J. L. de Boer

Valence fluctuations in the incommensurately modulated structure of calaverite AuTe2

compatible with A1-Fe phases in the same composition range; (3) the m value is ~4.0-4.4; (4) the strongest diffraction spots are fundamental, i.e. most atoms scatter in phase. From these assumptions, a framework of 126 atoms in the unit cell was set up, which produced the strongest diffraction spots. Vacancies and Fe atoms were distributed in the framework by trial and error to obtain the correct strong and weak reflections for the other diffraction spots. The model gave qualitative, but not fully satisfying, agreement with diffraction patterns. The model could explain the experimental HREM images, in the (100} as well as the (110) projection. This suggests that the main features of the model may be correct. On the other hand, the contrast of HREM images depended strongly on crystal thickness, questioning whether a projected structure interpretation of HREM images is applicable, even for the thinnest crystals. [0011 I 60 k

Structure, electrical transport, and magnetic properties of the misfit layer compound (PbS)1.13TaS2

Abstract (PbS)1.13TaS2, formerly designated “PbTaS3,” has been found by single-crystal X-ray diffraction to be a misfit layer compound characterized by two face-centered orthorhombic unit cells each with space group Fm2m. One belongs to the PbS part of the structure (a = 5.825A, b = 5.779A, c = 23.96A, Z = 8), the other belongs to the TaS2 part of the structure (a′ = 3.304A, b′ = 5.779A, c′ = 23.96A, Z = 4). The corresponding axes are parallel; the ratio of the lengths of the misfit a axes, a a′ is irrational, but close to 7 4 . The compound is built of alternately double layers of PbS with distorted NaCl-type structure and TaS2 sandwiches, with Ta in distorted trigonal prisms of sulfur. Along the c axes of length 23.96 A, four units are stacked; units of the same type but ( 1 2 )c apart are displaced with respect to each other over ( 1 2 )b . The symmetry of the complete structure is analyzed in terms of a four-dimensional superspace group. The electrical transport and magnetic properties are related to those of 2Hue5f8TaS2. The conduction is metallic and strongly anisotropic, the in-plane resistivity being about 105 smaller than the resistivity along the c axis. The Hall coefficient, with positive sign, corresponds with an electron donation from PbS to TaS2. The Seebeck coefficient is negative like that in other intercalates of 2Hue5f8TaS2. The compound is Pauli-paramagnetic.

Determination of the modulated structure of the inorganic misfit layer compound (PbS)1.18TiS2

Single-crystal X-ray diffraction results (Mo K-alphaBAR radiation, lambda = 0.71073 angstrom) are presented for the inorganic misfit layer compound titanium sulfide (PbS)1.18TiS2 which can be described as a two-component structure. The first subsystem (TiS2, nu = 1) has space-group symmetry C2(1)/m, and a basic structure unit cell given by a11 = 3.409 (1), a12 = 5.880 (2), a13 = 11.760 (2) angstrom and alpha-1 = 95.29 (2)-degrees. The modulation wavevector is q1 = a21* = alpha-a11*, with alpha = 0.5878 (3). Its subsystem superspace group is [GRAPHICS] (alpha, 0, 0). The second subsystem (PbS, nu = 2) has space group C2/m and a basic structure unit cell given by a21 = 5.800 (2), a22 = 5.881 (2), a23 = 11.759 (2) angstrom and alpha-2 = 95.27 (2)-degrees. The modulation wavevector is q2 = a11*. The subsystem superspace group is [GRAPHICS] (alpha-1, 0, 0). The relation between the two unit cells is defined by the common (a-nu-2*, a-nu-3*) plane. The symmetry of the complete system is described by the single superspace group G(s) = [GRAPHICS] (alpha, 0, 0). Reciprocal lattice parameters for this superspace embedding are a1* = a11*, a2* = a12*, a3* = a13* and a4* = a21*. Refinements on 1449 main reflections, with I > 2.5-sigma(I), converged smoothly to R(F2) = 0.064 (R(F) = 0.069). The final structure model included displacive modulation parameters up to second harmonics for Pb and first harmonics for the other atoms. The largest modulation amplitudes are on both atoms of the PbS subsystem. They mainly desribe displacements parallel to the layers, along the commensurate direction a-nu-2. A detailed analysis is given of the coordination of the Pb (nu = 2) and S (nu = 1) atoms by plotting interatomic distances as a function of the fourth superspace coordinate.

