Structural Phase Transitions and Magnetic Characterization of Ba2GdNbO6 for Low-Temperature Magnetocaloric Refrigeration.

Abstract

Ba2GdNbO6 has previously been reported to adopt either monoclinic, tetragonal, or cubic symmetry at room temperature. Using high-resolution synchrotron X-ray diffraction, neutron diffraction and neutron pair distribution function analysis we find that the compound adopts a tetragonal I4/m double-perovskite structure at room temperature (with a weak, temperature-independent second-order Jahn-Teller distortion in the NbO6 octahedra) and undergoes a phase transition to a monoclinic P21/n symmetry upon cooling to 2.4 K. Only upon heating above room temperature to T ≈ 450 K does Ba2GdNbO6 reversibly transition to a cubic Fm 3̅ m symmetry. Magnetic susceptibility measurements indicate predominant paramagnetic behavior down to 1.8 K, with minimal ferromagnetic short-range correlations (θ = 0.20(5) K) and a small exchange interaction (J 1 = -0.0032(8) K). At 2 K and 9 T, the compound exhibits a maximum magnetic entropy change of -ΔS m = 15.75 J K-1 mol-1 and an adiabatic temperature change of ΔT ad = 21 K, making it a promising candidate for low-temperature magnetocaloric applications. Heat capacity measurements confirm a rigid crystal lattice (T D = 267(3) K) and a corresponding small lattice entropy contribution in the low-temperature regime, highlighting the potential of Ba2GdNbO6 for effective cooling capability in magnetocaloric devices at cryogenic temperatures. This study elucidates the structural and magnetic characteristics of Ba2GdNbO6 and attests to its promise for low-temperature magnetocaloric refrigeration.