The combination of lanthanoid nitrates with 18‐crown‐6 (18‐c‐6) and tetrahalocatecholate (X4Cat2−, X=Cl, Br) ligands has afforded two compound series [Ln(18‐c‐6)(X4Cat)(NO3)]⋅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.