Selective metathesis synthesis of MgCr₂S₄ by control of thermodynamic driving forces

MgCr₂S₄ thiospinel is predicted to be a compelling Mg-cathode material, but its preparation via traditional solid-state synthesis methods has proven challenging. Wustrow et al. [Inorg. Chem., 2018, 57, 14] found that the formation of MgCr₂S₄ from MgS + Cr₂S₃ binaries requires weeks of annealing at 800 °C with numerous intermediate regrinds. The slow reaction kinetics of MgS + Cr₂S₃ → MgCr₂S₄ can be attributed to a miniscule thermodynamic driving force of ΔH = -2 kJ mol^-1. Here, we demonstrate that the double ion-exchange metathesis reaction, MgCl₂ + 2NaCrS₂ → MgCr₂S₄ + 2NaCl, has a reaction enthalpy of ΔH = -47 kJ mol^-1, which is thermodynamically driven by the large exothermicity of NaCl formation. Using this metathesis reaction, we successfully synthesized MgCr₂S₄ nanoparticles (<200 nm) from MgCl₂ and NaCrS₂ precursors in a KCl flux at 500 °C in only 30 minutes. NaCl and other metathesis byproducts are then easily washed away by water. We rationalize the selectivity of MgCr₂S₄ in the metathesis reaction from the topology of the DFT-calculated pseudo-ternary MgCl₂-CrCl₃-Na₂S phase diagram. Our work helps to establish metathesis reactions as a powerful alternative synthesis route to inorganic materials that have otherwise small reaction energies from conventional precursors.