Structural evolution and skyrmionic phase diagram of the lacunar spinel GaMo4Se8

The lacunar spinel family, which contains multiferroic skyrmion hosts GaV4S${}_{8}$ and GaV${}_{4}$Se${}_{8}$, is characterized by unique metal--metal bonding leading to strong coupling between crystal structure and magnetism. Synchrotron diffraction analysis of the low temperature crystal structure of GaMo${}_{4}$Se${}_{8}$, a less studied member of the family, reveals coexistence of the reported rhombohedral crystal structure and a new metastable orthorhombic structure. Computational and experimental characterization of the magnetic phase diagram of GaMo${}_{4}$Se${}_{8}$ indicates that slight crystal structure differences lead to distinct magnetic properties; the reported $R3m$ phase hosts skyrmions similar to GaV${}_{4}$Se${}_{8}$, while the novel $I\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}m2$ phase is a strongly uniaxial ferromagnet.