Borate single crystals are promising materials for non-linear optics, acoustoelectronics, piezotechnique, dosimetry and scintillation materials for detection of thermal neutrons.

The main structural units in borate single crystals are boron-oxygen complexes (B_nO_m) with direct covalent bonds shared by common oxygen atom in different ways. Cations of metals are linked with oxygen atoms by ionic bonding. Crystallochemical peculiarities of borate single crystals allow to realize all fours structural types known for compounds with different types of bonds (islet, chain, layer, sceleton) in these crystals. Therefore, in this borate system we can observe the role of crystal structure peculiarities in the formation of their phisico-chemical properties.

The perspective of the practical use of borate crystals require the study of their physico-mechanical properties. In this study we investigated the peculiarities of the deformation and fracture of borate crystals of different structural types under mechanical stresses.

It is found that all the types of borate crystals are characterized by a strong temperature dependence of dislocation motion. Slipping in this crystals starts at the temperature T ~ 0.9T_melt. It is established that slip in islet type Li₆REB₃O₉ (RE=Gd, Eu, Y) borate single crystals occurs in the planes most closely packed with respect to oxygen ions. The system of cleavage plane in these single crystals is associated with the break of RE-O bridging bonds and the longest Li-O bonds in five-vertex polyhedra of Li. Slipping in Li₂B₄O₇ single crystals of sceleton type takes place along the cavities of the boron-oxygen sceleton. And it is found that crack propagation in this crystal occurs in each case along the atomic layers linked by bridging oxygen atoms between the main structural units (B₄O₉). It is shown that chain- and layer-type borate single crystals LaB₃O₆ and beta-BaB₂O₄ are damaged practically without deformation.

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