C. Manolikas

Continuous series of one-dimensional structures in the compounds Bi2+xSe3, Bi2+xTe3, Sb2+xTe3, (Bi2Te3)nGeTeand (Sb2Te3)nGeTe

Abstract Electron microscopy techniques were used to characterise the commensurate and incommensurate superstructures formed after the addition of cations to the isostructural compounds Bi2Se3, Bi2Te3 and Sb2Te3. The observed electron diffraction patterns were analysed, using a method closely related to the “cut and projection” method, hereby assuming the building blocks of the superstructures to be 5-layer and 7-layer lamellae. This assumption was confirmed by high resolution electron microscopy images.

Continuous series of one-dimensional structures in compounds based onM2X3 (M =Sb, Bi; X =Se, Te)

Abstract For compounds of the type M 2 X 3 (Bi 2 Te 3 , Bi 2 Se 3 , Sb 2 Te 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 δ Bi 2 Te 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.

Electron microscopic study of digenite-related phases (Cu2−xS)

Abstract A study by means of electron microscopy and electron diffraction of synthetic digenite reveals the existence of a series of closely related structures with basic spacings 2a o , 3a o , 4a o , 5a o and 6a o , where a o is the (111) spacing of the cubic close packed sulphur sublattice. In all these structures copper atoms occupy tetrahedral interstices in a cubic close packed matrix of sulphur atoms. By means of high resolution microscopy non-rational spacings are shown to be the result of a mixture of spacings. It is shown that the particular set of systematic extinctions in the diffraction pattern discussed in previous papers, is related to a particular state of disorder and does not imply the presence of twinning.

The “local” structure of domain boundaries in ferroelastic lead orthovanadate☆

Abstract High resolution transmission electron microscopy is used to study the local structure of twin boundaries and anti-phase boundaries in the β-phase of lead orthovanadate. The local structure along twin boundaries and anti-phase boundaries is found to be γ-like. This γ-like region extends over two or three unit cells of the β-phase. The anti-phase boundaries have a displacement vector which is not exactly half a lattice vector; some additional displacement is present. A new intermediate phase was found around 100°C; it occurs between γ and β.

A 2a2a3c superstructure in hexagonal Ni1−xS: a study by means of electron diffraction and HRTEM

The high temperature phase of Ni1-xS has the NiAs-type structure. The coexistence of two superstructures, “3a3a3c” and “2a2a3c” with the basic phase is confirmed by means of electron diffraction. The 2a2a3c superstructure is studied by means of electron diffraction and high resolution electron microscopy. A structure model is proposed based on the periodic insertion of stacking faults in the NiAs-type basic structure and the ordering of vacancies in alternate metal-atom layers. Microtwinning in very narrow slabs is found to be a main feature of the 2a2a3c regions and two defect models are discussed.

Electron microscopic study of the superstructure, the domain structure and the transition state of CuIn5Te8

The structure and microstructure of the compound CuIn5Te8 have been examined. The lattice parameters as deduced from electron microscopy diffraction patterns were found to be a = 0,61 nm and c = 1.22 nm in good agreement with those determined by X-rays. Furthermore the crystals of this compound present a domain structure, consisting of three non-coaxial orientation variants. The examination of this domain structure allowed us to conclude that this compound has a thiogallate-like structure.

Incommensurate phases in lead potassium niobate and lead sodium niobate, homologues of barium sodium niobate

Abstract Electron diffraction and high resolution electron microscopy reveal the presence of incommensurate structures in lead potassium niobate, lead sodium niobate and in mixed crystals. The incommensurability can be described either as being due to the presence of quasi-periodic “diffuse” anti-phase boundaries or as being due to the presence of deformation waves, the deformation mode being shearing of the NbO 6 octahedra. The incommensurability depends on composition as well as on temperature.

A study of polytypism in AgIn5Se8 by combined electron microscopy techniques

Abstract Three different polytypes are found to exist in AgIn 5 Se 8 . They have been studied by means of conventional electron microscopy and diffraction as well as by high-resolution microscopy. The three different structures have tetragonal symmetry and can be derived from each other by means of periodic antiphase boundaries; they correspond to c a = 4, c a = 2 , and c a = 1 , respectively. Regions of c a = 4 often occur as microsyntactic intergrowths into c a = 2 areas, and a large number of nonperiodic antiphase boundaries also occur. After fast cooling from above T c a transition state characterized by the presence of diffuse intensity has been observed and analyzed.

On the formation mechanism of antiphase boundaries with well defined crystallographic orientations

Abstract Direct observations by the electron microscope show that the orientation of the antiphase boundaries, along well defined crystallographic directions in crystals of y-phase of cuprous telluride, is a secondary effect. It is associated with the strain minimization process, along the interfaces between coexisted σ- and y-phase in a narrow transition temperature range.