The recently discovered lithium-rich antiperovskites $\left({\mathrm{Li}}_2\mathrm{Fe}\right)\mathrm{SeO}$ and $\left({\mathrm{Li}}_2\mathrm{Fe}\right)\mathrm{SO}$ host lithium and iron ions on the same atomic position which octahedrally coordinates to central oxygens. In a cubic antiperovskite these sites form kagome planes stacked along the $\langle 111\rangle$ directions which triangular motifs induce high geometric frustration in the diluted magnetic sublattice for antiferromagnetic interactions. Despite their compelling properties as high-capacity Li-ion battery cathode materials, very little is known about the electronic and magnetic properties of lithium-rich antiperovskites. We report static magnetization, Mössbauer, and NMR studies on both compounds. Our data reveal a Pauli paramagnetic-like behavior, a long-range antiferromagnetically ordered ground state below $\approx 50$ K, and a regime of short-range magnetic correlations up to 100 K. Our results are consistent with a random Li-Fe distribution on the shared lattice position. In addition, Li-hopping is observed above 200 K with an activation energy of $E_{\mathrm{a}}=0.47$ eV. Overall, our data elucidate static magnetism in a disordered magnetically frustrated and presumably semimetallic system with thermally induced ion diffusion dynamics.

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• Diffusion
• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Order parameters
• Spin dynamics
• Kagome lattice
• Magnetization measurements
• Mössbauer spectroscopy
• Nuclear magnetic resonance