Infrared Spectra of Borohydride Ions in Alkali Halide Single Crystals

Abstract

Solid solutions of NaBH₄ in NaCl, NaBr, and NaI, of KBH₄ in KCl, KBr, and KI, of CsBH₄ in CsBr and CsI, of KBD₄ in KCl, KBr, and KI, and of CsBD₄ in CsBr and CsI have been prepared. The borohydride was diffused into the alkali halide single crystals as it was produced by action of the alkali metal and hydrogen upon the borates of a borosilicate glass capsule. The site symmetry was found to be tetrahedral for salts of both bcc and fcc structure. For NaX:BH₄^– and KX:BH₄^–, ${\text{2ν}}_4{\mathrm{\hspace{0.17em}<\hspace{0.17em}\nu }}_3{\mathrm{\hspace{0.17em}<\hspace{0.17em}\nu }}_2{\mathrm{\hspace{0.17em}+\hspace{0.17em}\hspace{0.17em}\nu }}_4$ for ${\mathrm{KX:BD}}_4^{–}{\nu }_3{\text{\hspace{0.17em}<\hspace{0.17em}2ν}}_4{\mathrm{\hspace{0.17em}<\hspace{0.17em}\nu }}_2{\mathrm{\hspace{0.17em}+\hspace{0.17em}\nu }}_4;$ and for ${\mathrm{CsX:BH}}_4^{–}{\text{2ν}}_4{\mathrm{\hspace{0.17em}<\hspace{0.17em}\nu }}_2{\mathrm{\hspace{0.17em}+\hspace{0.17em}\nu }}_4{\mathrm{\hspace{0.17em}<\hspace{0.17em}\nu }}_3.$ These variations are accounted for by isotopic shifts and by the nature of the repulsive interaction of the borohydride ion with the solvent ions.