Magnetocaloric effect in Gd₈P₂O₁₇ and Gd₃PO₇

The gadolinium-rich phosphates, Gd₈P₂O₁₇ and Gd₃PO₇, were successfully synthesized by the high-temperature solid-phase reaction method. Gd₈P₂O₁₇ and Gd₃PO₇ crystallize in the monoclinic system. Their magnetic and magnetocaloric properties were systematically investigated by measuring magnetic susceptibility (χ) and magnetization (M). The phase transition temperatures for Gd₃PO₇ and Gd₈P₂O₁₇ were observed to be below 2 K. The max magnetic entropy change (-ΔS_M^max) of Gd₈P₂O₁₇ reaches 21.7 J·kg⁻¹ K⁻¹ at 2 K and 5 T, which is comparable to that of the commercial Gd₃Ga₅O₁₂ under the same conditions. Gd₃PO₇ exhibits a -ΔS_M^max of 10.3 J·kg⁻¹ K⁻¹ at 6 K and 5 T, similar to the value of LiDyP₄O₁₂ under the same conditions. Moreover, the relative cooling capacity (RCP) and refrigeration capacity (RC) values reach 165.05 and 123.94 J/kg for Gd₃PO₇ and 152.14 and 108.99 J/kg for Gd₈P₂O₁₇ under the magnetic field changes of 5 T. These results indicate that Gd₈P₂O₁₇ and Gd₃PO₇ may be promising candidates for ultra-low temperature magnetic refrigeration applications.