Transition-metal incorporation and Co-Sr/Mn-Sr mixed-modifier effect in metaphosphate glasses

Modifier-mixing provides a well-established tool for generating non-linear trends in physical properties of multi-component glasses. While typically, mixtures of alkali or alkaline earth ions are considered, here, we report on the structural incorporation of Co2 + and Mn2 +, respectively, in strontium metaphosphate glasses, and on the associated Co-Sr/Mn-Sr mixed-modifier effect. Structural variations in the short and intermediate range were investigated with vibrational spectroscopy and synchrotron X-ray diffraction, elucidating the coordination states of the two transition metals as a function of mixing ratio. For the join of Co-Sr, we find that the octahedral coordination dominates for lower contents of cobalt, whereas the tetrahedral state is predominantly found at higher cobalt concentration. The analogue decrease in the coordination number is weaker when mixing manganese and strontium ions. Coordination numbers are shown to correlate clearly with vibrational spectroscopic data and macroscopic physical properties.