Geometric frustration and competing interactions in selected chalcogenide spinels

Quantum materials often exhibit exotic ground states that are of fundamental interest with potentialapplication in next generation technologies. Particularly important emerging areas of interest areskyrmion lattices, quantum spin liquids, and spin spirals. These quantum phases of matter originatewith frustrated interactions, the interplay of spin and lattice degree of freedom, and quantumfluctuations. These properties are readily found in AB2X4spinel materials which are the focus ofthis presentation. In chalcogenide spinel compounds, the A-sites form a diamond lattice structurethat can exhibit frustration due to the competing nearest neighbor and next neasrest neighborexchange interactions. Moreover, the B sites form a pyrochlore lattice which is a prototypefor geometrical frustration. The interactions withing the pyrochlore sublattice can be tuned byintroducing an alternating ordered vacancy on the A-site or by the ordered substitution of twoinequivalent ions on the A-site. For example, in LiGaCr4S8the different size of the Li+andGa3+ions results in the formation of an alternating array of larger and smaller Cr4tetrahedra or abreathing pyrochlore lattice.To demonstrate the approach that will be used for the proposed studies of chalcogenide spinels,I will present the results of neutron diffraction, X-ray diffraction, magnetic susceptibility, and heatcapacity measurements of LiGaCr4S8. The neutron diffraction and synchrotron X-ray diffractiondata reveal negative thermal expansion (NTE) below 111(4) K. The magnetic susceptibilitydeviates from Curie-Weiss behavior with the onset of NTE. At low temperature a broad peak in themagnetic susceptibility at 10.3(3) K is accompanied by the return of normal thermal expansion.These results indicate strong magnetoelastic coupling in LiGaCr4S8.