STRESSES GENERATED BY IMPURITIES IN DIAMOND

Single dissolved nitrogen, boron or hydrogen atoms cause the diamond l attice around them to expand by 40%, 33.7% and 31%, respectively. Such large atomic strains generate short-range stresses around each dissol ved impurity atom. If the impurity is dissolved uniformly in a diamond crystal, the atomic strains around each impurity atom will produce an overall uniform expansion that will not generate macro stresses in th e diamond crystal. However, if the impurity is not dissolved uniformly , incompatible macrostrains will result that will generate large elast ic stresses in the diamond crystal. These stresses may strengthen or w eaken a diamond crystal depending on their magnitudes, signs (i.e. com pression or tension) and spatial distributions in the crystal. An inho mogeneous distribution of impurities can be generated either by the ap pearance of higher order growth facets during crystal growth, which ge nerate differential segregation of impurities between the high- and lo w-order crystal growth sectors or by temporal changes in growth condit ions which generate radial impurity gradients. Since growth conditions control the distribution of impurities in the diamond, diamonds can b e strengthened or weakened by different growth conditions.