A CLASSIFICATION OF METABORATE CRYSTALS BASED ON RAMAN-SPECTROSCOPY

The Raman spectra of crystals built solely of metaborate triangles provide fingerprint identification of three distinct network types. Classified according to increasing cation field strength these are: rings with degenerate intra-annular bonds and D3h symmetry, distorted rings with alternating intra-annular bonds and C3h symmetry, as well as chains. The occurrence of each network type has been associated with a characteristic range of cationic field strength. This approach led to the discovery of a hitherto unknown C3h ring strontium metaborate crystal, with Sr2+ cations in 9- or 10-fold coordination to oxygen atoms. The Raman spectra of the mixed cation metaborates Ba2Ca(B2O4)3 and Ba2Mg(B2O4)3 confirm their C3h-ring structure and clearly point to the fact that the synergetic effect of dissimilar cations to the metaborate network cannot be predicted by the additivity of their field strengths.