ESR DETERMINATION OF TIO2 AND SNO2 TAMMANN TEMPERATURES

The thermal behaviour of vanadium(IV) paramagnetic ions deposited by impregnation on two rutile compounds (TiO2 and SnO2) has been investigated by ESR. Beyond a critical temperature, these probe V(IV) ions migrate from the surface into the bulk of the oxide. The most significant ESR features of the intralattice and extralattice V(IV) species are briefly analysed. For the V/TiO2 system. the amount of detectable bulk V(IV) ions depends strongly on the annealing temperature; their signal intensity drastically increases above 865 +/- 15 K. This sudden change indicates the temperature at which cations and cationic vacancies begin to move inside TiO2, namely the Tammann temperature of this oxide. This value agrees with the empirical range generally admitted for the Tammann temperature of oxides (between 0.3T(m) and 0.5T(m) where T(m) represents the oxide melting temperature, i.e. between 634 and 1057 K in the case of TiO2). It is also consistent with a previous experimental determination (870 K) obtained by specific surface measurements. A poor resolution hampers a similar analysis for the V/SnO2 system. However, near 650 K, the ESR spectra are strongly modified and indicate that diffusion mechanisms have begun. A 650 +/- 50 K value appears therefore as a reasonable estimate of the SnO2 Tammann temperature. Such a small value agrees with the low T(m) of this oxide (1400 K) which leads to a 400-700 K empirical range.