The real (incommensurate interface modulated) structure of Ni_6±xSe₅

The high-temperature Ni_6±xSe₅ phase has been carefully reinvestigated via electron diffraction and TEM imaging. Electron diffraction reveals an (in general) incommensurately modulated, continuously variable (within narrow limits) reciprocal lattice, indirectly confirming the notion of a continuously variable (within narrow limits) Ni_6±xSe₅ solid solution phase. The superspace group symmetry of the incommensurate interface modulated structure is determined and used to predict the allowed conventional three-dimensional space group symmetries when the primary modulation wavevector locks in to 1/2a*. The mechanism for accommodating nonstoichiometry is suggested to be the introduction into a 2 × 1 × 1 superstructure of variably spaced, nonconservative (100) APB's characterized by the displacement vector R = [1/4, 1/2, 0].