Structural investigation of ternary La/alkaline earth phosphate (La(1−x)MxP3Oy) (M = Ba, Ca, Sr) glasses

The structure of La-based metaphosphate glasses modified with aliovalent alkaline earth elements (M = Ba, Ca, Sr), with P:(M + La) ratio kept approximately at 3, were analyzed using Raman and IR spectroscopy. Increasing the concentration of the alkaline earths monotonically decreased the glass density and glass transition temperature, and shifted some vibrational modes by as much as 40 wavenumbers. The Raman frequencies of the symmetric POP and the PO vibrations depended on the M/La ratio, but not on the type of alkaline earth addition; however, the type of alkaline earth cation significantly changed the symmetric PO2 vibration. The average cation-oxygen coordination number remained near ∼7 except for a slight decrease for high concentrations of Ca and Sr substitutions. The POP/PO2 intensity ratio increased with increasing alkaline earth content. The replacement of divalent elements for trivalent La in a metaphosphate glass structure was partially accommodated by incorporation of protons as charge compensating defects. The protons were found to be in close proximity to the divalent cations.