Mechanochemical Synthesis and Magnetic Properties of the Mixed-Valent Binary Silver(I,II) Fluorides, AgI2AgIIF4 and AgIAgIIF3

Abstract

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^I₂Ag^IIF₄ (Ag₃F₄) and Ag^IAg^IIF₃ (Ag₂F₃). While the Ag^I₂Ag^IIF₄ phase was obtained at room temperature, the Ag^IAg^IIF₃ phase is metastable and required milling under cryogenic conditions. Characterization by synchrotron powder X-ray and neutron diffraction revealed that Ag^I₂Ag^IIF₄ crystallizes in the P2₁/c space group and is isostructural to β-K₂AgF₄. In this crystal structure, [Ag^IIF₂F_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^IAg^IIF₃ phase adopts the P1̅ space group, is isostructural to AgCuF₃ and also shows features of a one-dimensional antiferromagnet. This mechanochemical approach, also successfully applied to synthesize β-K₂AgF₄, 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.