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 D_3h symmetry, distorted rings with alternating intra-annular bonds and C_3h 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 C_3h 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 C_3h-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.