Formation and stability of Gd, Y, Yb and Lu disilicates and their solid solutions

The synthesis and the phase stability regions of the disilicates of Y, Lu, Yb and Gd have been investigated at temperatures between 1300 and 1600 °C. The mean ionic radius of the rare-earth element ion including Y turns out to be the key parameter to govern the stabilities of polymorph types of both pure disilicates and their solid solutions. Both are predicted correctly by the phase stability diagram of Felsche [J. Felsche, Struct. Bond. 13 (1973) 99–197]. Furthermore, it correlates with the reactivity, with increasing radius a faster reaction is observed at a given temperature. A fast reactivity is assumed to create difficulties in the densification of disilicates with ionic radius exceeding ∼0.88 Å. In the reaction from oxide powders monosilicates are formed in a first step. Disilicates are formed from those initially in the form of low-temperature modifications, which transform in a sequence to high-temperature modifications.