On the nature of fracture of SrB 4 O 7 and PbB 4 O 7 single crystals
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ICAL PROPERTIES OF CRYSTALS
On the Nature of Fracture of SrB4O7 and PbB4O7 Single Crystals E. F. Dolzhenkovaa, V. N. Baumera, A. V. Tolmacheva, and Yu. S. Oseledchikb a Institute
for Single Crystals, National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkiv, 61001 Ukraine e-mail: [email protected] b Zaporozh’e National Engineering Academy, Zaporozh’e, 69063 Ukraine Received March 10, 2006; in final form, January 22, 2007
Abstract—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. PACS numbers: 62.20.-x DOI: 10.1134/S1063774507050197
INTRODUCTION SrB4O7 (SBO) single crystals are new promising materials for nonlinear optics, in particular, for electrooptic modulators in the far UV spectral region [1–3]. SBO has a very wide range of transparency. The efficiency of conversion of fundamental radiation into the second harmonic in SBO exceeds the corresponding characteristic of such unique nonlinear optical crystals as barium β-borate β-BaB2O4, lithium triborate LiB3O5, and potassium dihydrogen phosphate KH2PO4 and approximately corresponds to the conversion efficiency of potassium titanyl phosphate (KTiPO4). At the same time, the radiation resistance of SBO is unprecedently high and significantly exceeds the corresponding values for the above-mentioned materials. Application of SBO in nonlinear optics is limited only by the absence of phase matching. PbB4O7 (PBO) single crystals are isostructural to SBO single crystals. Moreover, although the efficiency of PBO is lower than that of the known nonlinear optical crystals, SBO and PBO were proposed as materials for design of nonlinear optical film waveguides in [3, 4]. Similarity of the unit-cell parameters and a significant difference in the refractive indices of SBO and PBO (∆n = 0.2) allows successful design of promising structures on the basis of PBO films grown on SBO substrates. Variation in the PBO film thickness in such structures would make it possible to obtain a desired value of the refractive index and satisfy the phase-matching condition during generation of harmonics.
In addition, strontium tetraborate crystals activated with Eu2+ are currently applied as phosphors
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