Phase Equilibrium Relations in the Binary System Bismuth Sesquioxide-Niobium Pentoxide

The phase equilibrium diagram for the binary system bismuth sesquioxide-niobium pentoxide has been constructed from observations of fusion characteristics and X-ray diffraction data. In the system five binary compounds were observed with Bi2O3:Nb2O5 ratios of 5:3, 1:1, 4:9, 1:5, and 1:6. The 1:1 compound was found to transform irreversibly (in laboratory time) from the orthorhombic bismutotantalite type structure to a triclinic form at about 1,020 °C and melt congruently at 1,245 °C. The 5:3 compound melts incongruently at 1,193 °C the 4:9 at 1,183 °C and the 1:6 at 1,242 °C. The 1:5 compound has a maximum temperature of stability at 1,095 °C and the 4:9 and 1:6 compounds have minimum temperatures of stability at 1,070 °C and 1,002 °C respectively. Nb2O5 was found to enter into solid solution in Bi2O3, up to about 23.5 mole percent Nb2O5. The melting point is increased and the monoclinic-cubic phase transformation temperature is decreased. A morphotropic phase change occurrs at about 19.5 mole percent Nb2O5 from the cubic to a pseudocubic structure.