Structure and cryogenic magnetocaloric effects of NaREO2 (RE = Er, Ho) compounds

Rare-earth (RE) based oxides have presented great application prospects in low-temperature magnetic refrigeration technology. Thus far, studies on magnetic and especially magnetocaloric properties of alkali-metal RE oxides are limited, largely due to the difficulty in synthesizing these oxides. In this work, NaREO2 (RE = Er, Ho) compounds with both the rhombohedral (R 3¯ m space group) and cubic (Fm 3¯ m space group) structures were prepared by a facile ball-milling-aided sintering approach. The cubic structural compounds exhibit a spherical microstructure composed of vertically-grown irregular hexahedra, which sharply contrast with a loose lamellar microstructure of the rhombohedral structural compound. The as-prepared NaHoO2 or NaErO2 compound exhibits an antiferromagnetic ordering of RE3+ moments at a temperature of 2.4 K or lower than 2 K respectively. An excellent comprehensive magnetic refrigeration performance at cryogenic temperatures in a low magnetic field change of 0–20 kOe was obtained in these compounds, including a complete reversibility of the magnetocaloric effect, a maximal magnetic-entropy change ΔSM at 2 K of 12 J/kg·K for the rhombohedral NaErO2 and a relative cooling power (RCP) of 79.7 J/kg for the cubic NaHoO2.