Effect of mixed transition-metal ions in glasses. I. The P2O5–V2O5–Fe2O3 system

Non-linear variation of certain physical properties with composition, similar to the mixed alkali effect (MAE), has been observed in phosphate glasses containing iron and vanadium ions in the xV2O5 · (50 − x)Fe2O3 · 50P2O5 series. With the total concentration of transition ions (TIs) constant, gradual substitution of one TI (e.g. Fe in the binary composition Fe2O3–P2O5) by the other (viz., V) increases dc resistivity (ρ), activation energy for dc conduction (W) and glass transition temperature (Tg) until these properties reach their maxima. Beyond the maxima, ρ, W and Tg are observed to decrease with increasing TI-ratio, V/(V + Fe), until the binary P2O5–V2O5 composition is reached. The non-linear variation of the physical properties with TI-ratio has been termed the ‘mixed transition-ion effect’ or MTE. The similarities between MTE and MAE are remarkable because the former is electronic in nature while the latter has its origin in ionic motion. In the absence of a comprehensive theory, tentative arguments suggestive of the origin of MTE have been presented.