Effect of mixed transition-metal ions in glasses. Part III: The P2O5–V2O5–MnO system

Electrical and optical properties of phosphate glasses containing vanadium and manganese ions in the xP₂O₅–[(100 − x)(V₂O₅ + MnO)] (PVM) system have been investigated. This is the last article of a III-part series devoted to the electronic properties of phosphate glasses containing a mixture of transition ions. The first article was devoted to the electrical conductivity of glasses having the general composition: xP₂O₅–[(100 − x)(V₂O₅ + Fe₂O₃)] (PVF). Competitive transport of small polarons on V and Fe ion sites was found to contribute to a mixed transition-ion effect (MTE) in PVF glasses. Several features of MTE were found to be similar to the well known mixed alkali effect, observed in glasses containing two alkali ions. In the second article, optical absorption and electronic conduction of xP₂O₅–[(100 − x)(Fe₂O₃ + MnO)] (PFM) glasses were reported. In the absence of competitive transport between the two transition ions (since Mn ions were determined not to contribute to dc conduction), MTE was not observed. The most important feature of PFM glasses was a sharp increase in resistivity at a critical concentration of iron ions, similar to ‘metal–insulator transition’ (MIT). In the present article, we report a resistivity transition in PVM glasses which is similar to that exhibited by the glasses of the PFM series. While Fe ions contributed the carriers in the PFM glasses, V ions serve the same purpose in the PVM compositions. As the concentration of vanadium ions, n_V, is decreased in the composition range 0.82 > n_V > 0.40, resistivity (ρ) increases marginally. For glasses with 0.2 < n_V < 0.40, resistivity and the activation energy for dc conduction (W) increase sharply with decreasing n_V, marking the incidence of an MIT-type transition. As in the PFM glasses, the observation of MIT coincides with the transformation of small polarons to small bipolarons, which is confirmed by the shifting of the small polaron optical absorption band to higher energies with decreasing V concentration.