Mechanochemical Synthesis\nand Magnetic Properties\nof the Mixed-Valent Binary Silver(I,II)\nFluorides, Ag^I₂Ag^IIF₄ and\nAg^IAg^IIF₃

Abstract

Fluoridoargentates(II) represent a fascinating class\nof silver(II)\ncompounds with structural and magnetic similarities to cuprate superconductors.\nHowever, their synthesis is challenging, leaving their properties\nlargely underexplored and hindering the discovery of new phases. This\nstudy introduces mechanochemistry as a novel approach for the synthesis\nof fluoridoargentates(II), avoiding the use of anhydrous HF or elemental\nfluorine and employing readily available equipment. Notably, ball\nmilling of commercially available precursors successfully produced\nthe\nlong-sought-after first two examples of binary mixed-valent silver(I,II)\nphases, Ag^I₂Ag^IIF₄ (Ag₃F₄) and Ag^IAg^IIF₃ (Ag₂F₃). While the Ag^I₂Ag^IIF₄ phase was obtained at room temperature,\nthe Ag^IAg^IIF₃ phase is metastable\nand required milling under cryogenic conditions. Characterization\nby synchrotron powder X-ray and neutron diffraction revealed that\nAg^I₂Ag^IIF₄ crystallizes\nin the <i>P</i>2₁/<i>c</i> space group\nand is isostructural to β-K₂AgF₄. In this\ncrystal structure, [Ag^IIF₂F_4/2]^2– distorted octahedral units with 4 + 2 coordination,\nextend parallel to <i>a</i>-crystallographic axis giving\na quasi-one-dimensional canted antiferromagnetic character, as shown\nby magnetic susceptibility. The triclinic perovskite Ag^IAg^IIF₃ phase adopts the <i>P</i>1̅\nspace group, is isostructural to AgCuF₃ and also shows\nfeatures of a one-dimensional antiferromagnet. This mechanochemical\napproach, also successfully applied to synthesize β-K₂AgF₄, is expected to expand the field of silver(II) chemistry,\naccelerating the search for silver analogs to cuprate superconductors\nand potentially extending to other cations in unusual oxidation states.