Impact of lattice distortion and vacancies on magnetism and magnetocaloric effect in Ho₃B_xC_4-x compounds for hydrogen liquefaction

Hydrogen is increasingly recognized as a clean and sustainable energy carrier, essential for the transition to a low-carbon economy. Efficient storage and transportation of hydrogen necessitate its liquefaction, which requires extremely low temperatures. Traditional hydrogen liquefaction methods, such as the Claude cycle based on Joule-Thomson expansion, are energy-intensive and complex. This study successfully synthesizes Ho₃B_xC_4-x compounds and systematically investigates their crystal structure, electronic structure, magnetic properties, and magnetocaloric effects (MCEs). Through a combination of theoretical calculations and experimental validation, we explore the impact of precise elemental regulation on the magnetocaloric properties of these compounds. Our findings demonstrate that adjusting the boron and carbon content significantly enhances the MCE and effectively controls the magnetic transition temperature. This improvement is attributed to the synergistic effects of lattice distortion, electronic structure modifications, and lattice vacancies. Additionally, varying the carbon content modifies lattice vacancies, further optimizing the magnetic transition temperature. These results present a novel approach for developing sustainable cooling technologies. Furthermore, the tunable elemental composition allows for targeted adjustments to meet specific cooling requirements, thereby broadening the application scope of these materials.