Plasticity and slip system of plate-shaped crystals of L-lysine monohydrochloride dihydrate

Purpose. To identify the slip system of L-lysine monohydrochloride dihydrate (LH) and to relate it to the deformation behavior under uniaxial compression. Methods. The indentation hardness of the {100} face and the indentation moduli of the {100} and {011} faces of plate-shaped single crystals of LH (LHP) were determined using Knoop and Berkovich indenters, respectively. The deformation behavior during uniaxial compression was studied by the punch-stress vs. punch-displacement profile and by electron microscopy of the deformed crystals within cracked compacts. Results. The different indentation (elastic) moduli of the {100} and {011} faces of the crystals are consistent with the molecular packing density along these planes and suggest anisotropy. The existence of the {001} [100] slip system is proposed based on the pattern of changing indentation hardness with varying orientation of the Knoop indenter. A jagged uniaxial compression profile suggests deformation by mechanical twinning and not simple slip. The hypothesis of deformation by mechanical twinning is supported by the appearance of twin bands along the crystal faces as observed by electron microscopy of the cracked compacts. Conclusions. During compression, most LHP crystals have their {100} faces oriented normal to, or inclined to, the compression axis, thereby facilitating plastic deformation along the {001} [100] slip system by mechanical twinning. Due to the low attachment energy between them, the {001} planes can also act as cleavage planes. This study demonstrates that knowledge of the crystal structure and slip systems can be used to model the tableting and compaction behavior of molecular crystals, such as LH.