PHASE-RELATIONSHIPS IN THE SYSTEMS RE2O3-AL2O3-SIO2 (RE = RARE-EARTH ELEMENT, Y, AND SC)

A literature survey and recent results on phase relationships in the q uasi-ternary systems RE2O3-Al2O3-SiO2 are given, The investigated syst ems exhibit extended ternary solid solutions, RE9.33+2x(Si1-xAlxO4)(6) O-2 (with x up to similar to 0.33) and/or RE4Al2(1-x)Si2xO9+x (with x up to similar to 0.3), which are based on the quasi-binary phases RE9. 33(SiO4)(6)O-2 and RE4Al2O9, respectively, The former is encountered o nly inn systems with large RE3+ ions (e.g., La3+), whereas the latter is found in systems with small RE3+ ions (e.g., Yb3+); in systems with medium-sized RE3+ ions (e.g., Gd3+) both types exist. Quasi-ternary c ompounds are known only in the La, Ce, and Sc systems. Severe discrepa ncies in reported ternary eutectic temperatures led to a need for thei r accurate redetermination.