The Real (Incommensurate Interface Modulated) Structure of Ni6±xSe5

The high-temperature Ni6±xSe5 phase has been carefully re-investigated 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) Ni6±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) APBʹs characterized by the displacement vector R=[1/4, 1/2, 0].