A Spin Frustrated Hourglass {Gd9} Molecular Nanomagnet with Unusual Magnetocaloric Properties

ABSTRACT: We report a highly symmetric {Gd9} molecular nanocage with the formula [Gd9(BA)16(OH)10]Cl·3(C2H5OH)· 4(H2O), which crystallizes in the cubic space group Pn3̅n. The structure features two crystallographically distinct Gd3+ ions, forming highly regular triangular Gd3+ arrangements leading to a geometrically frustrated magnetic network. Magnetization meas- urements at 2 K reveal a broad plateau between 1.5 and 4 T, while zero-field heat capacity shows a Schottky anomaly centered at 0.6 K�indicative of low-lying excited states and competing magnetic interactions. The magnetocaloric effect, evaluated through both direct and indirect methods, exhibits a re-entrant profile in the isentropic curves, pointing to a nontrivial evolution of magnetic entropy under applied fields. To probe the origin of this behavior, we employed the finite-temperature Lanczos method antiferromagnetic exchange between Gd3+ ions, combined with the frustration inherent to the {Gd9} geometry, leads to a degenerate ground state. An external field lifts this degeneracy, producing a regime with a sharply reduced density of states between 1.5 and 4 K, which underlies the unconventional magnetocaloric response. The {Gd9} cage thus represents a rare example of a spin-frustrated arrangement arising from competing antiferromagnetic interactions between the Gd3+. These findings demonstrate how frustrated topologies and tunable low-energy excitations can be exploited to modulate the magnetothermal properties, with potential implications for cryogenic magnetic cooling technologies.