Polymorphism in the Sc2Si2O7-Y2Si2O7 system |
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Authors: | Alberto Escudero |
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Institution: | Departamento de Química Inorgánica, Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Avda. Américo Vespucio, s/n. 41092 Sevilla, Spain |
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Abstract: | This paper examines the structural changes with temperature and composition in the Sc2Si2O7-Y2Si2O7 system; members of this system are expected to form in the intergranular region of Si3N4 and SiC structural ceramics when sintered with the aid of Y2O3 and Sc2O3 mixtures. A set of different compositions have been synthesized using the sol-gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1750 °C during different times. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the β-RE2Si2O7 polymorph, with γ-RE2Si2O7 and δ-RE2Si2O7 showing very reduced stability fields. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of the components. Although, the XRD data show a complete solid solubility of β-Sc2Si2O7 in β-Y2Si2O7 at 1300 °C, the 29Si MAS-NMR spectra indicate a local structural change at x ca. 1.15 (Sc2−xYxSi2O7) related to the configuration of the Si tetrahedron, which does not affect the long-range order of the β-RE2Si2O7 structure. Finally, it is interesting to note that, although Sc2Si2O7 shows a unique stable polymorph (β), Sc3+ is able to replace Y3+ in γ-Y2Si2O7 in the compositional range 1.86?x?2 (where x is Sc2−xYxSi2O7) as well as in δ-Y2Si2O7 for compositions much closer to the pure Y2Si2O7. |
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Keywords: | Rare earth disilicate Solid-state chemistry Phase diagram X-ray diffraction MAS NMR |
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