On the nature of fracture of SrB4O7 and PbB4O7 single crystals

The crack resistance and peculiarities of the internal structure of isostructural SrB₄O₇ and PbB₄O₇ single crystals of the framework type have been studied. It is shown that the cleavage of these crystals, which is atypical of this type and inherent in strontium and lead tetraborates, is due to the presence of boron-oxygen layers (bound by a relatively small number of covalent bonds) in their 3D boron-oxygen frameworks; crystals are cleaved along these layers. It is established that cracks propagate in SrB₄O₇ and PbB₄O₇ single crystals as a result of breakage of both the bonds between bridge atoms and the bonds in B₃O₃ boron-oxygen cycles—the main elements of the boron-oxygen framework. The break of bonds in the boron-oxygen cycles is explained by the presence of an unusual oxygen position in these cycles, which is shared by three boron-oxygen tetrahedra and whose B-O bonds are much weaker in comparison with the bonds typical of BO₄ groups.