Corresponding relationship between characteristic birefringence, strain, and impurities in Zimbabwean mixed-habit diamonds revealed by mapping techniques

Birefringence in diamond is an optical phenomenon related to strain and various defects in crystal lattices. Despite extensive investigations being done to characterize and quantify it, there is still controversy about its origin in diamond lattices. Here we report the relationship between the distribution of birefringence patterns observed under cross-polarized light, strain features analyzed by Raman mapping, and the impurity characteristics revealed by Fourier transform infrared spectroscopy (FTIR) mapping in natural mixed-habit diamonds. It was deduced that the plastic deformation was enhanced with higher tensile residual stress, and nitrogen and VN3H defects were more enriched as a result of the temperature increase during crystallization, at growth bands showing straight birefringence patterns and the relative enrichment of graphite inclusions. These results provided solid data and insights for birefringence-related properties in diamond and correlated the occurrence of birefringence with diamond spectroscopic properties, which promoted the understanding of the formation of birefringence in natural diamonds and would be helpful for the synthesis of high-quality, birefringence-free diamonds.