Where the extraordinaries meet: a cascade of isosymmetrical superionic phase transitions and negative thermal expansion in a novel silver salt-inclusion borate halide

A new silver iodide borate, Ag3B6O10I, is presented as a promising solid-state electrolyte with a delta <-> gamma <-> beta <-> alpha cascade of the isosymmetric superionic phase transitions. Its crystal structure is composed of two sublattices, the host B-O covalent part from B6O13 6B:3 Delta 3:[phi]||| corner-sharing hexaborate groups, and the guest Ag-I ionic part. The transitions are associated with structural re-arrangements of the latter with a stepwise melting of silver sublattice and give rise to an increase in conductivity to 0.005 S cm(-1) at 99 degrees C. Exceptionally large room-temperature anharmonicity of silver vibration has been described. The migration of silver ions leads to negative thermal expansion (NTE) at 25-400 degrees C. The sodium analog, Na3B6O10I, with a normal perovskite-like structure was obtained. The structural diversity in the M3B6O10X family can be rationalized considering the difference in the ionic radii of X- anions and M+ cations.

Automated summary

This article introduces a new silver iodide borate as a promising solid-state electrolyte with multiple isosymmetric superionic phase transitions, which significantly improves conductivity and exhibits large room-temperature anharmonicity vibration and negative thermal expansion. Furthermore, the structural diversity can be explained by considering the difference in ionic radii.