${\mathrm{Ba}}_4{\mathrm{SbRu}}_3{\mathrm{O}}_{12}$ crystallizes with a structure that consists of clusters of three face-sharing Ru-centered octahedra $\left({\mathrm{Ru}}_3{\mathrm{O}}_{12}\right)$ 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 $R\overline{3}m$ symmetry to a monoclinic structure with $\mathrm{C}2/m$ symmetry. The phase transition is driven by displacements of $\mathrm{B}{\mathrm{a}}^{2+}$ cations and, as such, only subtly perturbs the geometry of the ${\mathrm{Ru}}_3{\mathrm{O}}_{12}$ 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 $\mathrm{S}{\mathrm{b}}^{5+}$ ions with a [Kr] $4d^{10}$ electron configuration minimizes interlayer superexchange interactions between clusters, thereby maintaining the frustration of the 2D triangular network. ${\mathrm{Ba}}_4{\mathrm{SbRu}}_3{\mathrm{O}}_{12}$ shows no signs of magnetic ordering down to 0.3 K, yet Curie-Weiss fitting of high-temperature susceptibility suggests strong antiferromagnetic interactions $({\theta }_{\mathrm{CW}}=-378 \mathrm{K})$. 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 ${\mathrm{Ba}}_4{\mathrm{SbRu}}_3{\mathrm{O}}_{12}$ an intriguing quantum spin liquid candidate that merits further study.

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• Frustrated magnetism
• Magnetic coupling
• Magnetism
• Molecular magnetism