S Vansmaalen

MISFIT LAYER COMPOUNDS (MS)NTS2 (M = SN, PB, BI, RARE-EARTH ELEMENTS - T = NB, TA - N = 1.08 - 1.19), A NEW CLASS OF LAYER COMPOUNDS

Abstract A study by X-ray diffraction and electron microscopy is reported of the type of compounds that is usually designated as ternary sulphides “MTS 3 ” ( M = Sn, Pb, Bi, rare earth metals; T = Nb, Ta). They are misfit layer compounds built of alternating double layers of MS with M in distorted square pyramidal coordination by sulphur and sandwiches TS 2 with T is distorted trigonal-prismatic coordination by sulphur. One intralayer lattice constant of MS is incommensurate with that of TS 2 . Recent progress in the understanding of the crystallographic description is discussed and a unifying view based on about ten compounds is presented. Results of electrical transport measurements are briefly summarized.

Superspace group descriptions of the symmetries of incommensurate urea inclusion compounds

Urea inclusion compounds are a class of incommensurate composite crystals. The urea molecules form a three-dimensionally connected network, with approximate space group symmetry P6122. This network contains tunnels (channels), which accommodate guest molecules. The periodicities of the urea subsystem and the guest subsystem along the tunnel axis are mutually incommensurate. Depending on temperature, the guest subsystem exists with various degrees of disorder. This paper reports the application of the superspace formalism to describe the symmetries of incommensurate urea inclusion compounds. A list of superspace groups is given, compatible with the urea subsystem structure, and describing the possible guest subsystem structures. Particular attention is given to the relation between superspace symmetry, the various types of (dis)order in the guest subsystem, and the point-symmetry of the guest molecules. Incommensurate urea inclusion compounds containing n-alkane, diacyl peroxide and α, ω-dibromoalkane guest molecules are analysed in detail.

CHANNELING IN QUASI-CRYSTALS

Ion-beam channeling has been observed in quasicrystals. For 1 MeV 4He+ ions in icosahedral AlCuFe the maximum effect found is 36%. The full width at half maximum of the observed dips is 1.3°. The effect persists up to great depths (> 200 nm), thus showing a high degree of ordering in this phase. The channeling effect is sensitive to radiation damage.

CONTRIBUTIONS OF ONE-PHONON AND 2-PHONON PROCESSES TO THE RESISTIVITY OF TTF-TCNQ

Abstract There is a long-standing disagreement on the relative contributions of one-phonon (1p) and two-phonon (2p) scattering processes to the resistivity ϱ TTF - TCNQ in the temperature range from 300K down to 100 to 150K. The group that believes 2p processes, in particular two-libron (2λ) processes, determine ϱ has argued that their predominance is due to the fact that they cause a larger modulation of the transfer integral t than 1p processes. We have calculated 1/t times the first and second derivatives of t with respect to the displacements due to the basic external lattice modes; these are the quantities that enter into the resistivity. The calculations were done for the TCNQ chain, which contributes most of the conductivity of TTF - TCNQ. Using the resulting values, we find that the contribution of 2λ, or indeed any 2p, modes to the resistivity of the TCNQ chain is negligible. The calculated coupling constants for 1p processes involving the external modes are large enough so that, in combination with the contribution of the internal modes, they could account for the observed ϱ.

ANISOTROPIC THERMAL-EXPANSION IN TCNQ SALTS

Abstract Crystals containing stacks of TCNQ-ions have roughly thermal expansion coefficients along the stack, which are three times the values at right angles to it. It is shown that a simple degree of freedom argument, taking into account the librational modes can explain this phenomenon.