The crystal structure of the intermetallic Nb₂Co₇ phase was identified [1] to be similar to monoclinic Zr₂Ni₇ [2,3]. Parallel layers with a complex internal atomic structure, arranged in a close packed fashion, can be recognised.

The X-ray synchrotron diffraction pattern (ID31 beamline, ESRF, Grenoble -France, λ = 0.400094 Å) of a Nb₂Co₇ powder containing a minor fraction of C15-NbCo₂ Laves phase (employed to identify the crystal structure of Nb₂Co₇ [1]), exhibits complex line broadening features (cf. Figure):

(a) 0kl and 5kl reflections are narrow. All other reflections are broad.

(b) some unindexed intensity maxima remain.
Those features can be described by an irregular stacking of the above mentioned layers, forming faults in the ideal monoclinic stacking. These faults correspond to local occurrence of the stacking sequence of orthorhombic Yb₂Ag₇ [4], which contains the same type of layers as Nb₂Co₇. Such type of faulting should indeed leave the 0kl and 5kl reflections unbroadened. Moreover, the unindexed intensity maxima can be understood as low-angle cut-offs of diffuse hk intensity bands (see Figure, italic numbers) parallel to c*. A combined refinement of the crystal and fault structure was performed using the DIFFaX+ software [5], correctly reproducing the mentioned features in the powder diffraction pattern.

[1] A. Leineweber, D. Grüner, F. Stein et al. to be published.

[2] F. R. Eshelman, J. F. Smith, Acta Cryst. B 28 (1972) 1594.

[3] E. Parthé, R. Lemaire, Acta Cryst. B 31 (1975) 1879.

[4] G. Cordier, R. Henseleit, Z. Kristallogr. 194 (1991) 146.

[5] M. Leoni, A. F. Gualtieri, N. Roveri, J. Appl. Cryst. 37 (2003) 166.

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