Superstructure and stacking faults in hydrothermal-grown KBe{sub 2}BO{sub 3}F{sub 2} crystals

The structure of the hydrothermal-grown nonlinear optical crystal KBe{sub 2}BO{sub 3}F{sub 2} was investigated. A new structure of the R3-bar c space group with cell parameters of a=4.422(1) A and c=37.524(3) A was obtained by powder X-ray diffraction and Rietveld refinement. The new structure is a 1x1x2 superstructure of the previously reported R32 structure with a different stacking sequence of (Be{sub 2}BO{sub 3}F{sub 2}){sub {infinity}} layers along the c axis. The relationship between the refined structure and the experimental results is discussed. A stacking fault mechanism is proposed for the formation of the superstructure as well as the nonuniformity of the hydrothermal-grown KBBF crystals. - Graphical Abstract: A new structure of the R3-bar c space group with cell parameters of a=4.422(1) A and c=37.524(3) A was obtained from hydrothermal-grown KBe{sub 2}BO{sub 3}F{sub 2} crystals by powder XRD and Rietveld refinement. Highlights: > Structural investigation on hydrothermal-grown KBe{sub 2}BO{sub 3}F{sub 2} crystals by powder XRD and Rietveld refinement. > Relationship between structures and SHG properties. > Superstructure formed by stacking faults during hydrothermal crystal growth.