The real (incommensurate interface modulated) structure of Ni_(6 +- x)Se_5

摘要

The high-temperature Ni_(6 +- x)Se_5 phase has been carefully reinvestigated via electron diffraction and TEM imaging. Electron diffraction reveals an (in genral) incommensurately modulated, continuously variable (within narrow limits) reciprocal lattice, indirectly confirming the notion of a continuously variable (within narrow limits) Ni_(6 +- x)Se_5 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 X 1 X 1 superstructure of variably spaced, nonconservative (100) APB's characterized by the displacement vector R [1/4, 1/2, 0).