Proton spin–lattice relaxation in cerous magnesium nitrate hydrate

The proton spin−lattice relaxation in powdered Ce2Mg3(NO3)12⋅24H2O has been measured in the temperature range 65−335 K. In contrast to other paramagnetic salts it has been found that the nuclear relaxation due to nuclear dipole−dipole interactions is comparable in magnitude with the relaxation due to the electronic moments on the paramagnetic ions in this temperature range. Below 160 K the relaxation times are independent of the resonance frequency and have a temperature dependence given by τ1 = 0.375 exp(−182/T) sec. This is attributed to relaxation via the Ce3+ ions and hence makes it possible to calculate a value for their relaxation times. It is also concluded that the second excited doublet of the Ce3+ ions is at an energy of 200±18 K. At higher temperatures two relaxation time minima are observed. At 14 MHz the minimum values are 71 and 69 msec at 244 and 303 K, respectively. The weak temperature dependence of the NMR second moment indicates that 180° flips of the water molecules provide the relaxation mechanism in this temperature range. Calculated values of τ1, using the published crystallographic data, and assuming that there are two types of water molecule, are in reasonable agreement with experiment. The observed activation energies are approximately 6.8 and 9.5 kcal/mole.