Structure–Property Correlations in Solid Solutions of (CuI) 8 P 12− x As x , 2.4≤ x ≤6.6

A series of P/As mixed pnicogen phases of composition (CuI) 8 P 12− x As x , in which x=2.4, 4.2, 4.8, 5.4, and 6.6, have been synthesized and characterized by X‐ray single crystal and powder diffraction, solid‐state NMR spectroscopy, thermal gravimetric analysis, and impedance spectroscopy. These materials are isostructural to (CuI) 8 P 12 and consist of neutral, tubular P/As mixed pnicogen chains associated with Cu I and I − ions. The As is distributed throughout the pnicogen chains; however, the “roof” sites of the [P8] cage show preferred occupation by As relative to the other sites. Accordingly, the change in cell volume is not a linear function of the As incorporation. Solid‐state 31 P NMR spectroscopy of the 40 % As incorporated sample are consistent with the X‐ray structural model, with extensive broadening due to 31 P– 75 As coupling and site disorder, and a change in the chemical shifts of the resonances due to the As substitution into the lattice. The degree of copper ion site disorder, probed by single‐crystal X‐ray diffraction, increases with increasing As content. Although very little change is observed in the copper ionic conductivity of polycrystalline samples, which ranges from 1.8–5.1×10 −6 S cm −1 for (CuI) 8 P 12− x As x , x =0, 4.2, 5.4; a single crystal ( x =4.8) measured along the needle axis has a conductivity of 1.7×10 −3 S cm −1 at 128 °C. This represents an order of magnitude improvement in conductivity over (CuI) 8 P 12 at the same temperature.