Rare-earth-based frustrated magnets have attracted significant attention as excellent candidates for magnetic refrigeration operating at subkelvin temperatures. Herein we investigate the magnetic ground states and low-temperature magnetocaloric effect (MCE) of frustrated Shastry-Sutherland ${\mathrm{Gd}}_2{\mathrm{Be}}_2\mathrm{Ge}{\mathrm{O}}_7$ (GBGO) and Lu-diluted ${\left({\mathrm{Gd}}_{0.36}{\mathrm{Lu}}_{0.64}\right)}_2{\mathrm{Be}}_2\mathrm{Ge}{\mathrm{O}}_7$ (GLBGO) compounds, both of which possess spin $S=7/2$ moment. Compared to the isostructural $S_{\mathrm{eff}}=1/2$ analog ${\mathrm{Yb}}_2{\mathrm{Be}}_2\mathrm{Ge}{\mathrm{O}}_7$ (YBGO), GBGO exhibits a volumetric entropy density ($S_V$) approximately three times larger ($429\mathrm{mJ}{\mathrm{K}}^{-1}\mathrm{c}{\mathrm{m}}^{-3}$ versus $149\mathrm{mJ}{\mathrm{K}}^{-1}\mathrm{c}{\mathrm{m}}^{-3}$), while GLBGO shows a comparable $S_V$ ($158\mathrm{mJ}{\mathrm{K}}^{-1}\mathrm{c}{\mathrm{m}}^{-3}$ versus $149\mathrm{mJ}{\mathrm{K}}^{-1}\mathrm{c}{\mathrm{m}}^{-3}$). Magnetic and specific heat measurements reveal that GBGO forms an antiferromagnetic ordered state at $T_N=0.85$ K, whereas GLBGO shows spin freezing behavior below $T_f\sim 0.22$ K. Analysis of magnetic entropy $S_M$ ($T, B$) reveals that GBGO yields a substantially larger volumetric entropy change (–$\mathrm{\Delta }{S}_M$) in temperature windows of 0.6–2 K though it is outperformed by YBGO below $\sim 0.6$ K. However, YBGO can realize much lower minimum temperature ($T_{\min }$) than both GBGO and GLBGO. Importantly, a critical entropy value $S_V\sim 185\mathrm{mJ}{\mathrm{K}}^{-1}\mathrm{c}{\mathrm{m}}^{-3}$ is identified through comparison with other Gd/Yb-based subkelvin magnetic refrigerants. Above this threshold, Gd-based magnets have an advantage in achieving larger MCE at temperatures near or above their $T_N$; below it, Yb-based ones possess better subkelvin cooling performance featuring larger MCE and lower $T_{\min }$.

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• Antiferromagnetism
• Frustrated magnetism
• Magnetic phase transitions
• Magnetic susceptibility
• Magnetism
• Magnetocaloric effect