NUCLEAR-QUADRUPOLE ALIGNMENT OF LU-176(M) AND LU-177 IN A LUTETIUM SINGLE-CRYSTAL AT LOW-TEMPERATURES AND SYSTEMATICS OF ELECTRIC-FIELD GRADIENTS IN PURE HEXAGONAL TRANSITION-METALS

The quadrupole frequencies for Lum176 and Lu177 nuclei in a Lu single crystal have been determined by nuclear orientation at temperatures down to 6 mK as -128(16) MHz and +294(37) MHz, respectively. From the observed γ anisotropies several γ-ray multipole mixing ratios could be derived. With the known ground-state quadrupole moment of Q=3.39(2) b for Lu177 the electric field gradient of LuLu is found to be +3.6(5) × 1017 V/cm2. This value does not fit into the frame of Raghavan's universal correlation. Taking into account recent data on other systems, a new classification of electric field gradients in hexagonal metals is proposed: The electronic contribution to the total electric field gradient is proportional to the ionic part, the proportionality constant being positive for all group-IIIb and -IVb hexagonal metals and negative for all group-IIb, -VIIb, and -VIIIb metals. © 1979 The American Physical Society.