Synthesisand Magnetism of Silver Chromium SelenideSpinels

Abstract 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.