Ernest M. Levin

The system niobium pentoxide-phosphorus pentoxide

Abstract A partial phase diagram of the system Nb 2 O 5 P 2 O 5 has been determined experimentally, using the quenching technique and identifying the phases by X-ray powder diffractometry. The major phase, NbPO 5 , has a melting point above 1650°C. It transforms at about 1253°C from a low-temperature, tetragonal phase to a high-temperature, monoclinic phase, isostructural with TaPO 5 . The 9Nb 2 O 5 ·P 2 O 5 phase shows a limited solid solution range and melts incongruently at 1435°C. The eutectic between 9:1 and NbPO 5 is at 1365°C and 67 mole % Nb 2 O 5 . At 96.4 mole % Nb 2 O 5 another phase was found, stable between ∼1340 and 1420°C and isostructural with the 9:1 phase. A new monoclinic phase, “22:1” ss was found, with a limited stability field in both temperature and composition. It is most likely composed of 3 × 3 and 3 × 4 ReO 6 -type blocks of octahedrally oxygen-coordinated niobium. In the subsystem NbPO 5 P 2 O 5 liquidus values could not be determined even in sealed Pt tubes because of reactivity and high vapor pressure of P 2 O 5 . One compound was identified of probable composition Nb 2 O 5 ·2P 2 O 5 , with pseudocubic symmetry and melting incongruently at about 1025°C. X-ray powder diffraction data and unit cell dimensions are listed for the phases.