STRUCTURE, ELECTRICAL TRANSPORT, AND MAGNETIC-PROPERTIES OF THE MISFIT LAYER COMPOUND (CES)1.16NBS2, CENBS3

(CeS)1.16NbS2 is a misfit layer compound built of alternate double layers of CeS, approximately a {100} slice of NaCl type CeS, and sandwiches of NbS2 with Nb in slightly distorted trigonal prisms of sulfur. The two structural units in the composite crystal are described in centered orthorhombic lattices: a1 = 5.727(1), A, b1 = 5.765(1), A, c1 = 11.41(1), A, space group Cm2a, Z = 4 for the CeS lattice and a2 = 3.311(1), A, b2 = 5.765(1), A, c2 = 22.82(2), A, space group Fm2m, Z = 4 for the NbS2 lattice. Corresponding axes are parallel and the stacking direction is c. Refinements of the CeS part, the NbS2 part, and the common projection using 889, 288, and 135 reflections, respectively, converged to RF = 0.048, 0.069, and 0.072, respectively. The electrical conduction is metallic with a strong anisotropy, the resistivity along the c axis, ϱc, being about 104 larger than the in-plane resistivity ϱab. The conduction is by holes in an almost filled 4dz2 band of the NbS2 part of the structure, indicating electron transfer from the CeS part to the NbS2 part of the structure. The magnetic properties show down to 80 K a Curie Weiss behavior with a Curie constant close to that of CeS with the scheme, Ce3+(e−)S2−; χ−1 goes to zero with decreasing temperature after a small discontinuous drop at about 50 K.

Persistent organic pollutants

Protocol to the 1979 Convention on Long-range Trans-boundary Air Pollution on Persistant Organic Pollutants, Keith Bull The 1998 Agreement for the UN/ECE region The Development of a Global Treaty on Persistant Organic Pollutants (POPs), John Buccini Criteria for Additional New POPs, Bo Wahlstrom Chlorinated Pesticides - Aldrin, DDT, Endrin, Dieldrin and Mirex, Vladimir Zitko Hexachlorbenzen e, Vladimir Zitko Dioxins and Furans (PCDD/PCDF), Heidelore Fiedler Releases of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans to land and water and with products, Patrick H. Dyke Toxicology and risk Assessment of POPs, Stephen Safe Multimedia Models for Global Transport and Fate, Frank Wania, Martin Scheringer Background Contamination of Humans with Dioxin, Dioxin-Like PCB and other banjo - Organochlorines in Nigeria and the African Region, Olaf Papke, Peter Furst Sources, fates and Effects of Persistant Organic Pollutants in China, with Emphasis on the Pearl River Delta, Ming H. Wong DDT in Mexico, Fernando Diaz-Barriga et al Dioxin and Furan Reduction Technologies for Combustion and Industrial Thermal Process Facilities, Hans-Ulrich Hartenstein Alternative Technologies for Destruction of PCB and other POPs, Ian D. Rae.

Structural relationship between the orthorhombic, monoclinic and triclinic misfit layer compounds (MS)nTS2 (M = Sn, Pb, rare earth metals, T = Ti, V, Cr, Nb, Ta; 1.13 < n < 1.21)

Abstract Unit cell dimensions of the two structural units MS and TS2 are reported for the monoclinic misfit layer compounds (PbS)1.18TiS2 and (PbS)1.13VS2, and for the triclinic compound (SnS)1.20TiS2. They are compared with the unit cells of the orthorhombic misfit layer compounds and with triclinic (LaS)1.2CrS2. In all cases it is found that the reciprocal lattices of the substructures MS and TS2 have a common ( b ∗, c ∗)-plane. A building principle is formulated, which states that a stabilizing interlayer interaction is obtained if the M atoms of the MS double layers fall in the groves formed by sulfur atoms of TS2 sandwiches. Using this principle, the occurence of orthorhombic, monoclinic and triclinic symmetries is explained.

