Mixed valence tungsten phosphate glasses: ESR and diffuse reflectance investigation

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The synthesis of mixed valence glasses containing large amounts of W V in the system K 2OP 2O 5WO 3− x two ways: a direct reduction of tungsten trioxide by hydrogen-rich gases, resulting from the decomposition of diammonium phosphate or melting under inert atmosphere with an oxide of composition close to “WO 2.5” as starting material. The evaluation of W V concentration was performed by thermogravimetric oxidation and by double integration of the ESR signal, but measures being in good agreement: the relative W V concentration ranges from 76% to 0.1% for glasses synthesized in air and stands around 85% for glasses synthesized under argon. From a quantitative analysis of ESR and reflectance spectra, the variations of the distortion of the polyhedra W VO n aronnd W 5+ ions was studied as a function of tungsten V and total tungsten concentration. The decrease of the polyhedral distortion when W V concentration increases is to be compared with a similar behaviour in crystallized tungsten bronzes; but the optical spectra showed that the W V ions are no longer located at the center of a regular octahedron but exhibit a pyramidal coordination WO 5, involving a tungstyl ion WO 3+ (C 4v symmetry) characterized by a very short W-O distance ( ≅1.6 A ̊ . At high potassium contents, especially in glasses synthesized under argon, optical properties suggest the existence of blocks of octahedra linked by edges and corners called “tungsten clusters”.