The real (incommensurate interface modulated) structure of Ni6 +/- xSe5

The high-temperature Ni6 +/-xSe5 phase has been carefully reinvestigated vi a 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 continuou sly variable (within narrow limits) Ni6 +/-xSe5 solid solution phase. The s uperspace group symmetry of the incommensurate interface modulated structur e is determined and used to predict the allowed conventional three-dimensio nal 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, non conservative (100) APB's characterized by the displacement vector R = [1/4, 1/2, 0]. (C) 2001 Academic Press.