We report proximate quantum spin liquid behavior in ${\mathrm{K}}_2{\mathrm{Co}}_2$(${\mathrm{SO}}_4{)}_3$ with the magnetic ${\mathrm{Co}}^{2+}$ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at $T_{\mathrm{t}}\simeq 125\mathrm{K}$ from high-temperature cubic to low-temperature monoclinic phase with the threefold superstructure. Magnetization and heat capacity consistently show the formation of the $J_{\mathrm{eff}}=1/2$ state of ${\mathrm{Co}}^{2+}$ below 50 K. In zero field, ${\mathrm{K}}_2{\mathrm{Co}}_2$(${\mathrm{SO}}_4{)}_3$ shows signatures of static magnetic order formed below $T^{*}\simeq 0.6\mathrm{K}$, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of $\sim 1\mathrm{T}$, and low-temperature heat capacity demonstrates the $T^2$ behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render ${\mathrm{K}}_2{\mathrm{Co}}_2$(${\mathrm{SO}}_4{)}_3$ a promising pseudospin-$\frac{1}{2}$ material for studying quantum magnetism in the double trillium lattice geometry.

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• Frustrated magnetism
• Magnetic interactions
• Magnetic phase transitions
• Quantum spin liquid