Experimental determinations and quantum-chemical calculations of the vibrational spectra of β-ZnB4O7 and β-CaB4O7.

Abstract The two oxoborates β -ZnB 4 O 7 and β -CaB 4 O 7 were synthesized and investigated by FTIR- and Raman spectroscopy and ab initio quantum chemical calculations. Maximum and mean deviations between experimentally determined bands and calculated modes ranged between 15–36 cm −1 and 5–7 cm −1 , respectively, allowing band assignments to vibrational modes in most cases. The complex network structures with tetrahedral BO 4 and planar OB 3 groups are mirrored by the spectra and numerous vibrational modes, not assignable by standard borates classification schemes. It was confirmed that OB 3 units, despite similar force constants and geometry, do not absorb in the same range as BO 3 units. Bands in the high wavenumber range are rather caused by B–O–(Zn/Ca), O–B–O, B–O–B, and B–O stretching and bending vibrations. The experimental observation of inactive or Raman-active modes in the absorption spectra indicates defects or structural distortions in both compounds.