Superstructure and stacking faults in hydrothermal-grown KBe2BO3F2 crystals

The structure of the hydrothermal-grown nonlinear optical crystal KBe2BO3F2 was investigated. A new structure of the R3̄c space group with cell parameters of a=4.422(1) Å and c=37.524(3) Å was obtained by powder X-ray diffraction and Rietveld refinement. The new structure is a 1×1×2 superstructure of the previously reported R32 structure with a different stacking sequence of (Be2BO3F2)∞ 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.