The star lattice, which can be visualized as a honeycomb network with each vertex replaced by a triangle, provides a rare platform for realizing exotic quantum states such as quantum spin liquids and disorder-driven random-singlet (RS) states. Herein, we investigate the ground-state properties of the three-dimensional (3D) stuffed hyper-star lattice ${\mathrm{Li}}_2{\mathrm{Cu}}_2$(${\mathrm{MoO}}_4{)}_3$, which exhibits a crossover from short-range spin correlations to a disorder-driven RS-like state below $T^{*}\sim 15.8\mathrm{K}$. Thermodynamic and microscopic measurements capture this crossover through a change in the power-law behavior of various observables, from $\sim T^{0.25}$ for $T>T^{*}$ to $\sim T^{-0.50}$ for $T<T^{*}$. Upon further cooling, a quasifrozen state emerges near $T_f=0.32\mathrm{K}$, likely associated with weakly coupled spin chains within the hyper-star spin network. Our results underscore the crucial role of orphan spins and weak residual interactions in stabilizing a disorder-driven quantum-disordered state in three dimensions.

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• Magnetic susceptibility
• Spin dynamics
• Antiferromagnets
• Electron spin resonance
• Muon spin resonance