Importance of d-Spacing in Diffraction of Crystals

Braggs’ equation suggests that the interplanar d-spacing is an important and integral part of a diffraction process. Therefore, keeping in mind the fact that B-Z is the W-S of reciprocal lattice, the interplanar spacing calculations for low index planes for an arbitrary value of cell dimension in three cubic crystal lattices; sc, bcc and fcc and similarly in primitive cases of some elemental and MX-crystals have been made to check the nature of occurrence of Braggs’ planes in them. The results obtained from these calculations are found not only to be in consonance with the experimental data available for crystals of metallic elements and MX-systems but they also appear to take care of the ‘systematic absences’ arising due to the so called ‘lattice centring’, screw axes and glide planes, without actually taking them into consideration. This allows us to conclude that the set of parallel planes (of atoms) of crystals with the least interplanar spacing diffract first (by using the beam of X-rays, electrons or neutrons) due to its shortest reciprocal lattice vector and then continue in a systematic and spherically symmetric manner with increasing order of interplanar spacing in reciprocal space. This newly found criterion is extremely important for the work of crystal structure determination using any diffraction technique.