For a structure with a rhombohedral lattice, the positions of X-ray diffraction spots are not symmetrical about the zero layer line because the hexagonal unit cell is non-primitive causing the hkl reflections to be absent when - h + k + l is not equal to 3 n ( + or - n = 0, 1, 2 ....). For the 213 l row shown the permitted reflections above the zero layer line are 21.1, 21.4, 21.7 etc. while below this line for bar (-ve) only the reflections 21.2, 21.5, 21.8 etc are visible. The l=0 line will conveniently divide the spots on each side in the ratio 1:2, on visual inspection this allows quick identification of a rhombohedral lattice.
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