Yb-based triangular lattice materials have attracted significant attention due to their potential for hosting exotic magnetic quantum states. In this paper, we report the structural, magnetic, and thermodynamic properties of two Yb triangular lattice compounds with vacancy disorder at nonmagnetic Cu sites: ${\mathrm{CuYbSe}}_2$ and ${\mathrm{Cu}}_{2.73}{\mathrm{YbP}}_2$. Single-crystal x-ray diffraction determines notable Cu-site vacancies in both materials. Unlike a single Cu site with 0.5 occupancy in ${\mathrm{CuYbSe}}_2, {\mathrm{Cu}}_{2.73}{\mathrm{YbP}}_2$ only presents 27% vacancies on one of three Cu sites. Magnetic susceptibility measurements indicate dominant antiferromagnetic interactions between ${\mathrm{Yb}}^{3+}$ ions. The effective magnetic moment is consistent with an effective spin-1/2 state of ${\mathrm{Yb}}^{3+}$. ${\mathrm{CuYbSe}}_2$ exhibits strong magnetic anisotropy between in-plane and out-of-plane directions with the estimated exchange interactions $J_a/k_{\mathrm{B}}=6.48\mathrm{K}$ and $J_c/k_{\mathrm{B}}=2.55\mathrm{K}$. No magnetic ordering is observed down to 0.4 K, supporting its quantum disordered ground state and candidacy as a quantum spin liquid. In contrast, ${\mathrm{Cu}}_{2.73}{\mathrm{YbP}}_2$ shows clear evidence of spin freezing at 4.5 K in both magnetization and heat capacity data, indicating a spin glass state. The field-induced magnetic orders are found in ${\mathrm{CuYbSe}}_2$. These results highlight the critical role of structural vacancies in perturbing Yb-based triangular lattices. We demonstrate that while the intrinsic Yb triangular lattices are comparable, the presence of Cu vacancies can fundamentally alter the magnetic ground state—tuning the system between quantum disordered and spin glass behavior.

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• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Magnetism
• Specific heat