The crystal structure of NbSe3 has been refined from single crystal X-ray diffraction data. It has a monoclinic symmetry with lattice parameters: a=10.009 AA, b=3.4805 AA, c=15.629 AA, beta =109.47 AA, space group P21/m and six formulae per unit cell. The crystal structure of NbSe3 as determined at 100K shows that the 145K transition in the electrical resistivity is not accompanied by a structural distortion. Except for the decrease due to thermal contraction all interatomic distances are found to be the same. This is compatible with the proposed model of the charge-density-waves formation which has been put forward in order to explain the physical properties of the 145K transition. Electron diffraction pictures taken above and below the transition give direct evidence of the charge-density-waves formation at the transition. At 120K in agreement with Tsutsumi et al. (1977) the pictures contain superstructure spots at the (h,k+or-0.243, l) positions. Contrary to what has been reported by the same authors, the authors have found that at 3K above the transition the electron diffraction pictures do not contain the diffuse scattering streaks corresponding to the planes at k'=k+or-0.243, which would indicate fluctuations of the CDW above the transition.

Electron diffraction patterns are obtained of the orthorhombic and monoclinic structures of TaS3. For the orthorhombic structure one set of superlattice spots is observed at (l ± 0.5) a*, (m ± ± 0.125) b*, (n ± 0.25) c* below 210 K. For the monoclinic structure two sets of superlattice spots are observed, the first one at la*, (m ± 0.253) b*, nc* below 240 K, the second one at (l ± 0.5) a*, (m ± 0.245) b*, (n ± 0.5) c* below 160 K. Diffuse scattering lines are present for the two structures. Resistivity measurements are performed on crystals with the two structures, which show strong increase of the resistivity indicating metal‐semiconducting transitions at the same temperatures where the superlattice spots appear. These transitions are interpreted as successive Peierls transitions on the different types of chains of TaS3. Also electron diffraction patterns are shown of NbSe3 at very low temperatures where the two charge density waves that occur at 145 and 59 K are formed. A comparison is given between TaS3 and NbSe3.

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