Characterization of the electronic properties of YB${}_4$ and YB${}_6$ using ${}^{11}$B NMR and first-principles calculations

Abstract

Two compounds, tetragonal YB${}_4$ and cubic YB${}_6$, have been investigated by electric-field gradient (EFG) and Knight shift measurements at the boron sites using the ${}^{11}$B nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. In YB${}_6$ ${}^{11}$B ($I=3/2$) NMR spectra reveal patterns typical for an axially symmetric field gradient with a quadrupole coupling frequency of ${\nu }_Q=600\pm 15$ kHz. In the second boride (YB${}_4$) three different EFGs were observed corresponding to the three inequivalent crystallographic sites for the boron atoms ($4h$, $4e$, and $8j$). They correspond to: ${\nu }_Q(4h)=700\pm 30$ kHz with an asymmetry parameter $\eta =0.02\pm 0.02$, ${\nu }_Q(4e)=515\pm 30$ kHz, $\eta =0.00+0.02/-0.00$, and ${\nu }_Q(8j)=515\pm 40$ kHz, $\eta =0.46\pm 0.08$. The Knight shifts measured by Magic-Angle Spinning (MAS) NMR at room temperature are very small being $0.6\pm 8$ ppm and $-1\pm 8$ ppm for YB${}_4$ and YB${}_6$, 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.

22 pages, 6 figures