Magnetic properties and neutron spectroscopy of lanthanoid-{tetrabromocatecholate/18-crown-6} single-molecule magnets

Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO3) (Br(4)Cat)]center dot X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br(4)Cat(2-) = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d(24))(NO3)(Br(4)Cat)]center dot CH3CN-d(3) (1-Ln(D); Ln = La, Tb, Dy) have been synthesised, with 1-Tb-D and the diamagnetic analogue 1-La-D measured by INS. The dynamic magnetic properties of 1-Tb-D and 1-Dy-D have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO3)(Br(4)Cat)]center dot X (2-Ln: X = CH2Cl2; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-Tb-D by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes havealso been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamicmagnetic measurements reveal significant particle-size dependence for the slow magnetic relaxationfor 1-Tb-D, while a previously reported anomalous phonon bottleneck effect in the 1-Dy-D analoguedoes not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln showthat the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects andcrystal packing, which has implications for the future use of Ln-SMMs in devices.

Automated summary

Lanthanoid single-molecule magnets exhibit slow magnetic relaxation at low temperatures, influenced by crystal packing effects. The study on a family of compounds sheds light on the impact of crystal lattice on the slow magnetic relaxation of Tb and Dy compounds, providing insights for the future development of Ln-SMMs in devices.