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To investigate the pyramid–prism (PY–PR) boundary of a rapidly grown KDP crystal, the exact position of the PY–PR boundary of an X-cut rapidly grown KDP sample is determined by measuring the refractive-index non-uniformity. The peak-valley value of Δ(ne–no) near the PY–PR boundary is approximately 3 × 10−6, and the width of the distortion area near the PY–PR boundary is approximately 0.6 mm. The FWHM values of the high-resolution X-ray diffraction peaks in the pyramidal sector, the prismatic sector, and the region containing the PY–PR boundary are all 0.008°, indicating the fine crystalline quality of the crystal. Moreover, the existence of the PY–PR boundary does not affect the fine crystalline quality. From the pyramidal sector to the prismatic sector, the transmittance in the ultraviolet region decreases gradually when crossing the PY–PR boundary. The fluences corresponding to 50% laser-induced damage (LID) probability are 11.9, 12.8, and 17.0 J cm−2 (3 ns, 355 nm) for the PY–PR boundary, the pyramidal sector, and the prismatic sector, respectively. By identifying different types of defects in each sector, it is found that the PY–PR boundary exhibited the worst LID performance. This is because there are more bulk microdefects in the PY–PR boundary. These defects are more likely to be inclusions induced by the instability of the steps near the PY–PR boundary. [ABSTRACT FROM AUTHOR]

The first non-centrosymmetric mixed-alkali-metal borophosphates, Na4MB2P3O13 (M = Rb 1, Cs 2), were obtained using a low-temperature flux method. Single-crystal X-ray diffraction studies of 1 and 2 reveal that the two compounds are isostructural, both crystallizing in the orthorhombic space group Pna21; their structures consist of novel 1D borophosphate chains constructed from B2P3O14 fundamental building units, assembled into a 3D framework by alkali metal cations. Second-harmonic generation (SHG) measurements show that 1 and 2 are type-I phase-matchable, with SHG responses ca. 0.35 and 0.42 times that of KH2PO4, respectively. The cut-off edges of 1 and 2 are ca. 276 and 267 nm, respectively, which suggests that they are potential ultraviolet nonlinear optical materials. Density functional theory calculations were employed to shed light on the band structure and density of states as well as the electron density distribution. [ABSTRACT FROM AUTHOR]