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 degrees C during different times. Isotherms at 1300 and 1600 degrees C have been analysed in detail to evaluate the solid solubility of Yb2Si2O7 in beta-Y2Si2O7 and gamma-Y2Si2O7. Although Yb2Si2O7 shows a unique stable polymorph (beta), Yb3+ is able to replace Y3+ in gamma-Y2Si2O7 and delta-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 degrees in the Si2O7 unit across the system. The temperature-composition diagram of the system, obtained from powder XRD data, is dominated by the beta-RE2Si2O7 polymorph, with gamma-RE2Si2O7 and delta-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 beta-gamma phase boundary is slightly shifted towards higher radii. (C) 2011 Elsevier Inc. All rights reserved.