The crystal structure of MoSe2 has been refined from single‐crystal X‐ray diffraction data. MoSe2 is isotypic with hexagonal MoS2 (C7 type); a = 3.289(1) Å, c = 12.927(4) Å, z(Se) = 0.6210(3). It is pointed out that the structure of MoS2 is incorrectly given in many books, although it had already been correctly reported by Dickinson and Pauling [4]. Corrected interatomic distances are given; the trigonal prism about Mo is almost regular.

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type seen in Au22Mn6 (Hiraga et aL, 1982). In the present case, there is a sliding offset of Mn-column positions as indicated, for example, by the arrows in Fig. 7(b) and the width of Dla along the long-period axis is different. Careful observation of these images enables a schematic model for this lattice 'side-step' to be obtained, as drawn in Fig. 7(c) where the difference in brightness of spots is indicated by the lined, half-filled or solid circles (and the regions of D la type are shown in screen-tone). The structure model of Au22Mn6 is also shown in Fig. 7(d) for comparison. Some apparent nearest-neighbour MnMn pairs, the brightness of which is lower than that of other parts, are visible at the side-step. However, no nearest-neighbour Mn-Mn pairs are known to exist in this alloy. Moreover, the recorded image comes from the projection of the structure along the incident-beam direction. Thus, it can be concluded that the occupation probability of the Mn atoms in the columns is less than unity in order to avoid true nearest-neighbour Mn-Mn pairs, and this causes the lower contrast of the pairs, as discussed for the 'superstructure' image condition (Shindo, Hiraga & Hirabayashi, 1984). It is not possible to distinguish where the Mn atoms are located in the columns, whether there is perfect order along the tilt boundary or if the Mn atoms occupy the columns randomly along the beam direction. It should be noted that the superstructure shown in Fig. 7 AbstractA complete analysis of the modulated structure of NbTe4 is presented, using the newly developed formalism of de Wolff [Acta Cryst. (1974 up to the fifth harmonic in the modulation wave. However, the values found for higher harmonics do not describe the real modulation wave, but assume values to account for the variation in intensities due to experimental errors. Therefore, only the amplitudes up to the second order are considered to be reliable.

The incommensurately modulated structure of tantalum tetratelluride (TaTe4) has been determined by X-ray diffraction on the basis of a 2a x2a x3c supercell. Mo Ka radiation, A=0.71069/~, /z= 405.0 cm -~. The space group of the subcell is P4/mcc, that of the supercell P4/ncc. a=6.5154(5), c= 6-8118 ~, RF =0.064 for 6422 observed reflections.The structure is characterized by TaTe4 chains along the c axis. These chains are modulated in such a way that Ta3 clusters are formed. The Te atoms respond to the displacements of the Ta atoms, keeping the bonds of the Te dimers, which link the chains, equal in length. The clusters in neighbouring chains are shifted with respect to each other with a phase shift of ~, causing the doubling of the a axis. A simple model is proposed to account for the observed modulations in NbTe4 and TaTe4.