Mechanochemical synthesis and magnetic properties of the mixed-valent binary silver(I,II) fluorides, Ag$^I{}_2$Ag$^{II}$F$_4$ and Ag$^I$Ag$^{II}$F$_3$

Fluoridoargentates(II) represent a fascinating class of silver(II) compounds with structural and magnetic similarities to cuprate superconductors. However, their synthesis is challenging, leaving their properties largely underexplored and hindering the discovery of new phases. This study introduces mechanochemistry as a novel approach for the synthesis of fluoridoargentates(II), avoiding the use of anhydrous HF or elemental fluorine and employing readily available equipment. Notably, ball milling of commercially available precursors successfully produced the long-sought-after first two examples of binary mixed-valent silver(I,II) phases, Ag$^I$$_2$Ag$^{II}$F$_4$ (Ag$_3$F$_4$) and Ag$^I$Ag$^{II}$F$_3$ (Ag$_2$F$_3$). While the Ag$^I$$_2$Ag$^{II}$F$_4$ phase was obtained at room temperature, the Ag$^I$Ag$^{II}$F$_3$ phase is metastable and required milling under cryogenic conditions. Characterization by synchrotron powder X-ray and neutron diffraction revealed that Ag$^I$$_2$Ag$^{II}$F$_4$ crystallizes in the P2$_1$/c space group and is isostructural to β-K$_2$AgF$_4$. In this crystal structure, [Ag$^{II}$F$_2$F$_{4/2}$]$^{2−}$ distorted octahedral units with 4 + 2 coordination, extend parallel to a-crystallographic axis giving a quasi-one-dimensional canted antiferromagnetic character, as shown by magnetic susceptibility. The triclinic perovskite Ag$^I$Ag$^{II}$F$_3$ phase adopts the P$overline{1}$ space group, is isostructural to AgCuF$_3$ and also shows features of a one-dimensional antiferromagnet. This mechanochemical approach, also successfully applied to synthesize β-K$_2$AgF$_4$, is expected to expand the field of silver(II) chemistry, accelerating the search for silver analogs to cuprate superconductors and potentially extending to other cations in unusual oxidation states.