Designing magnetocaloric materials for hydrogen liquefaction with light rare-earth Laves phases

Magnetocaloric hydrogen liquefaction could be a "game-changer" for liquid hydrogen industry. Although heavy rare-earth-based magnetocaloric materials show strong magnetocaloric effects in the temperature range required by hydrogen liquefaction (77 ~ 20 K), the high resource criticality of the heavy rare-earth elements is a major obstacle for upscaling this emerging liquefaction technology. In contrast, the higher abundances of the light rare-earth elements make their alloys highly appealing for magnetocaloric hydrogen liquefaction. Via a mean-field approach, it is demonstrated that tuning the Curie temperature ($T_C$) of an idealized light rare-earth-based magnetocaloric material towards lower cryogenic temperatures leads to larger maximum magnetic and adiabatic temperature changes ($\Delta S_T$ and $\Delta T_{ad}$). Especially in the vicinity of the condensation point of hydrogen (20 K), $\Delta S_T$ and $\Delta T_{ad}$ of the optimized light rare-earth-based material are predicted to show significantly large values. Following the mean-field approach and taking the chemical and physical similarities of the light rare-earth elements into consideration, a method of designing light rare-earth intermetallic compounds for hydrogen liquefaction is proposed: tunning $T_C$ of a rare-earth alloy to approach 20 K by mixing light rare-earth elements with different de Gennes factors. By mixing Nd and Pr in Laves phase $(Nd,Pr)Al_2$, and Pr and Ce in Laves phase $(Pr,Ce)Al_2$, a fully light rare-earth intermetallic series with large magnetocaloric effects covering the temperature range required by hydrogen liquefaction is developed, demonstrating a competitive maximum effect compared to the heavy rare-earth compound $DyAl_2$.