Structural, magnetic, and magnetocaloric properties of triangular-lattice transition-metal phosphates

The recent discovery of the spin supersolid candidate Na₂BaCo(PO₄)₂ has stimulated a great deal of research on triangular-lattice transition-metal phosphates. Here we report a comprehensive study on the structural, magnetic, and magnetocaloric properties of polycrystalline Na₂AT(PO₄)₂ (A = Ba,Sr; T = Co,Ni,Mn). X-ray and neutron diffraction measurements confirm that Na₂BaT(PO₄)₂ (NBTP) crystallizes in a trigonal structure, while Na₂SrT(PO₄)₂ (NSTP) forms a monoclinic structure with a slight distortion of the triangular network of T²⁺ ions. The dc magnetization data show that all six compounds order antiferromagnetically below 2 K, and the N\'eel temperatures of NSTP are consistently higher than those of NBTP for T = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinical distortions. Furthermore, magnetocaloric measurements show that trigonal NBTP can reach a lower temperature in the quasiadiabatic demagnetization process and thus it demonstrates a better performance in the magnetic refrigeration, compared with monoclinic NSTP. Our findings highlight the outstanding magnetocaloric performances of the trigonal transition-metal phosphates and disclose two necessary ingredients for a superior magnetic coolant that can reach an ultralow temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions.