Electron paramagnetic resonance of terbium in a zirconium fluoride glass

Electron paramagnetic resonance of lanthanides

Many applications of lanthanides exploit their electron spin relaxation properties. Double electron-electron measurements of distances are possible because of the relatively long relaxation times of Gd³⁺. Relaxation enhancement measurements of distance are possible because of the much shorter relaxation times of other lanthanides. Magnetic resonance imaging contrast agents use the long relaxation time of the S-state Gd³⁺ ion, and NMR shift reagents use the fast relaxation of selected other lanthanides. Other than Gd³⁺ and the isoelectronic Eu²⁺ ion, spin relaxation of the lanthanides is so fast that their EPR spectra can be observed only in the liquid helium temperature range. In this chapter the EPR properties of each of the lanthanides is briefly summarized, with an emphasis on electron spin relaxation.

Electron Spin Relaxation of Tb3+ and Tm3+ Ions

Electron spin relaxation times T₁ and T_m of Tb³⁺ and Tm³⁺ in 1:1 water:ethanol and of Tb³⁺ doped (2%) in crystalline La₂(oxalate)₃ decahydrate were measured between about 4.2 and 10 K. Both cations are non-Kramers ions and have J = 6 ground states. Echo-detected spectra are compared with CW spectra and with field-stepped direct-detected EPR spectra. Due to the strong temperature dependence of T₁, measurements were not made above 10 K. Between about 4.2 and 6 K T₁ is strongly concentration dependent between 1 and ~50 mM. T₁ values at 4.2 K are in the μs range which is orders of magnitude faster than for 3d transition metals. Phase memory times, T_m, are less than 500 ns, which is short relative to values observed for 3d transition metals and organic radicals at 4 K. T_m is longer in the oxalate lattice which is attributed to the lower proton concentration in oxalate than in the organic solvent, which decreases nuclear spin diffusion. The rigidity of the crystalline lattice also may contribute to longer T_m.