Quadrupole Coupling and Knight Shift Anisotropy in "hcp" Lanthanum

Abstract

The nuclear magnetic resonance of ${\mathrm{La}}^{139}$ has been observed in hcp polycrystalline lanthanum metal at 25.6 kOe and 300\ifmmode^\circ\else\textdegree\fi{}K. The recorded pattern at 15.5 Mc/sec displays a central line and one pair of satellite lines. The position and shape of the central line yields an isotropic Knight shift of (0.685\ifmmode\pm\else\textpm\fi{}0.020)% and an axial shift of (0.046\ifmmode\pm\else\textpm\fi{}0.020)%. The satellite spacing yields a lowest pure quadrupole frequency ${\ensuremath{\nu}}_{Q}=101\ifmmode\pm\else\textpm\fi{}4$ kc/sec or $\frac{{e}^{2}\mathrm{qQ}}{h}=(1.41\ifmmode\pm\else\textpm\fi{}0.06)$ Mc/sec. A calculation of the lattice electric field gradient is made for the $\mathrm{ABAC}$ stacking sequences, yielding 0.540\ifmmode\times\else\texttimes\fi{}${10}^{22}$ and 0.487\ifmmode\times\else\texttimes\fi{}${10}^{22}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ for the so-called $A$ and $B$ (or $C$) layer sites, respectively, by the method of plane-wise summation of de Wette. Improved values of the nuclear electric quadrupole moment, $Q({\mathrm{La}}^{139})=0.21$ b, and the Sternheimer antishielding factor, ${\ensuremath{\gamma}}_{\ensuremath{\infty}}({\mathrm{La}}^{3+})=\ensuremath{-}73.5$, are adopted and discussed. The sign of the conduction-electron electric field gradient is shown to be negative.