The noncentrosymmetric optical borate crystals are the basic materials of modern high-power laser systems because of appropriate nonlinear optical (NLO) coefficients, wide transparency window including visible and UV ranges and high optical damage thresholds. Orthorhombic lead tetraborate PbB₄O₇ (PBO) was discovered in binary system PbO-B₂O₃ many years ago. The crystal structure of PBO belongs to mm2 point group, space group being Pmn2₁. The PBO structure is formed by the rigid three-dimensional network of corner-linked BO4 tetrahedrons, and lead atoms occupy the large cages of the structure and are coordinated by nine oxygen atoms. PBO is a nonlinear crystal with rather large nonlinearity of the second order and large Raman-type nonlinearity of the third order.

The PbB₄O₇ single crystal has been grown from the melt by Czochralski method. The charge has been prepared from 2PbCO₃•Pb(OH)₂ and H₃BO₃ taken with stoichiometric ratio. Crystal growth was performed in the air from a platinum crucible 50 mm in diameter and 70 mm in high with 300 grams of the charge. The seed oriented in [100] direction was used. The plate-like transparent, colorless PbB₄O₇ single crystal with dimensions 2×22×38 mm along [100], [010] and [001] directions correspondingly was grown after three days. The growth process included starting growth without pulling (the rotation rate was 8 rpm) during two days and growth with pulling rate 1.2 mm/day during one day. This crystal had several rather well developed facets; especially pronounced were the facets corresponding to the planes [100], [010] and [101]. From as grown crystal was cut off the sample which was subjected to etching in diluted nitric acid (5 weight %) at the temperature of 90°C during 10 minutes. A system of defects has been revealed by optical microscopy. Growth steps and boundaries of polar growth twins can be observed on as-grown surfaces. Etching of as-grown (100) surface revealed etching pits, being the dislocations outcrop onto the surface. The etching patterns observed on this surface reveal the stripes of different etching behavior that means the existence of twins, despite (100) is not a polar surface. Twin boundaries probably correspond to (010) planes, in accordance with isostructural crystal SBO. The twin boundary nature of observed stripes is admitted by the difference of etching pits symmetry in twin-free regions and twin boundaries. 

Structural properties and micromorphology of etched PbB4O7(100) surface have been observed by RHEED using EFZ4 device at electron energy of 50 keV and AFM measurements performed with Solver P-47H device in noncontact mode, respectively. A system of Kikuchi lines has been recorded from PbB₄O₇ (100) surface.

Acknowledgements: This study is supported by SB RAS, Projects 28.13 and 43.12, and by PSB RAS Grant 2.5.2. 

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