Nuclear Magnetic Resonance in Hexagonal Lanthanum Metal: Knight Shifts, Spin Relaxation Rates, and Quadrupole Coupling Constants

Abstract

The ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ nuclear magnetic resonances in the double-hexagonal close-packed (dhcp) form of metallic lanthanum have been studied by spin-echo techniques in the frequency range 6-30 MHz and temperatures between ∼1 and 210°K. A seven-line powder pattern due to electric quadrupole interactions is observed with $\left|e^{2}q{Q}^{\mathrm{}\left(139\right)\mathrm{}}{h}^{-1\mathrm{}}\right|=7.8\mathrm{}\pm 0.3$ MHz. The quadrupole splittings are identical within the experimental uncertainty for the two crystallographic sites. The Knight shifts are very different, however, $K\approx +0.63\% \mathrm{and} +1.02\%$ at 4°K, and $K\approx +0.29\% \mathrm{and} +0.92\%$ at 210°K. The anisotropic Knight-shift contribution is estimated to be less than 10% of the measured shifts. The average spin-lattice relaxation time for the two sites is $T_{1}T=0.56\mathrm{}\pm 0.05$ sec °K. The relaxation time for the site having the smaller Knight shift exceeds that for the other site by a factor of approximately 1.7. A comparison of the average Knight shifts and spinlattice relaxation rates in dhcp lanthanum with those in hcp scandium and yttrium leads to the conclusion that the direct $s$-contact interaction is the dominant magnetic-hyperfine mechanism in the Group-IIIB transition metals.