Disappearance of βAgI in (AgI)_0.7–(NaPO₃)_0.3 composite at room temperature

Abstract

We report crystalline phases in the melt-quenched (AgI)_0.8–(NaPO₃)_0.2 and (AgI)_0.7–(NaPO₃)_0.3 superionic composites at room temperature as studied by VEGA powder neutron diffraction of KEK. (AgI)_0.8–(NaPO₃)_0.2 contains γAgI and βAgI with a 70:30 ratio, which is similar to that found in pure AgI. On the other hand, (AgI)_0.7–(NaPO₃)_0.3 contains AgI and Ag₅IP₂O₇ with a 53:47 ratio, without any indication of βAgI presence. Due to the overall broad line widths and the hexagonal-related orthorhombic unit cell of Ag₅IP₂O₇, VEGA was indispensable to probe its existence in (AgI)_0.7–(NaPO₃)_0.3 at room temperature. The disappearance of βAgI in (AgI)_0.7–(NaPO₃)_0.3 in favour of Ag₅IP₂O₇ containing more oxygens indicating that the phosphate chains have been oxidized to P₂O₇ pairs of phosphate tetrahedra.