Te for two: Ordering phenomena in doped Ni_1+xM_yTe₂ (M = Ag, Cu, In)

A detailed TEM and XRD study has been carried out on B8 related ternary solid solution phases in the Ni_1+xM_yTe₂ (M = Ag, Cu, In) systems. Despite previous reports of a large B8 ternary compositional homogeneity range in the Ni_1+xM_yTe₂ (M = Ag) system, no evidence for incorporation of Ag into Ni_1+xTe₂ could be found. In the case of M = Cu, several Ni_1+xM_yTe₂ (M = Cu) compositions were found to be single-phase and part of a genuine ternary solid solution, but only if the Cu content was at much lower concentrations than previously suggested. In the case of M = In, two roughly determined solid solution fields were observed, an expected one based on the Ni_1+xTe₂ solid solution (SS1) and a second unexpected, quite distinct and compositionally broad solid solution (SS2) centred on the nominal composition Ni_2.80In_0.80Te₂. The latter SS2 solid solution was shown to be incommensurately modulated with a composition-dependent primary modulation wavevector. The superspace group symmetry of the latter is determined and the observed B8 parent cell dimensions used to discuss possible substitution mechanisms. The origin of the incommensurate modulation in the case of the SS2 solid solution is attributed to the regular alternation of In-rich and Te-rich layers along the c direction.