Sol-gel synthesis of Ln 2 (Ln = La, Nd)Ti 2 O 7
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Lanthanum and neodymium titanates were prepared by a sol-gel route. Synthesis of La 2 Ti 2 0 7 from pure alkoxide precursors yielded an intermediate perovskite type phase, La ( i-^)TiO 3 , which partially transformed to La 2 Ti 2 07 on heating to 1500 °C. Substitution of the lanthanum acetylacetonate for alkoxide precursor yielded La 2 Ti 2 0 7 without any intermediate phase at a very low temperature of 700 °C. Sintering of the La 2 Ti 2 0 7 gel powder yielded a highly dense ceramic with —97% theoretical density. Similar sintering treatment resulted in ~ 9 2 % theoretical density for Nd 2 Ti 2 O 7 .
I. INTRODUCTION Interest in lanthanide titanates with the general formula, Ln 2 Ti 2 O 7 , arose as a result of the observation that they might be ferroelectric by analogy with Cd 2 Nb 2 0 7 , which is a unique and unusual ferroelectric material at low temperatures.1 Cd2Nb2O7 has a cubic structure of the mineral pyrochlore (Na, Ca) (Nb, Ta)O6F. Roth was the first to synthesize and investigate a series of lanthanide titanates by solid-state reactions.2 Among the rare earth titanates, those with Ln = Sm to Lu are isostructural to pyrochlore whereas lanthanum and neodymium titanates are monoclinic with a space group P2i.3A Further investigations revealed that both lanthanum and neodymium titanates are effective ferroelectric substances with both high Curie temperatures and coercive fields.5'6 Their temperature stability and low dielectric loss at microwave frequencies make them good candidate materials for high frequency applications. Indeed, La 2 Ti 2 0 7 and Nd 2 Ti 2 0 7 are being used as major components in high K microwave dielectric formulations.7'8 Also, La 2 Ti 2 0 7 has been found to have good piezoelectric properties with possible use as a high temperature transducer material.9'10 Nevertheless, studies on single crystals of these materials are limited because of their high melting points which make crystal growth difficult. The synthesis of single crystal fibers of these compounds by laser heated pedestal growth has recently been reported.11 Though some methods for the synthesis of La 2 Ti 2 0 7 and Nd 2 Ti 2 0 7 in the polycrystalline form were reported based on coprecipitation of hydroxides,12'13 thermal decomposition of nitrates,12 liquid mix techniques,14 and hydrothermal methods,15 these ceramics are by far fabricated only by conventional solid state reactions of the oxides. However, for high quality products, pure powders consisting of uniform
a
^On leave from Andhra University, Visakhapatnam, India. 'Also with the Department of Agronomy.
b
J. Mater. Res., Vol. 7, No. 10, Oct 1992
http://journals.cambridge.org
Downloaded: 27 Nov 2014
particles are desirable for good sinterability and finegrained microstructures. In recent years, the sol-gel process has become a method of interest for the synthesis of nanocomposites and electroceramics.16 The advantages of the sol-gel process compared to conventional methods for polycrystalline ceramics are better control of stoichiometry and homogeneity, lower react
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