Knowledge Bank

The metaborate ion, $BO2-$, has been incorporated in dilute solid solution in crystals of KCl, KBr, and other alkali halides. The ion is stable and soluble in fused alkali halide, and upon cooling occupies a halide lattice site. Infrared spectra of solid solutions have been studied at $300\circ K$ and at $77\circ K$. and bands observed from species containing $B10,B11$, and $O18$ in normal abundances, and from species containing isotopically enriched $O17$. Half-widths of the bands are approximately one $cm-1$ at $300\circ K$, allowing precise measurement of isotope shifts. Analysis of the spectra shows $BO2-$ to be linear, and comparable to $CO2$. Approximate vibrational frequencies for $B11O216-$ (values vary somewhat with the host lattice) are $\nu 3$ (asymmetric stretch) $1970cm-1,\nu 2$ (bend) $610cm-1$, and $\nu 1$ (symmetric stretch) $1070cm-1$. No infrared absorption of $\nu 1$ and $2\nu 3$ can be observed at high concentrations, indicating that the $BO2-$ environment is highly symmetric. The asymmetric stretch, $\nu 3$, absorbs strongly in the infrared and may be easily observed at concentrations below a part per million. The absorptions of $BO2-$ are affected by cation impurities, through the formation of ''ion pairs'' in the lattice.