Below detection, limit symmetry

There are two weak Bragg peaks in this diffraction pattern, which could not be indexed since they do not belong to the hexagonal crystal lattice with the established unit cell. As we will find out later (Chapters 6 and 7), these two peaks manifest the presence of a small amount of an impurity phase - a solid solution of Sn in Ni, which has a face-centered cubic crystal structure with the space group symmetry Fm3m and a = 3.543 A. Only one theoretically possible Bragg peak of the major hexagonal phase (003) is unobserved in this powder diffraction pattern because its intensity is below the limits of detection. 

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