An electron and X-ray diffraction investigation of Ni_1+xTe₂ and Ni_1+xSe₂ Cdl₂/NiAs type solid solution phases

The reported wide-range nonstoichiometric Ni_1+xTe₂, 0 ≤ × ≤0.83, and Ni_1+xSe₂, 0.57 ≤×≤0.92, CdI₂/NiAs structure type solid solution fields have been carefully reinvestigated via electron and X-ray powder diffraction to search for evidence of Ni/vacancy ordering and superstructure phases. In the case of telluride, evidence is thereby found for a Ni₃Te₄ superstructure phase attempting to condense out at × ̃ 0.5 while, in the case of selenide, three quite distinct phases have been identified within the Ni_1+xSe₂ solid solution field. An intriguing characteristic honeycomb diffuse-intensity distribution has also been observed across the solid solution field in the case of the Ni_1+xTe₂ system. Its presence has been attributed to low frequency phonon modes along certain very specific directions of reciprocal space.