Molecular structure of (AgPO3)$_{1-x}$ (AgI)$_{x}$ glasses

Melt-quenched AgPO$_{3}$ glasses were synthesized by dry ( Ag$_{3}$PO$_{4}$ + P$_{2}$O$_{5}$, prep. 1) and wet (NH$_{4}$H$_{2}$PO$_{4}$ + AgNO$_{3}$, prep. 2) routes. Glass transitions were examined in MDSC at a scan rate of 3$^{o}$C/min. Prep. 1 samples display \textit{bimodal} glass transition temperatures, with T$_{g}^{low}$ = 220$^{o}$C and T$_{g}^{high}$ = 238$^{o}$C and with the T$_{g}^{low}$ endotherm higher in strength than the T$_{g}^{high}$ one. In contrast, prep. 2 samples show a single T$_{g}$ = 203$^{o}$C that is significantly lower in temperature. These results are consistent with the notion that prep. 2 probably yields samples with bonded water while prep 1 gives pure AgPO$_{3}$ glasses that are intrinsically \textit{phase separated}. The nature of the two phases in the latter is less obvious at present, but we note that upon alloying AgI, the additive selectively bonds in the T$_{g}^{low}$ phase at low x ($<$0.20) with T$_{g}^{low}$ steadily decreasing, and with the T$_{g}^{high}$ phase remaining largely unaffected. At higher x ($>$0.20) a major structural reorganization occurs, and we observe the opening of a \textit{reversibility window} in the 0.22 $<$ x $<$ 0.37 range. As in the chalcogenides, we identify the \textit{window }with the intermediate phase with glasses at x $<$ 0.20 stressed-rigid, while those at x $>$ 0.37 as floppy. A percolation threshold for electrical conduction occurs$^{1}$ near x $\sim $ 0.3 and falls in the reversibility window as expected. \newline \newline 1. M. Mangion and G.P. Johari, Phys. Rev. \underline {B36}, 8845 (1987) \newline Supported by NSF grant DMR 04-56472