Cs₂Fe₂(MoO₄)₃─A Strongly Frustrated Magnet with Orbital Degrees of Freedom and Magnetocaloric Properties

We report an in-depth study of the thermodynamic and magnetocaloric properties of a strongly frustrated magnet, Cs₂Fe₂(MoO₄)₃. The underlying structure belongs to the double trillium lattice, which consists of two Fe^II (S = 2) sites with easy-axis and easy-plane single-ion anisotropy. Detailed ⁵⁷Fe Mössbauer spectroscopic investigations along with ligand-field calculations support the existence of disparate ground states. The antiferromagnetic ordered structure is presented by the propagation vector k = (0,0,0) with noncollinear magnetic moments of 2.97 μ_B (Fe1) and 0.17 μ_B (Fe2), respectively. Strong and disordered magnetic correlations exist in the temperature regime between T_N ≈ 1.0 K and |θ_CW| ≈ 22 K. The large degeneracy of the ground state is investigated in terms of its magnetocaloric response. Magnetization and specific heat measurements indicate a significant magnetocaloric cooling efficiency, making this rare-earth-free compound a promising candidate for cryogenic magnetic refrigeration applications, with refrigeration capacity of 79 J kg^-1 for Δ(μ₀H) = 8 T.