X-ray microanalysis and high-resolution transmission electron microscopy of the reduced titanium-niobium oxides

Abstract

Phase relationships in TiNb 2O 7 and Ti 2Nb 10O 29 reductions at 1400°C were investigated by means of X-ray microanalytical electron microscopy and high-resolution transmission electron microscopy (TEM). Compositions of phases present in equilibrium were obtained by applying thin-crystal approximation by which Nb Ti ratios in different phases were determined; their oxygen content was inferred from structural considerations. In this manner, phase relationships in that portion of the TiO 2NbO 2NbO 2.5 equilibrium diagram with 2.417 ≥ x (in MeO x ) ≥ 2 were defined. Data obtained, in combination with high-resolution electron microscopy observations, confirmed that the reduction reaction, in part, is a heterogeneous process controlled by outward diffusion of both metal and oxygen atoms. Recombination of the diffused particles leads to the formation of separate crystals. The original block structure phase undergoes transformation in a quasihomogeneous manner either to an isomorphous phase in the binary NbO system or to a structurally related lower composition oxide. A new superstructure Me 25O 60(Ti 7.16Nb 42.84O 120) has been detected as an intermediate metastable phase, generated in the reduction of TiNb 2O 7 to stable Me 12O 29(Ti 1.53Nb 10.47O 29) and MeO 2(Ti 0.52Nb 0.48O 2) phases. Consideration of phase relationships among Me 25O 60, Me 12O 29, and MeO 2 suggests a chemical mechanism for the reaction concerned. The Me 25O 60 superstructure has a monoclinic symmetry with cell parameters a = 19.0 Å, b = 3.8 Å, c = 26.6 Å, α = 90°, β = 90°, γ = 78.5°, as determined from the structure image calculations.