Influence of K+ introduction on ultra-low-temperature magnetocaloric effects in rare-earth fluorides

The development of ultra-low-temperature magnetocaloric materials (MCMs) is crucial for advanced cryogenic refrigeration technologies. Rare-earth fluorides exhibit promising magnetocaloric properties, but the high magnetic ordering temperatures ( T 0 ) limit their performance below 2 K. Single-phase KLnF 4 (Ln = Tb, Dy, Ho, Er, Tm) compounds with a trigonal structure were successfully synthesized by introducing the diamagnetic K⁺ ions into LnF 3 via solid-state sintering. Structural analysis confirms that the K⁺ incorporation increases the distance between Ln 3+ ions, suppressing dipolar interactions and reducing T 0 to below 2 K. Notably, KHoF 4 exhibits a 16.5 % higher magnetic entropy change (-∆ S M ) than HoF 3 , attributed to the lower magnetic anisotropy of the trigonal KHoF 4 with higher symmetry. The effects of these two aspects overcome the negative impact caused by the decrease of the magnetic ion concentration. These findings demonstrate that K⁺ incorporation is a dual-functional strategy for simultaneously modulating transition temperatures and magnetocaloric performance of rare-earth fluorides. This work provides a scalable approach to designing ultra-low-temperature MCMs with various refrigeration windows.