TEMPERATURE-INDUCED DIFFUSION OF PROBE VANADIUM(IV) IONS INTO THE MATRIX OF TITANIUM-DIOXIDE AS INVESTIGATED BY ESR TECHNIQUES

Heat treatments of multicomponent oxides are known to induce an interdiffusion of the cations. In the case of V2O5/TiO2 (rutile) mixed compounds, vanadium(IV) ions can be included as dopants inside the TiO2 matrix. This reaction leads formally to a VxTi1-xO2 solid solution. Indeed, when an impregnated sample containing 1% of vanadium (V/Ti atomic ratio) is submitted to increasing annealing temperatures, the ESR signals characteristic of vanadyl ions located at the TiO2 surface are progressively replaced by an orthorhombic signal attributed to some vanadium ions at the four oxidation state stabilized in substitutional sites inside the TiO2 matrix. The line width of this signal is not modified upon heat treatment. Therefore, the direct between its height and that of a reference compound (V0.01Ti0.99O2 solid solution obtained by coprecipitation) provides a direct measurement of the intralattice vanadium amount. Isothermal evolution of this amount for various annealing temperatures gives an activation energy of 2 eV for the bulk cationic diffusion in agreement with the calculated values previously published. In absence of sintering effect, the temperature at which this mechanism (865 +/- 15 K) becomes suddenly important indicates the mobility onset of the cations and the cationic vacancies of the TiO2 SUPport and this temperature can be related to the Tammann temperature of this oxide.