Continuous series of one-dimensional structures in compounds based on M 2X 3 (M =Sb, Bi; X =Se, Te)

Abstract

For compounds of the type M 2X 3 (Bi 2Te 3, Bi 2Se 3, Sb 2Te 3), doping with Ge (Sn or Pb), as well as addition of excess cations, leads to the formation of continuous series of 1 D superstructures which can now be characterized by means of electron microscopy. The systems Ge δBi 2Te 3 and Bi 2+δ are discussed as model systems. Analysis of the electron diffraction patterns with the fractional shift method suggests the superstructures are built up of sequences of five- and seven-layer lamellae. This interpretation is confirmed by direct (high resolution) images. From a comparison between experimental high resolution images and computed images, a model for the five- and seven-layer lamellae is presented. A modified “cut and projection” method is described and applied to derive the sequences of “5” and “7” bands from the (approximate) q-values of the (quasi)commensurate diffraction patterns.