Oxidation State Of Mn Ions Research Articles

The change in oxidation state of Mn ions in semiconducting BaTiO3 and SrTiO3 around the phase transition temperatures

The oxidation states of Mn in low concentration (< 0.1 atom%) in donor doped BaTiO3 and SrTiO3 have been studied using EPR, optical absorption spectra and electrical resistivity measurements. Mn substituting at the Ti-sites exists as Mn2+ in the cubic and rhombohedral phases and as Mn3+ in the tetragonal and orthorhombic phases of BaTiO3. The change in oxidation state arises from the charge redistribution among singly- or doubly- ionised acceptor states and the conduction electron, depending upon the structural changes in the host lattice. The gain in Jahn-Teller energy stabilises Mn3+ in the tetragonal and orthorhombic phases. In SrTiO3, co-existence of Mn2+, Mn3+ and Mn4+, together with Mn2+ − VO is observed. Only Mn2+ in SrTiO3 converts to Mn3+ as the lattice approaches TC (106 K). The electron loss (or hole gain) by the corresponding acceptor states may involve vibronic interactions and is influenced by the soft phonon modes. The acceptor states are more sensitive to local structural deviations and the facile redistribution of charges at the acceptors leads to the changes in resistivity around the phase transitions.