Crystallization of ThSiO4 from structurally and/or compositionally diphasic gels

The use of structurally and/or compositionally diphasic gels has been shown earlier to have profound effects in lowering the crystallization temperature and enhancing densification in several systems. In this study of solid-state epitaxy the ThSiO4 composition has been investigated. The use of compositional diphasicity allows one to lower the huttonite (/?-ThSiO4) crystallization temperature by as much as 200 °C. Using crystalline thorite (a-ThSiO4) as second phase nuclei partially stabilizes this phase (hypothetically metastable), while the introduction of huttonite nuclei completely inhibits the formation of the a form, which appears in detectable quantities in the unseeded precursors. This confirms the structural (epitaxial) control of the solid-state reaction. The use of structural or compositional nanoheterogeneity clearly helps to lower the crystallization temperature and to stabilize a particular polymorph in ceramics processed by the sol-gel route, and this principle can certainly be utilized more generally.