A comment on the use of symmetry arguments in the interpretation of X-ray diffraction patterns from two-phase systems

It is pointed out that, when determining the crystal structure of a precipitate in a two-phase alloy, it is not sufficient to consider only the overall symmetry of the combined precipitate and matrix reciprocal lattices. If several precipitate variants exist in the parent crystal, and if all variants have an equal probability of being present, then the combined reciprocal lattice will always demonstrate the symmetry of the parent crystal. Recently, [Lee (1970). J. Appl. Cryst. 3, 413] the symmetry of the reciprocal lattice was used to deduce that the ω-phase precipitate formed in the b.c.c. matrix of a Ti–30 %V alloy has a b.c.c. structure. The extra information that is available for the ω-phase can be used to demonstrate that this conclusion is incorrect, and that the ω unit cell has hexagonal symmetry.