Slow Magnetic Relaxation in Lanthanoid Crown Ether Complexes: Interplay of Raman and Anomalous Phonon Bottleneck Processes

Abstract The combination of lanthanoid nitrates with 18‐crown‐6 (18‐c‐6) and tetrahalocatecholate (X 4 Cat 2− , X=Cl, Br) ligands has afforded two compound series [Ln(18‐c‐6)(X 4 Cat)(NO 3 )] ⋅ MeCN (X=Cl, 1‐Ln ; X=Br, 2‐Ln ; Ln=La, Ce, Nd, Gd, Tb, Dy). The 18‐c‐6 ligands occupy equatorial positions of a distorted sphenocorona geometry, whereas the charged ligands occupy the axial positions. The analogues of both series with Ln=Ce, Nd, Tb and Dy exhibit out‐of‐phase ac magnetic susceptibility signals in the presence of an applied magnetic field, indicative of slow magnetization relaxation. When diluted into a diamagnetic La host to reduce dipolar interactions, the Dy analogue exhibits slow relaxation up to 20 K in the absence of an applied dc field. Concerted magnetic measurements, EPR spectroscopy, and ab initio calculations have allowed elucidation of the mechanisms responsible for slow magnetic relaxation. A consistent approach has been applied to quantitatively model the relaxation data for different lanthanoid analogues, suggesting that the spin dynamics are governed by Raman processes at higher temperatures, transitioning to a dominant phonon bottleneck process as the temperature is decreased, with an observed T −6 rather than the usual T −2 dependence ( T is temperature). This anomalous thermal dependence of the phonon bottleneck relaxation is consistent with anharmonic effects in the lattice dynamics, which was predicted by Van Vleck more than 70 years ago.