19F nuclear magnetic resonance in CeF 3

Abstract

A 19F pulsed NMR investigation of single crystals of CeF 3 and CaF 2-doped CeF 3 has been undertaken in the temperature range 300–570K at a frequency of 14 and 60 MHz. It was found that the free-induction decay is non-linear and could be fit by a sum of exponentials using a nonlinear least square procedure to an appropriate functional form. The results of this analysis indicate that there are three distinct spin-spin relaxation times T 2, which is consistent with the existence of three inequivalent fluorine sublattices. At room temperature, fluorine ions on two of the sublattices were found to be in motion, with the fastest ion specified as the F3-type in the Oftedal structure. The temperature dependence of the spin-spin relaxation time T 2 for the fastest ions was found to undergo an inversion at 400 and 385K for the pure and doped samples, respectively. The inversion indicates an increase in the interchange rate of ions between two sublattices. Activation energies are 0.29 ± 0.02 eV in the low temperature region and 0.21 ± 0.01 eV in the high temperature region for the pure sample, with corresponding values of 0.24 ± 0.01 eV and 0.23 ± 0.02 eV for the doped sample. T 1 was approximately the same for both samples, and essentially temperature independent, indicating an exchange-dominated spin-lattice decay mechanism.