Characterization of the electronic properties of YB4 and YB6 using 11B NMR and first-principles calculations

[摘要] Two compounds, tetragonal YB4 and cubic YB6, have been investigated by electric-field gradient (EFG) and Knight shift measurements at the boron sites using the B-11 nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. In YB6 B-11 (I = 3/2) NMR spectra reveal patterns typical for an axially symmetric field gradient with a quadrupole coupling frequency of v(Q) = 600 +/- 15 kHz. In the second boride (YB4) three different EFGs were observed corresponding to the three inequivalent crystallographic sites for the boron atoms (4h, 4e, and 8j). They correspond to: v(Q) (4h) = 700 +/- 30 kHz with an asymmetry parameter eta = 0.02 +/- 0.02, v(Q) (4e) = 515 +/- 30 kHz, eta = 0.00 + 0.02/ - 0.00, and v(Q)(8j) = 515 +/- 40 kHz, eta = 0.46 +/- 0.08. The Knight shifts measured by magic-angle spinning (MAS) NMR at room temperature are very small being 0.6 +/- 8 and -1 +/- 8 ppm for YB4 and YB6, respectively. For the theoretical calculations structure optimizations were performed as a first step. For the obtained structural parameters the EFGs were computed within the local-density approximation. Very satisfactory agreement between experimental and theoretical results is obtained both for the structural parameters and the B EFGs, thus confirming the underlying structural models. In addition to the EFGs, band structures, densities of states, and valence-electron densities are presented and the bonding situation in the two yttrium borides is discussed. The band-structure results are compatible with the very low values for the Knight shifts mentioned above. (C) 2004 Elsevier B.V. All rights reserved.

[发布日期] 2004-11-30

[效力级别]  Proceedings Paper

[关键词] metal borides;electronic band structure;electric-field gradient;chemical bonding;NMR