Unexpected coexisting solid solutions in the quasi-binaryAg(II)F2/Cu(II)F2 phase diagram.

Abstract

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 ca. 2-fold 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.