Figure 2
(a) Part of the crystal structure in the monoclinic phase at 80 K. The yellow (brown) arrows represent the bonds that become shorter (longer) below the transition. Interatomic Ta-Ni and Ta-Se3/S3 distances in Ta\(_\textrm{2}\)NiSe\(_\textrm{5}\) (c and d), Ta\(_\textrm{2}\)Ni\(_\mathrm {0.93}\)Co\(_\mathrm {0.07}\)Se\(_\textrm{5}\) (f,g) and Ta\(_\textrm{2}\)NiS\(_\textrm{5}\) (i and j) as a function of temperature. Error bars reflect the statistical errors from the refinement and are smaller than the symbol sizes. A clear splitting of the Ta-Ni and Ta-Se3 bond lengths can be observed for Ta\(_\textrm{2}\)NiSe\(_\textrm{5}\) and Ta\(_\textrm{2}\)Ni\(_\mathrm {0.93}\)Co\(_\mathrm {0.07}\)Se\(_\textrm{5}\) which is totally absent for Ta\(_\textrm{2}\)NiS\(_\textrm{5}\) where only a small reduction with decreasing temperature is found, simply reflecting the standard thermal expansion behavior. Panels (b) and (e) are parts of the x-ray precession images for the reciprocal (5 3 l) series (l is changing from 4 to -3) reconstructed from single-crystal XRD data collected at 360 and 80 K for Ta\(_\textrm{2}\)NiSe\(_\textrm{5}\), and at 280 and 80 K for Ta\(_\textrm{2}\)Ni\(_\mathrm {0.93}\)Co\(_\mathrm {0.07}\)Se\(_\textrm{5}\) show the splitting of the reflections below the orthorhombic-to-monoclinic phase transition. Panel (h) shows the precession images for Ta\(_\textrm{2}\)NiS\(_\textrm{5}\) again for the reciprocal (5 3 l) series (l is from 4 to -3) at 360 and 80 K from our single-crystal XRD data, and for the reciprocal (5 1 l) series (l is from 4 to 11) at 2 K from Synchrotron single-crystal XRD data, where no splitting of the reflections is seen in the whole temperature range. More details and complete precession plots are given in the SM.
