Effect of oxide additions on the polymorphism of tantalum pentoxide. IV. The system Ta2O5Ta2WO8

The low-temperature form of pure Ta 2 O 5 has been found to exist in two slightly different modifications. The lowest temperature form has a b axis multiplicity of 14 whereas the highest temperature form (about 1350°C) has a multiplicity of 11. At intermediate temperatures an “infinite” number of at least partially ordered sequences of these two modifications exist in equilibrium. The addition of WO 3 to Ta 2 O 5 causes the “stabilization” of an “infinite” number of phases similar in structure to the low temperature form of Ta 2 O 5 . From just less than 10 mol % WO 3 to the last compound in the series at 26 2 3 mol % WO 3 these phases are in equilibrium with liquid, and the high temperature structure type of pure Ta 2 O 5 is eliminated from the phase diagram. One compound, 15Ta 2 O 5 ·2WO 3 with a multiplicity of 8, was found to melt congruently at about 1815°C and the last phase, 11Ta 2 O 5 ·4WO 3 with a multiplicity of 13, melts incongruently at about 1605°C.