Infrared Spectra of Borohydride Ions in Alkali Halide Single Crystals

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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–, 2ν4 < ν3 < ν2 + ν4 for KX:BD4–ν3 < 2ν4 < ν2 + ν4; and for CsX:BH4–2ν4 < ν2 + ν4 < ν3. These variations are accounted for by isotopic shifts and by the nature of the repulsive interaction of the borohydride ion with the solvent ions.