Modulations in Intermetallic Families of Compounds

Härtill 7 uppsatser Diss. (sammanfattning) Stockholm : Stockholms universitet, 2004 filosofie doktorsexamen Doctor philosophiae doctorat ès lettres Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm This thesis is based on a study of five distinct intermetallic systems with the aim of expanding the general knowledge of aperiodically modulated crystal structures. Families of compounds that contain a variety of superstructures together with incommensurately modulated structures have been investigated mainly by means of single crystal X-ray diffraction and higher dimensional structure models. A uniform (3+1)-dimensional structure for Bi-Se phases was developed with the composition as a single variable. The structure description is based on a cubic NaCl type structure with homoatomic layer stackings. It is shown by computational modelling that the formation energies of bismuth selenides with more than 40 at. % Bi are close to zero, a result that supports the idea of a continuous series of stackings corresponding to an ordered solid solution of Bi in Bi 2 Se 3 . The Nowotny chimney-ladder structures are described with a (3+1)-dimensional composite structure, valid for all such compounds regardless of the included elements, the composition or the valence electron concentration. A new member is added to this family by the ZrBi 1.62 compound. The modulation is believed to arise as a secondary effect of the criteria of a fixed electron count. A symmetry analysis is presented for the RE 1+ε (MB) 4 ( RE = rare earth elements, M = iron metal elements) family of compounds and a uniform (3+1)-dimensional composite structure description has been developed. The modulation may be due to the presence of unusually short contacts between the RE channel atoms, giving rise to a rotational modulation of the (MB) 4 tetraederstern chains. A (3+1)-dimensional incommensurate structure has been determined for the novel δ 1 – CoZn compound. The structure displays a unique assembly of fused icosahedra and the modulation is induced by geometric strain. The structure of the K(PtSi) 4 compound was re-determined. Despite a close kinship with the RE 1+ε (MB) 4 compounds, this structure is not modulated. Degree of Doctor of Philosophy This thesis is based on a study of five distinct intermetallic systems with the aim of expanding the general knowledge of aperiodically modulated crystal structures. Families of compounds that contain a variety of superstructures to