Redetermination of the structure of hessite, Ag2Te-III

This single-crystal study confirms the space-group symmetry reported previously [Frueh (1959). Z. Kristallogr. 112, 44-52], but proves that interatomic distances were miscalculated, leading to incorrect assignment of coordination number for one Ag atom and incorrect structure description. The structure can be regarded as a strongly distorted antifluorite structure with both independent Ag atoms approximately tetrahedrally coordinated by Te. The Ag-Te distances are in the range 2.8415 (7)-3.034 (1) angstrom. The shortest Ag-Ag distance is 2.841 (1) angstrom, shorter than in metallic silver (2.89 angstrom).

The incommensurately modulated structures of the blue bronzes K0.3MoO3 and Rb0.3MoO3

The incommensurately modulated structures of the isostructural blue bronzes of K and Rb with modulation wavevector q = a* + 0.748 (1)b* + 1/2c* at 100 K have been determined by X-ray diffraction. The lattice parameters of the C-centred monoclinic cell for K0.3MoO3 are: a = 18.162 (2), b = 7.554 (1), c = 9.816 (1) angstrom, beta = 17.393(6)degrees, V = 1195.7 angstrom3, Z = 20, mu = 55.7 cm-1, lambda = 0.7107 angstrom, M(r) = 156.9. For Rb0.3MoO3: a = 18.536 (2), b = 7.556 (1), c = 10.035 (5) angstrom, beta = 118.52 (1)degrees , V = 1234.9 angstrom3, Z = 20, mu = 110.0 cm-1, lambda = 0.7107 angstrom, M(r) = 172.4. The symmetry of the structure can be described as consisting of a one-dimensionally modulated system with the four-dimensional superspace group C(s 1BAR)C2/m (0beta1/2). Th final R(F) = 0.033 for 7985 reflections for the K bronze and 0.032 for 4458 reflections for the Rb bronze. In the modulated structure, valence calculations show that the phase transition to the semiconductor state is accompanied by ordering of Mo5+ along the infinite-chain direction. The metallic conductivity, with delocalization of 4d electrons between clusters by overlapping Mo-O-Mo orbitals along the infinite-chain direction, tums into semiconductor properties by localization of 4d electrons on individual Mo(2) and Mo(3) octahedra (not on Mo10O30 clusters as a whole), modulated with wavevector q. By a comparison of the structures of the blue and red bronzes, their physical properties can be interpreted.

THE MODULATED STRUCTURE OF THE COMMENSURATE MISFIT-LAYER COMPOUND (BISE)1.09TASE2

Single-crystal X-ray diffraction (Mo Kalpha radiation, lambda = 0.71073 angstrom) was used to determine the structure Of the commensurate inorganic misfit-layer compound (BiSe)alpha0TaSe2, alpha0 = 12/11 congruent-to 1.09. The structure is described as a (3 + 1)-dimensional intergrowth compound with two subsystems and with symmetry according to the superspace group P:Fm2m(alpha0,0,0)111BAR. The TaSe2 subsystem has a11 = 3.421 (1), a12 = 5.970 (1) and a13 = 24.34, (7) angstrom, with subsystem space group Fm2m and subsystem superspace group P.Fm2m(alpha0,0,0)111BAR. The BiSe subsystem has lattice parameters a21 = 3.135 (1), a22 = 2.984 (1) and a23 = 12.174 (4) angstrom, with subsystem space group Pm2m and subsystem superspace group P:Pm2m(alpha0(-1),1/2,1/2)111BAR. The (a(nu2)*,a(nu3)*) reciprocal-lattice plane is common to the two subsystem lattices, nu = 1, 2. Refinements on 1614 unique reflections converged smoothly to R = 0.050. The modulation was divided into a large block-wave occupational modulation describing the Bi/Se ordering and a relatively small displacement modulation. An interpretation is given of the effect of the latter using the bond-valence method. The structural features are compared to the incommensurate misfit-layer compounds, in relation to the inter-subsystem bonding.

The determination of the commensurately modulated structure of tantalum tetratelluride

Determination de la structure modulee incommensurablement de TaTe 4 par diffraction RX sur la base dune surmaille 2a×2a×3c. Le groupe despace de la sousmaille est P4/mcc, celui de la surmaille P4/ncc. Affinement de la structure jusqua 0,064. La structure se caracterise par des chaines TaTe 4 le long de laxe c. Ces chaines sont modulees de telle sorte que des agregats Ta 3 sont formes