Comparative NMR investigation of the Re-based borides

Abstract

We report a systematic study of the rhenium-based borides, ${\text{ReB}}_{2}$, ${\text{Re}}_{7}{\text{B}}_{3}$, and ${\text{Re}}_{3}\text{B}$, by means of the $^{11}\text{B}$ nuclear magnetic resonance (NMR) spectroscopy. While ${\text{Re}}_{7}{\text{B}}_{3}$ and ${\text{Re}}_{3}\text{B}$ are superconductors, ${\text{ReB}}_{2}$ exhibits no superconducting signature but is of current interest due to its superhard mechanical property. Since the major focus of this investigation is their electronic characteristics in the normal states, we performed the measurements at temperatures between 77 and 295 K. For ${\text{Re}}_{7}{\text{B}}_{3}$ and ${\text{Re}}_{3}\text{B}$, $s$-character electrons were found to be responsible for the observed $^{11}\text{B}$ NMR Knight shift and spin-lattice relaxation rate $(1/{T}_{1})$. From ${T}_{1}$ analysis, we thus deduce the partial $\text{B}\text{ }s$ Fermi-level density of states (DOS) of both borides. On the other hand, the relaxation rate of ${\text{ReB}}_{2}$ is mainly associated with $p$ electrons, similar to the cases of ${\text{OsB}}_{2}$ and ${\text{RuB}}_{2}$. In addition, the extracted $\text{B}\text{ }2p$ Fermi-level DOS is in good agreement with the theoretical prediction from band-structure calculations.