ON CROSSINGS OF TWINS IN LITHIUM-NIOBATE CRYSTALS

The crossing ranges of the twin layers, formed in lithium niobate (LiN bO3) single crystals under uniaxial loading with constant deformation rate (from 5 to 30 mu m min(-1)) in the temperature range from 450 to 850 degrees C are investigated, using optical methods. Crystallographi c analysis of possible twin crossings was carried out in a pseudocubic system to check on the mechanical joining (compatibility) of these cr ossings. It has been shown that, among the four possible types of cros sings of different types of layers (LiNbO3 has two conjugated twinning systems), only one type can lead to a coherent crossing. In the last case, according to the considered pseudo-cubic model, in structural do mains separated by this type of crossing, all four different orientati on states are realized. The other three types of crossing are not cohe rent, and they result in crack formation.