The study of defects and growth in organic crystals of thallium hydrogen phthalate (TAP)

Imperfections in the (010) and (001) crystals of thallium hydrogen phthalate (TAP) have been studied by means of X-ray diffraction topography. The experimental results show that two types of dislocations, spiral dislocations and ordinary dislocations, both distributed in an unexpectedly high density and running nearly normal to the (001) face, play an important role in increasing the growth rate of the (001) plane for TAP by providing self-perpetuating growth steps or growth centres, compared with crystals of KAP (potassium hydrogen phthalate) and RAP (rubidium hydrogen phthalate) of the same family, which show the lowest growth rates on the (001) surfaces. The spiral dislocations produce beautiful `white' Borrmann images and their contrast decreases with decrease in specimen thickness and Borrmann coefficient. The images lose their fine spiral details when the contrast inversion from `white' to `black' occurs.