Effect of mixing transition ions in glasses. II. The P2O5–Fe2O3–MnO system

The first article of this series reported electrical properties of phosphate glasses containing V and Fe ions, both of which contributed to conductivity. A mixed transition-ion effect was detected in such glasses which had several characteristics similar to the mixed alkali effect, observed in glasses containing two alkali ions. This article reports electrical and optical properties of phosphate glasses containing Fe and Mn ions, one of which (Mn) does not contribute directly to conductivity. DC resistivity by the Van der Pauw four-probe method and optical absorption (UV–VIS–IR) measurements have been performed on xP2O5 − (100 − x) (Fe2O3 + MnO) (PFM) glasses. The transport mechanism has been identified to be hopping of small polarons between Fe2+ and Fe3+ sites. For 0.3 < nFe < 0.5, where, nFe, the atomic fraction of Fe ions is given by (Fe/(P + Fe + Mn)), a marginal linear increase in resistivity has been observed with decreasing nFe. However, when nFe is reduced below ∼0.3, the resistivity increases exponentially with decreasing nFe, which is similar to ‘metal–insulator transition’ (MIT), observed in many crystalline systems. The resistivity transition in these glasses has been explained by a (small) polaron to (small) bipolaron transition (PBT) in the composition range 0 ⩽ nFe ⩽ 0.3. The occurrence of PBT is confirmed by the shifting of the polaronic optical absorption band to higher energies with increasing small bipolaron concentration.