Barium polyphosphate glasses, from structure to thermochemistry
Abstract In order to understand the effects of BaO addition on the structure and thermochemistry of polyphosphate glasses, a series of vitreous compositions having the formula (100-x)NaPO3-xBaO was investigated with x ranging from 0 to 25 mol%. The glasses were synthesized using the melt quenching technique and reveal a limit of vitrification of 25 mol% of BaO. Fourier Transform Infrared Spectroscopy and 31P Magic Angle Spinning Nuclear Magnetic Resonance show the depolymerization of the metaphosphate chains, leading to Q1 and Q2 phosphate groups as main structural units together with a smaller average chain length. The substitution of Na2O and P2O5 by BaO increases the density, glass transition and crystallization temperatures indicating the shrinking and reticulation of the polyphosphate network. The enhancement of the chemical durability was related to the strengthening of the glasses towards water attack and the low mobility of the Ba2+ cations. Finally, the microcalorimetric investigations, pursued in 4.5 wt% H3PO4 solution, show a decrease of the dissolution enthalpy. This trend was correlated with the structural modification within the vitreous network and the reduction of the chemical degradability of the glasses.