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

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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 ¹¹B nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. In YB₆ ¹¹B (I=3/2) NMR spectra reveal patterns typical for an axially symmetric field gradient with a quadrupole coupling frequency of νQ=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: νQ(4h)=700±30 kHz with an asymmetry parameter η=0.02±0.02, νQ(4e)=515±30 kHz, η=0.00+0.02/-0.00, and νQ(8j)=515±40 kHz, η=0.46±0.08. The Knight shifts measured by Magic-Angle Spinning (MAS) NMR at room temperature are very small being 0.6±8 ppm 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.