Superstructure studies in hydrothermal-grown RbBe2BO3F2 crystals

Deep-UV nonlinear optical crystal RbBe2BO3F2 (RBBF) grown by the hydrothermal method was found to have a centrosymmetric structure of R3¯c by powder X-ray diffraction. The structure of R3¯c has cell parameters of a = 4.44223(11) Å and c = 39.6696(12) Å in the lattice where the (BO3)3− groups turn 60° along each (Be2BO3F2)∞ layer in the a-b plane and it is double size along the c-axis compared with standard R32 structure. As the examined RBBF shows a very low second harmonic generation capability, a stacking fault model was proposed that the hydrothermal RBBF is nonuniform with a mixture of both R32 and R3¯c structures using the DIFFaX program.