Determination of the structural distortions corresponding to the ${\mathbf{q}}^1$- and ${\mathbf{q}}^2$-type modulations in niobium triselenide ${\mathrm{NbSe}}_3$

Synchrotron-radiation single-crystal x-ray diffraction is used to study the modulations corresponding to both the ${\mathbf{q}}^{1\mathrm{-}}$and ${\mathbf{q}}^2$-type charge-density waves (CDW’s) in ${\mathrm{NbSe}}_3$. The ${\mathbf{q}}^1$-type modulation is shown to involve displacements of the atoms of column III. The ${\mathbf{q}}^2$-type modulation is analogous, but resides on column I. An important feature is that it is found that both CDW’s involve displacements of a Se atom of column II. This structure explains previous experiments, but appears to be at variance with the more recently obtained scanning-tunneling-microscopy images. The implications for the dimensionality of the ${\mathbf{q}}^1$ transition are discussed.