Crystal structure, stability and existence region of CsAg4Br3-xI2+x superionic conductors

CsAg₄Br_3-xI_2+x solid solutions, which are isomorphic to the well-known α-RbAg₄I₅ superionic conductor, exhibit very similar conductivity by Ag⁺ ions. However, the CsAg₄Br_3-xI_2+x electrolytes, promising low-temperature superionic conductors, have been little studied. Therefore, the present work set out to clarify their thermodynamic stability, define an existence region, and characterise the crystal structure parameters as a function of x. CsAg₄Br_3-xI_2+x samples were prepared by solid-phase synthesis over a wide range from x = –0.25 to x = 2.00. The phase composition of the samples was specified by X-ray diffraction and thermal analysis. Single-phase CsAg₄Br_3-xI_2+x was observed for freshly synthesised samples within the composition range of 0.25 < x ≤ 1.35; however, all solid solutions CsAg₄Br_3-xI_2+x (with the exception of x = 0.38) were found to be metastable at room temperature. Although the research samples can be easily prepared, they gradually decomposed when stored at 25 °C over a period of months. Solid solutions having a predominance of iodine decomposed most rapidly, while samples with a predominance of bromine decomposed much more slowly. Only one of the studied samples, having x = 0.38, underwent no changes; this sample remained single-phase throughout the entire storage period of up to >3 years. The occupancy of different crystallographic positions of Ag in freshly prepared samples as a function of x was established. The polythermal cross-section of the four-component system Cs – Ag – Br – I along the line between two non-existent compounds “CsAg₄Br₅” – “CsAg₄I₅” was used to represent the CsAg₄Br_3-xI_2+x solid solution field.