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 less than or equal to x less than or equal to 0.83, and Ni1+xSe2, 0.57 less than or equal to x l ess than or equal to 0.92, CdI2/NiAs structure type solid solution fields h ave been carefully reinvestigated via electron and X-ray powder diffraction to search for evidence of Ni/vacancy ordering and superstructure phases. I n the case of telluride, evidence is thereby found for a Ni3Te4 superstruct ure phase attempting to condense out at x similar to 0.5 while, in the case of selenide, three quite distinct phases have been identified within the N i1+xSe2 solid solution field. An intriguing characteristic honeycomb diffus e-intensity distribution has also been observed across the solid solution f ield in the case of the Ni1+xTe2 system. Its presence has been attributed t o low frequency phonon modes along certain very specific directions of reci procal space. (C) 2001 Academic Press.