Ba4SbRu3O12: A hexagonal perovskite with isolated magnetic clusters on a geometrically frustrated network

Ba4SbRu3O12 crystallizes with a structure that consists of clusters of three face-sharing Ru-centered octahedra (Ru3O12) connected via Sb-centered octahedra. Rietveld refinements of both x-ray and neutron powder diffraction data confirm the absence of Sb/Ru antisite mixing. Variable temperature neutron diffraction measurements reveal a phase transition on cooling below 100 K from the aristotype structure with R3¯m symmetry to a monoclinic structure with C2/m symmetry. The phase transition is driven by displacements of Ba²⁺ cations and, as such, only subtly perturbs the geometry of the Ru3O12 clusters. The Ru 4d orbitals overlap to form delocalized molecular orbitals that span the trioctahedral cluster. At high temperature (T ≥ 200 K), the cluster adopts an S=3/2 intermediate spin state, but upon cooling to low temperature, susceptibility data suggest a gradual transition to an S=1/2 low spin state. The presence of Sb⁵⁺ ions with a [Kr] 4d¹⁰ electron configuration minimizes interlayer superexchange interactions between clusters, thereby maintaining the frustration of the 2D triangular network. Ba4SbRu3O12 shows no signs of magnetic ordering down to 0.3 K, yet Curie-Weiss fitting of high-temperature susceptibility suggests strong antiferromagnetic interactions (θCW=−378 K). The low-temperature specific heat evolves linearly with temperature, suggestive of a gapless quantum spin liquid. The combination of small magnetic quantum number, minimal chemical disorder, geometric frustration, and lack of long-range magnetic order makes Ba4SbRu3O12 an intriguing quantum spin liquid candidate that merits further study.

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Author Sara Haravifard Physics