Vibrational studies of BH−4 and BD−4 isolated in alkali halides

Single crystals of alkali halides doped with BH^?₄ and BD^?₄ were grown from the melt. Previously unreported bands in the infrared spectra of BH^?₄ and BD^?₄ isolated in different alkali halides are interpreted in terms of summation bands of internal and external modes of vibration. This has allowed the torsional and translational modes of the impurity ion to be identified. The tetrahedral symmetry of the borohydride ion is retained when it is isolated within alkali halides with the NaCl structure. A reduction of symmetry towards C_3v was observed when BH^?₄ (or BD^?₄) was isolated within lattices with CsCl structure.Raman and far infrared spectra of alkali halide/BH^?₄ systems will be reported for the first time, and high pressure infrared studies of these systems will be described. The effects of pressure in the internal mode, external modes, Fermi resonance and NaCl to CsCl structural phase changes will be discussed.