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

High-temperature solid-state reaction between orthorhombic AgF₂ and monoclinic CuF₂ (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 Ag_1-x Cu_x F₂ . Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF₂  : Cu) appears to be at an upper limit of Cu concentration of 30 mol % (Ag_0.7 Cu_0.3 F₂ ), while the monoclinic phase (CuF₂  : Ag) can form a nearly stoichiometric Cu : Ag=1 : 1 solid solution (Cu_0.56 Ag_0.44 F₂ ), preserving the CuF₂ crystal structure. Experimental data and DFT calculations showed that AgF₂  : Cu and CuF₂  : Ag solid solutions deviate from the classical Vegard's law. Magnetic measurements of Ag_1-x Cu_x F₂ showed that the Néel temperature (T_N ) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag_1-x Cu_x F₂ showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J_2D | in both structures. Electrical conductivity measurements of Ag_0.85 Cu_0.15 F₂ showed a modest increase in specific ionic conductivity (3.71 ⋅ 10^-13 ±2.6 ⋅ 10^-15  S/cm) as compared to pure AgF₂ (1.85 ⋅ 10^-13± 1.2 ⋅ 10^-15  S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample.