Influence of niobium on the charge-density-wave transition of Ni(Ta1-xNbx)2Se7

The influence of doping by niobium on the low-temperature charge-density-wave (CDW) state of NiTa2Se7 has been investigated by x-ray diffraction and measurements of the electrical conductivity. Doping concentrations in Ni(Ta1-xNbx)2Se7 varied between 0.5% and 30%. X-ray crystal structure refinements showed that the niobium atoms statistically replace the tantalum atoms, but with a ratio of 2:1 for the two crystallographically independent tantalum sites Ta1 and Ta2. Niobium was not found on the Ni sites nor intercalated between the layers. Previous investigations have established that the primary CDW is located on the Ni and Se2 atoms, and only a small influence on the CDW was expected from Nb doping of the Ta chains. Nevertheless, we found that the CDW was very sensitive to the level of doping x with dtc/dx=-16, where tc=Tc(x)/Tc(0) is the reduced transition temperature. Similar values for dtc/dx were calculated for different isoelectronically doped compounds with transition temperatures varying between Tc=59 K (NbSe3) and 263 K [(TaSe4)2I]. It is suggested that this can be explained by a common mechanism of weak pinning of the CDW to the impurities. The results are rationalized by considering the recently found 4kF component of the CDW, which is located on the Ta2 atoms. The large effects of doping on the CDW transition can then be understood in terms of the effect of doping on the 4kF CDW and the interactions between the primary CDW and the 4kF CDW