Solid solubility of Yb2Si2O7 in β-, γ- and δ-Y2Si2O7

This paper examines the structural changes with temperature and composition in the Yb2Si2O7–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 Yb2O3 and Y2O3 mixtures. A set of different compositions have been synthesised using the sol–gel method to obtain a xerogel, which has been calcined at temperatures between 1300 and 1650 °C during different times. Isotherms at 1300 and 1600 °C have been analysed in detail to evaluate the solid solubility of Yb2Si2O7 in β-Y2Si2O7 and γ-Y2Si2O7. Although Yb2Si2O7 shows a unique stable polymorph (β), Yb3+ is able to replace Y3+ in γ-Y2Si2O7 and δ-Y2Si2O7 at high temperatures and low Yb contents. IR results confirm the total solid solubility in the system and suggest a constant SiOSi angle of 180° in the Si2O7 unit across the system. 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 reduced stability fields. The diagram is in accordance with Felsche's diagram if average ionic radii are assumed for the members of the solid solution at any temperature, as long as the β–γ phase boundary is slightly shifted towards higher radii.

Graphical abstractPolymorphism, as a function of average radii, in the systems Yb2Si2O7–Y2Si2O7 (dotted symbols), Lu2Si2O7–Y2Si2O7 (solid symbols) and Sc2Si2O7–Y2Si2O7 (open symbols) compared to phase boundaries of Felsche for pure rare earth disilicates. Circles: β-RE2Si2O7. Squares: γ-RE2Si2O7. Triangles: δ-RE2Si2O7.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► β-Yb2Si2O7 and β-Y2Si2O7 form a complete solid solution at 1300 °C. ► Yb(III) replaces Y in γ- and δ-RE2Si2O7, at low Yb contents and high temperatures. ► Phase diagram of the Yb2Si2O7–Y2Si2O7 system is dominated by the β-RE2Si2O7 phase. ► γ- and δ-phases show very reduced stability fields. ► β–γ phase boundary in Felsche's diagram should be slightly shifted towards higher radii.