Studies of Silica‐Structure Phases: I, GaPO 4 , GaAsO 4 , and GaSbO 4

In the course of work on all compositions which would probably yield silica‐structure phases, a detailed study was made of the A 3+ B 5+ O 4 ‐type compounds with gallium as the trivalent element. Two forms of GaPO 4 exist corresponding to quartz and cristobalite; the quartz form has a thermal expansion of the low‐quartz type and no stable α‐β transition is found in this phase. It inverts stably to the cristobalite form at 933°± 4°C. The cristobalite form melts at 1670°C. and the liquid cannot be made to form a glass. The cristobalite form also has a metastable displacive transition at 616°± 2°C. GaAsO 4 exists only in a form corresponding to the lowquartz structure. It disproportionates into Ga 2 O 3 and As 2 O 5 at about 1000°C. GaSbO 4 could only be prepared in one form corresponding to the rutile structure. The influence of such phases on SiO 2 was determined by studying a few mixtures in the system SiO 2 ‐GaPO 4 . The SiO 2 end member appears to admit about 25% GaPO 4 into solid solution in the cristobalite phase at the highest temperature. Even under hydrothennal conditions, however, equilibrium cannot be easily attained at the lower temperatures in a few hundred hours. The results are presented in a not‐impossible diagram for the system GaPO 4 ‐SiO 2 .