The elastic constants and structure of the vitreous system Mo-P-O

Abstract

The preparation of glasses with compositions spanning the entire vitreous range obtainable from melting phosphorous pentoxide with molybdenum trioxide, and phosphoric acid with ammonium molybdate, in open crucibles, is described. The second order elastic constants of these Mo-P-O glass systems have been obtained from ultrasonic wave velocity measurements. The most striking features of the results are two discontinuities in the compositional gradient of each elastic modulus, at compositions of ∼53 and 63 mol% MoO3 content. These compositions approximate to those of molybdenum metaphosphate and molybdenum pyrophosphate crystals. A second noteworthy feature is that the elastic moduli display a substantial overall increase with molybdenum content, 36% for bulk and 83% for shear over the entire composition range, even though the Mo-O bond stretching force constant is less than half that of the P-O bond. A detailed qualitative interpretation of these features is made, in terms of the proposed compositional dependence of the crosslink densities of the molybdenum and phosphorus atoms, and the atomic ring sizes.