An Electron and X-Ray Diffraction Investigation of Ni1+xTe2 and Ni1+xSe2CdI2/NiAs Type Solid Solution Phases

The reported wide-range nonstoichiometric Ni1+xTe2, 0≤x≤0.83, and Ni1+xSe2, 0.57≤x≤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 x∼0.5 while, in the case of selenide, three quite distinct phases have been identified within the Ni1+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 Ni1+xTe2 system. Its presence has been attributed to low frequency phonon modes along certain very specific directions of reciprocal space.