Structural Phase Transitions and Magnetic Characterization of Ba ₂ GdNbO ₆ for Low-Temperature Magnetocaloric Refrigeration

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 <i>I</i>4/<i>m</i> 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 <i>P</i>2₁/<i>n</i> symmetry upon cooling to 2.4 K. Only upon heating above room temperature to <i>T</i> ≈ 450 K does Ba₂GdNbO₆ reversibly transition to a cubic <i>Fm</i> 3̅ <i>m</i> 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 (<i>J</i> ₁ = -0.0032(8) K). At 2 K and 9 T, the compound exhibits a maximum magnetic entropy change of -Δ<i>S</i> _m = 15.75 J K^-1 mol^-1 and an adiabatic temperature change of Δ<i>T</i> _ad = 21 K, making it a promising candidate for low-temperature magnetocaloric applications. Heat capacity measurements confirm a rigid crystal lattice (<i>T</i> _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.