Structural and chemical studies in the niobium oxides and related compounds

The chemistry and thermodynamics of the Nb-O system and the related Ti-Nb-O system were studied to elucidate: 1) Kinetic characteristics of block structure oxides at T>1300°C , 2) Thermo dynamic properties of block structure oxides at T>1300°C and 3) Phase equilibria in the metastable Nb2O5 systems. Single crystals of B- and P-Nb2O5. were prepared by the Chemical Vapour Transport Technique Tinder predominantly convection controlled conditions. Phase equilibria experiments were carried out in a Cahn microbalance system and a newly designed Horizontal Quench Furnace line. Controlled oxygen potentials were generated from CO2/CO or CO2/H2 gas mixtures. Structural studies of the oxides involved X-ray diffraction, electron diffraction and high resolution transmission electron microscopy (HRTEM). Results of phase equilibria studies in the NbO2 - (B-Nb2O5) system at 750°C <T<900°C indicated that the transformation of B- to the block structure phase takes place readily on the area of contact with the Pt. container, probably catalyzed by the platinum surface. The transformed oxides contained both disordered and ordered fragments. In diffraction studies, the disordered structures simulated the tetragonal symmetry of M-Nb2O5, the existence of which was contradicted by HRTEM results. A new phase Nb43O107 having monoclinic lattice parameters of a = 20.3 Ã, b = 3.8 Ã, c = 61.5 à and β = 117° was detected in the ordered crystals. Its structural relationships with the neighbouring phases in the Nb-O system is discussed. At 700°C and controlled oxygen potentials, B-Nb2O5 transformed directly into NbO2 on reduction. In the system NbO2 - (P-Nb2O5) at isotherm 750°C and controlled oxygen potentials, Nb12O29 existed as the only stable intermediate phase. Phase equilibria in the system (Ti-Nb-O) were determined by equilibrating TiNb2O7, Ti2Nb10O29 and TiNb24O62 at 1400°C. The oxide-gas equilibrium was attained rapidly (< 1 hr). The (p, x) isotherms showed complete bivariance in the systems (Ti,Nb)O2 - TiNb2O7, (Ti,Nb)O2 - Ti2Nb10O29 and (Ti,Nb)O2 - TiNb24O62. HRTEM results showed that the bivariance was due to coherent intergrowth of neighbouring phases in the host lattice, suggesting that Wadsley defects were thermodynamically stable. An abrupt process: TiNb2O7 â metastable transient Me25O60 â Ti2Nb10O29 was also detected. Associated with the reduction of Ti-Nb oxides is the generation of a rutile phase TixNb1-xO2, the composition of which is dependent upon the oxygen potential. The results are summarised in a ternary phase diagram.