Triangular-lattice compounds serve as a prototypical example of geometrically frustrated systems and can host a variety of quantum states, among which quantum spin liquids (QSLs) are particularly intriguing. However, structural disorder is often present in real QSL candidates and it may drive the system into a spin-glass state. Here, we have successfully grown the single crystals of a new triangular-lattice rare-earth compound ${\mathrm{K}}_2\mathrm{CsYb}$(${\mathrm{PO}}_4{)}_2$. X-ray diffractions confirm high phase purity with no detectable structural disorder. The dc magnetic susceptibility measurements reveal dominant magnetic anisotropy, with a stronger response in the $ab$ plane than along the $c$ axis. Notably, no phase transition or spin freezing is observed, as further corroborated by ac susceptibility measurements down to 400 mK. Moreover, specific heat measurements down to 50 mK show no hint of long-range magnetic order either, but only an anomaly that can be well fitted by a two-level Schottky function due to the splitting of the Kramers doublet ground state. Collectively, these findings point to a spin disordered ground state consistent with QSL behaviors. To gain deeper insight into the underlying physics, first-principles calculations based on density functional theory are carried out, suggesting a strong spin-orbit entangled ground state characterized by an effective spin $J_{\mathrm{eff}}$ = 1/2. Our work establishes ${\mathrm{K}}_2\mathrm{CsYb}$(${\mathrm{PO}}_4{)}_2$ as a pristine platform for exploring novel quantum states in a perfect triangular-lattice system.

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• Magnetic anisotropy
• Magnetic susceptibility
• Magnetism
• Quantum spin liquid
• Specific heat
• Spin-orbit coupling
• Spinon
• Antiferromagnets
• Rare-earth magnetic materials
• Specific heat measurements
• X-ray diffraction