Crystallization of ThSiO 4 from structurally and/or compositionally diphasic gels
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I. INTRODUCTION AND BACKGROUND The solution-sol-gel method is highly amenable for preparing nanocomposites. Among the nanocomposite materials there are compositionally diphasic materials1"8 in which the phases differ in composition and there are structurally diphasic, sometimes called seeded materials,9"14 in which the phases differ in structure. Of course, it is possible to combine these two types of diphasicities. Recently, structural and compositional diphasicities have been shown to reduce substantially the crystallization temperature of zircon (ZrSiO 4 ). 15 The purpose of the present work is to study the crystallization behavior of different monophasic and diphasic gels in the system ThO,-SiO 2 , and specifically the composition of ThSiO4. The special interest in this composition comes from the fact that there are two known structural polymorphs of this compound, related by a reconstructive transition: (1) a-ThSiO 4 (mineral name thorite) with the tetragonal zircon structure, and (2) flThSiO4 (mineral name huttonite) with the monoclinic monazite structure. The thermodynamics of the ThSiO4 phases is still an open question but it was believed that the solid-state reaction could only lead to the /? form, l6 whereas the a form could be obtained hydrothermally' 7 or by the molten salt method.18 According to Mumpton,19 there are four different conceivable hypotheses concerning the stability relations of ThSiO4, two of them being more in agreement with the following observations. (1) Huttonite could be the stable modification at all temperatures and pressures. This theory relegates thorite to a metastable role at all temperatures. 10 Also
associated with the Department of Agronomy.
J. Mater. Res. 2 (4), Jul/Aug 1987
http://journals.cambridge.org
(2) Thorite could be the stable polymorph of ThSiO4 up to a certain temperature (about 1200 °C) above which huttonite is the only stable form. The goal of the present study is not to verify the validity of these hypotheses, but to investigate the crystallization behavior of different diphasic gels using aand/S-ThSiO4 as "seeds." II. EXPERIMENTAL PROCEDURE A. Preparation of the sols (precursors and seeds) (1) A thorianite (ThO 2 ) sol was first prepared hydrothermally in a Parr bomb by treating a ThO 2 precipitate for 3 days at 200 °C. The ThO 2 precipitate was obtained by titrating a 0.3M Th(NO 3 ) 4 ,4H 2 O solution into concentrated NH 4 OH. The resulting sol was washed and dispersed in distilled water prior to phase characterization by x-ray powder diffraction and morphology and particle size determination by transmission electron microscopy (TEM). The hydrothermally prepared ThO 2 is poorly crystalline, the crystal size being between 10 and 20 nm [Fig. 1 (a) ]. (2) A thorite seed sol was obtained by treating a stoichiometric mixture of a Th(NO 3 ) 4 ,4H 2 O solution and tetraethylorthosilicate (TEOS) in ethanol in a Parr bomb. The temperature was maintained at 200 °C for 2 days. The thorite particles are quite crystalline and are approximately 30 nm in diameter [Fig.
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