The Mechanism And Key Factors Of Macroscopic Crystal Defects Formation In Organic Compounds

The arrangement of atoms or molecules in most crystalline materials is not perfect.The regular patterns are interrupted by crystal defects.Crystal defects are very common in the crystallization process.The presence of crystal defects can seriously affect crystal properties such as mechanical properties,optical behavior,and electrical conductivity.Macroscopic inclusion is an influential defect which affects purity of crystals during crystallization and remains not fully understood.In this work,we chose thiourea as the model material to investigate inclusion formation in detail.Different types of defects observed inside single crystals of thiourea are characterized.Confocal Raman spectroscopy and optical microscopy analyses revealed the coexistence of three types of defects:(i)liquid inclusions containing saturated solution without gas bubbles inside these inclusions,(ii)liquid inclusions containing saturated solution with gas bubbles,(iii)isolated gas bubbles inside the crystal.The Raman spectral analysis showed that the first two types of inclusion cavities were filled with saturated thiourea solution.Recrystallization of thiourea were performed in deionized water,ethylene glycol,methanol,ethanol,n-propanol,and n-butanol via cooling modes.Polar solvent was found to be promotive to the inclusion formation.The effect of supersaturation on inclusion formation was found to be complicated.Increasing supersaturation did not always stimulate the formation of inclusions.An evaporation experiment was designed to monitor the process of crystal growth and inclusion formation and a supersaturation window was found.Under the lower threshold of the window,inclusion can seldom form due to low crystal growth.Beyond the upper threshold,however,inclusion formation also decreased.This supersaturation effect was explained using the “hunger center” mechanism.