Structural and magnetic characterizations of antiferromagnetic RE3PO7 (RE = Gd, Tb, Dy, Ho) phosphates featuring large conventional and inverse cryogenic magnetocaloric effects

Abstract

The magnetocaloric (MC) properties in various type rare earth (RE)-based magnetic materials have been intensively characterized recently, aimed to develop high-performing materials for cryogenic magnetic cooling application and deepen our understanding of their intrinsic magneto properties. In this work, we synthesized the RE₃PO₇ (RE = Gd, Tb, Dy, Ho) phosphates using traditional high-temperature solid-state method and characterized their structure, elemental valence states, magnetic properties, and MC performances. All of these RE₃PO₇ phosphates possess monoclinic symmetry with the C2/m (No. 12) space group and order antiferromagnetically around the Neel temperatures (T_N) of 4.1, 3.9, 8.3 and 2.5 K for Gd₃PO₇, Tb₃PO₇, Dy₃PO₇ and Ho₃PO₇ phosphates, respectively. The consistent elements in these RE₃PO₇ phosphates are distributed uniformly and present as RE³⁺, P⁵⁺, and O²⁻ chemical states, respectively. Meanwhile, they exhibit large conventional and inverse cryogenic MC effects, attributed to their antiferromagnetic ground state and unique field-induced first-order magnetic phase transition below their own T_N. Moreover, under 0–7 T magnetic field variation, we identified their conventional MC performances using magnetic entropy change/temperature-averaged entropy change (5 K) and relative cooling power, which are 18.81/17.96 J/(kg·K) and 289.93 J/kg for Gd₃PO₇, 11.51/11.11 J/(kg·K) and 226.00 J/kg for Tb₃PO₇, 21.70/20.73 J/(kg·K) and 471.67 J/kg for Dy₃PO₇, and 18.88/18.42 J/(kg·K) and 434.56 J/kg for Ho₃PO₇, respectively. These conventional MC parameters are comparable to those of some reported cryogenic RE-based MC materials, indicating that RE₃PO₇ phosphates may have limited potential for low-temperature magnetic cooling applications under certain conditions.