Synthesis and Magnetism of Silver Chromium Selenide Spinels

Chalcogenide spinels exhibit electronic and magnetic properties that are tunable through lattice occupancy and composition. Here, we report the first synthesis of spinel-phase Ag2Cr2Se4 and AgCr2Se4 and demonstrate an oxidation-induced ferro-to-ferrimagnetic transition accompanied by a massive increase in TC of >250 K associated with changes in Ag+ site occupancy. The Ag spinels are synthesized through air-free cation exchange of CuCr2Se4 nanocrystals driven by excess AgNO3 to yield ferromagnetic Ag2Cr2Se4 (a = 10.838(4) Å, TC = 152 K, 2 K saturation = 3.1 μB/Cr) with Ag+ occupying both Td 8a and Oh 16c sites. Upon air exposure, Ag2Cr2Se4 converts to ferrimagnetic AgCr2Se4 (a = 10.6085(19) Å, TC > 400 K, 2 K saturation = 2.2 μB/Cr) through a loss of Oh-Ag+ alongside the formation of Ag2Se. We attribute the ferro-to-ferrimagnetic transition to the generation of holes in Ag-Se bands, which couple antiferromagnetically to Cr3+. This Ag-Cr-Se spinel system demonstrates the largest change in TC reported and provides an excellent framework for further exploration of magnetic coupling as a function of oxidation and structure.