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

The recent discovery of the spin supersolid candidate Na₂BaCo(PO₄)₂ stimulates numerous research interest on the 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\'{e}el temperatures of NSTP are consistently higher than those of NBTP for T = Co, Ni, and Mn, due to the release of geometrical frustration by monoclinic distortions. Further magnetocaloric measurements show that trigonal NBTP can reach a lower temperature in the quasi-adiabatic demagnetization process and thus shows 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 ultra-low temperature, including a perfect geometrically frustrated lattice and a small effective spin number associated with the magnetic ions.