis based on mixed powders requires long periods of heating at 1000 °C.6 Alternatively, when using precursors such as alkoxides, a defect pyrochlore phase, K2Ta2O6 crystallizes at 650 °C; the perovskite phase is obtained only after heating to 850 °C.7 Attempts to generate potassium tantalate powders by the hydrothermal method have, to date, produced materials with only a defect pyrochlore structure.8 This paper details the first report of the hydrothermal synthesis of KTaO3 powders with the perovskite structure at temperatures of 200 °C and lower.
II. EXPERIMENTAL
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