Supercritical Hydrothermal Synthesis, Magnetism, and Rotating Magnetocaloric Effect of Fluorocarbonates K4RE2(CO3)3F4 (RE = Ce, Pr, Nd, Sm, Eu, Tb, and Dy) with Rare-earth Dimers RE2O12F4

Strong magnetic anisotropy in low-dimensional magnets leads to a significant rotating magnetocaloric effect, presenting new opportunities for the exploration of more efficient and streamlined refrigeration systems. In this study, we have successfully synthesized a series of centimeter-scale rare-earth-containing fluorocarbonates, K 4 RE 2 (CO 3 ) 3 F 4 ( RE = Ce, Pr, Nd, Sm, Eu, Tb, and Dy) through a supercritical hydrothermal method, featuring unique RE 2 O 12 F 4 magnetic dimers. The above materials' fundamental magnetism has been systematically investigated, and no long-range magnetic transitions were captured above 1.8   K. Based on the low-temperature specific heat results, the long-range magnetic ordering was found around 0.84   K and 0.97   K for K 4 Tb 2 (CO 3 ) 3 F 4 and K 4 Dy 2 (CO 3 ) 3 F 4 , respectively. Furthermore, the anisotropic magnetism of K 4 Tb 2 (CO 3 ) 3 F 4 and K 4 Dy 2 (CO 3 ) 3 F 4 reveals significant magneto-crystalline anisotropy, with the magnetic field ( B ) aligned along the RE 2 O 12 F 4 magnetic dimer, which acts as the easy magnetic axis. The maximum rotating field entropy change, − Δ S R max for K 4 Tb 2 (CO 3 ) 3 F 4 reaches approximately 9   J   kg -1 K -1 around 17   K, while for K 4 Dy 2 (CO 3 ) 3 F 4 , it is approximately 5.5   J   kg -1 K -1 near 11   K, both under a 7   T field applied along the c -axis. These findings highlight the potential of these compounds for magnetic refrigeration applications through a rotating crystallographic direction.