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Electronic population effect in halogen nuclear spin - lattice relaxation in praseodymium trihalide compounds

Nuclear quadrupole resonance determinations of the spin - lattice relaxation rates of the , and nuclei in the praseodymium trihalide crystals and are reported. Data are presented in the temperature range 7 - 297 K. They are shown to be dominated by magnetic dipole interactions between the halogen nuclear spins and the paramagnetic ions. The relaxation rates display unusual temperature dependences. This behaviour is a consequence of the crystalline electric field splitting of the ground-state multiplet of the configuration into six states, and the resultant depopulation of the ground paramagnetic state as the temperature is raised. The qualitatively different temperature variation for the two compounds is a result of the different energy splittings of the multiplet levels in the two instances. The analysis utilizes an electron-spin correlation time described by a temperature-independent term, the characteristic time for spin exchange between neighbouring ions, plus a temperature-dependent exponential term.