A Geometrically Frustrated LiCaGd ₅ (BO ₃ ) ₆ Oxide with a Record Low‐Field Magnetocaloric Response at Ultra‐Low Temperature

ABSTRACT The development of magnetic refrigeration technology that based on the principle of magnetocaloric effect (MCE) has been constrained by the scarcity of high–performing magnetic refrigerants with large magnetic entropy changes under low applied field changes. Here, we report a geometrically frustrated LiCaGd 5 (BO 3 ) 6 oxide with a record low–field magnetocaloric response, which is related to the disruption of the long‐range order of magnetic Gd 3+ sublattice and weakens the nearest–neighbor magnetic dipole interactions by the disorder substitution of non‐magnetic Ca 2 + ions. The LiCaGd 5 (BO 3 ) 6 oxide crystallizes in the hexagonal structure with the space group P 6 5 22, in which magnetic Gd 3+ ions (partially and disorderly substituted by non–magnetic Ca 2 + ions) form a typical triangular geometrically frustrated arrangement within the ab plane. The LiCaGd 5 (BO 3 ) 6 oxide orders antiferromagnetically around 0.78 K and reveals a field–induced first–order magnetic phase transition character. Importantly, the LiCaGd 5 (BO 3 ) 6 oxide shows a record of maximum isothermal magnetic entropy change of 40.8 J·kg – 1 ·K − 1 under a low applied field change of 0−2 T among the Gd–based oxides, positioning the LiCaGd 5 (BO 3 ) 6 oxide as exceptionally promising for ultra–low temperature refrigeration applications. This work also provides a new avenue for the design of high–performing magnetic refrigerants by tuning geometrical frustration and magnetic exchange interactions.