ELECTRONIC POPULATION EFFECT IN HALOGEN NUCLEAR-SPIN-LATTICE RELAXATION IN PRASEODYMIUM TRIHALIDE COMPOUNDS

Nuclear quadrupole resonance determinations of the spin-lattice relaxa tion rates of the Cl-35, Br-79 and Br-81 nuclei in the praseodymium tr ihalide crystals PrCl3 and PrBr3 are reported. Data are presented in t he temperature range 7-297 K. They are shown to be dominated by magnet ic dipole interactions between the halogen nuclear spins and the Pr3paramagnetic ions. The relaxation rates display unusual temperature de pendences. This behaviour is a consequence of the crystalline electric field splitting of the ground-state multiplet of the Pr3+ configurati on into six states, and the resultant depopulation of the ground param agnetic state as the temperature is raised. The qualitatively differen t temperature variation for the two compounds is a result of the diffe rent energy splittings of the multiplet levels in the two instances. T he analysis utilizes an electron-spin correlation time described by a temperature-independent term, the characteristic rime for spin exchang e between neighbouring ions, plus a temperature-dependent exponential term.