Direct observation of the microstructure and defects in KBe2BO3F2 crystals by TEM----Technical Institute of Physics and Chemistry

We apply for the first time transmission electron microscopy to the direct observation of the deep-UV nonlinear optical crystal KBe₂BO₃F₂ (KBBF) grown using different methods. Two kinds of electron diffraction patterns were observed in the hydrothermally synthesized KBBF along the [120] direction, which resulted from the coexistence of two structures with space groups R32 and R [3 with combining macron]

c. As a comparison, the flux grown KBBF crystals show uniform R32 structures. Furthermore we observed a twin boundary in flux KBBF which corresponds to the (2 [1 with combining macron]

3)_R32 crystallographic plane. Two structure models are proposed here for the twin boundary. These observations on the microstructure and defects were not disclosed in previous powder XRD and optical microscopy experiments and shed new light on the understanding of the structure and defects of KBBF crystals.

Dalton Trans., 2014

Fig. 1 Structures of the R32 phase (left) and the R [3 with combining macron]

c phase (right) of KBBF crystals along [001] (a); along [120] (b); along [100] (c).

Fig. 2 HRTEM images of flux KBBF (a) and hydrothermal KBBF (b) along [001]. The inserts are the ED images. ED simulation image of KBBF for both R32 and R [3 with combining macron]

c phases along [001] (c).