19F nuclear magnetic resonance in CeF3

Abstract

A F19 pulsed NMR investigation of single crystals of CeF3 and CaF2-doped CeF3 has been undertaken in the temperature range 300°K to 570°K at a frequency of 1 4 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 T2, 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 T2 for the fastest ions was found to undergo an inversion at 400°K and 385°K for the pure and doped samples, respectively. The inversion indicates an increase in the interchange rate of ions between two sublattices. Activation energies are .29 ± .02 eV in the low temperature region and .21 ± .01 eV in the high temperature region for the pure sample, with corresponding values of .24 ± .01 eV and .23 ± .02 eV for the doped sample. T1 values were found to be approximately the same for both the pure and doped samples, and essentially temperature independent, ranging from 1.3 to 1.7 milliseconds at 14 MHz and from 1.6 to 1.9 milliseconds at 0 0 60 MHz, over a temperature range of 300°K to 540°K. This temperature independent spin-lattice relaxation time and the calculated T1 value of .75 millisecond, using the exchange integral, indicate an exchange-dominated spin-lattice decay mechanism.