Structure and Properties of Mixed Strontium−Manganese Metaphosphate Glasses

Abstract

Structural, optical, and physical properties of glasses prepared by melt reduction in the mixed Sr−Mn metaphosphate system xMnO−(1 − x)SrO−P2O5, 0 ≤ x ≤ 1, have been investigated by vibrational, optical, EPR, and thermal techniques. Mn ions were found mostly in the +2 oxidation state and in sites of octahedral symmetry. Such sites are formed by neighboring Mn−oxygen polyhedra, where the covalent character of Mn−O bonding increases with cation mixing. The phosphate structure was found to consist predominantly of metaphosphate tetrahedral species (Q2) with a minority of pyrophosphate (Q1) and neutral (Q3) phosphate tetrahedra, whose relative abundance changes nonlinearly with MnO content. The symmetric stretching vibration of terminal PO2− units in Q2 species was employed to probe the influence of mixed Sr/Mn environments on phosphate structure, and the results suggested a deviation from the homogeneous distribution of metal cations. This was attributed to the coordination numbers of Sr and Mn ions (i.e., 8 and 6, respectively) which exceed the available number of terminal oxygen atoms per metal ion, M (i.e., 4), and thus require the formation of neighboring M−oxygen polyhedra which are connected by P−O−M−O−M−O−P linkages. Nevertheless, each metal ion was found to form its own M−O site and to retain the identity of its site in both single and mixed cation glasses. While density and molar volume follow a linear decrease with MnO content, glass transition temperature Tg, thermal expansion coefficient, refractive index, and optical dispersion exhibit clear deviations from additivity. The increasing trend of Tg with cation mixing was attributed to a combination of the different cross-linking abilities by P−O−M−O−M−O−P bridges of the Sr and Mn ions with the relative proportion of metaphosphate Q2 units. The composition dependence of optical dispersion, as expressed by the Abbe number, was correlated with the average electronic band gap obtained from refractive-index dispersion data using the Wemple−DiDomenico single oscillator model. While all glasses in the Sr−Mn system were found to exhibit low optical dispersion, cation mixing was shown to increase dispersion because of increased covalency in Mn−O bonding.