Unexpected Coexisting Solid Solutions in the Quasi-Binary Ag^(II)F2/Cu^(II)F2 Phase Diagram**

High-temperature solid-state reaction between orthorhombic AgF and monoclinic CuF (y=0.15, 0.3, 0.4, 0.5) in a fluorine atmosphere resulted in coexisting solid solutions of Cu-poor orthorhombic and Cu-rich monoclinic phases with stoichiometry AgCuF. Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF : Cu) appears to be at an upper limit of Cu concentration of 30 mol % (AgCuF), while the monoclinic phase (CuF : Ag) can form a nearly stoichiometric Cu : Ag=1 : 1 solid solution (CuAgF), preserving the CuF crystal structure. Experimental data and DFT calculations showed that AgF : Cu and CuF : Ag solid solutions deviate from the classical Vegard's law. Magnetic measurements of AgCuF showed that the Néel temperature (T) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for AgCuF showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J| in both structures. Electrical conductivity measurements of AgCuF showed a modest increase in specific ionic conductivity (3.71 ? 10±2.6 ? 10 S/cm) as compared to pure AgF (1.85 ? 101.2 ? 10 S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample.