Structure and Properties of the Mo3Nb2O14 Oxide
Résumé
Mixed Nb-Mo oxides were prepd. by solid-state reaction of Nb(V) and Mo(VI) oxides at 973-1123 K. Optimal conditions were detd. for the formation of the Mo3Nb2O14 compd. As established by Rietveld refinement of the powder x-ray diffraction patterns, the Mo3Nb2O14 oxide has the tetragonal cell with a 23.150(6) and c 3.998(4) .ANG. and a tunnel structure similar to that of the Mo5O14 oxide. The solids were characterized by several phys. techniques, including scanning and TEM, FTIR, UV-visible diffuse reflectance spectroscopy, XPS, and ESR spectroscopy. The Mo3Nb2O14 solid prepd. in air at 973-1073 K after cooling to room temp. contains high amts. of Mo(V) species (.apprx.1% of total Mo). The presence of paramagnetic species correlates with the intense green color of the solids and the strong d-d transition band in the UV-visible spectra, typical for the d1 species. The amt. of paramagnetic species does not depend on the solid annealing and/or on the small variations of its compn. Neither is it related to the O release upon the solid heating, being therefore an intrinsic property of the Mo3Nb2O14 oxide. The unusual stabilization of reduced Mo species in the highly oxidizing conditions was explained by the substitution disorder between Nb and Mo atoms. Probably a configuration contg. Μ3 O bonded to three Mo(VI) atoms is unstable and decomps., leading to a Mo(V) center and a hole in the valence band.