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 Ag1−xCuxF2. 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 % (Ag0.7Cu0.3F2), while the monoclinic phase (CuF2 : Ag) can form a nearly stoichiometric Cu : Ag=1 : 1 solid solution (Cu0.56Ag0.44F2), 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 Ag1−xCuxF2 showed that the Néel temperature (TN) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag1−xCuxF2 showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J2D| in both structures. Electrical conductivity measurements of Ag0.85Cu0.15F2 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.