Adiabatic demagnetization refrigeration (ADR) based on frustrated rare-earth magnets provides a promising route toward helium-free sub-Kelvin cooling, while balancing magnetic entropy and interaction scales is essential for optimizing the ADR functionalities. Here we investigate the low-temperature magnetic and ADR properties of the alkali rare-earth double phosphates ${\mathrm{K}}_{3}Ln{\left({\mathrm{PO}}_4\right)}_2 (Ln$ = Yb, Gd), which crystallize in a monoclinic $P{2}_1/m$ structure containing distorted isosceles triangular lattices of rare-earth ions. Magnetization and specific heat measurements confirm the absence of long-range magnetic ordering down to 0.1 K in both compounds. Direct quasi-ADR measurements demonstrate strongly contrasting refrigeration behaviors: ${\mathrm{K}}_3\mathrm{Yb}{\left({\mathrm{PO}}_4\right)}_2$ reaches a minimum temperature of 29.8 mK from an initial state of 2 K and 6 T, whereas ${\mathrm{K}}_3\mathrm{Gd}{\left({\mathrm{PO}}_4\right)}_2$ exhibits extended hold times below 150 mK associated with its larger magnetic entropy reservoir. Crystal-electric-field calculations confirm a $J_{\mathrm{eff}}=1/2$ ground-state doublet in ${\mathrm{K}}_3\mathrm{Yb}{\left({\mathrm{PO}}_4\right)}_2$, while thermodynamic analysis based on magnetic entropy and interaction scales accounts for the distinct ADR performance of the two compounds. These results identify ${\mathrm{K}}_{3}Ln{\left({\mathrm{PO}}_4\right)}_2$ as a chemically robust platform for rare-earth-tunable solid-state ADR.

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• Frustrated magnetism
• Magnetocaloric effect
• Magnetic refrigeration
• Magnetization measurements
• Neutron diffraction
• Specific heat measurements