Total scattering measurements of a quantum spin-orbital liquid pyrochlore candidate

The pyrochlores Hg2M2F6S (M = Cu, Co, Ni, Mg, Zn) are a family of highly underexplored materials with frustrated pyrochlore lattices of corner-sharing tetrahedra. The Cu2+ variant, in particular, i.e. Hg2Cu2F6S, is especially interesting as previous reports have established a lack of magnetic order down to T = 2.4 K, as well as a lack of a Jahn-Teller distortion of the Cu2+ cations (d9). The latter is an especially unexpected result as Cu2+ cations in an octahedral coordination are amongst the most unstable cations toward a Jahn-Teller instability, and a lack of a distortion at all temperatures implies the presence of active orbital degrees of freedom. In conjunction with the lack of magnetic order, implying active spin degrees of freedom, the properties of Hg2Cu2F6S are consistent with those of a quantum spin-orbital liquid, where both spin and orbital degrees of freedom remain fluctuating albeit strongly interacting down to the lowest measurable temperatures. We seek to use GEM to perform total scattering measurements down to T = 1.5 K using a cryostat in order to unambiguously determine whether a Jahn-Teller distortion of the Cu2+ octahedra exists in Hg2Cu2F6S at both the average and local crystallographic scales.