S. van Smaalen

The structure of different phases of pure C70 crystals

Abstract Single crystal of pure C70 are grown from the vapour phase and the structure and morphology of these crystals is studied. By means of X-ray diffraction and TEM measurements five different phases are observed. The observed phases are (from high to low temperatures) fcc, rhombohedral, ideal hcp (c/a=1.63), deformed hcp (c/a=1.82) and a monoclinic phase. The occurrence of these different phases and the phase transitions is accounted for in a simple model. For the monoclinic structure a model for the stacking of the orientationally ordered molecules in the lattice is proposed. For both the hcp and fcc phases a Lennard-Jones type interaction potential is used to calculate bond strengths, lattice energies and the theoretical morphology.

Refinement of modulated structures against X-ray powder diffraction data with JANA2000

JANA is a computer program for the refinement and analysis of periodic and aperiodic (incommensurately modulated structures and composite crystals) crystal structures. Here a new module is introduced that allows Rietveld refinements against powder diffraction data. It is shown that JANA2000 provides a state-of-the-art description of the peak profiles. A re-analysis of the low-temperature structure of (CO)xC60 showed that the application of icosahedral symmetry restrictions to the C60 molecule leads to a better description of the electron density and to a corrected position of the CO molecule as compared with a rigid-body refinement. The incommensurately modulated structure of NbTe4 has been successfully refined against X-ray powder diffraction data. The structural parameters are equal to, but less accurate, than the parameters obtained from a single-crystal study.

Refinement of the crystal structure of tetragonal Al2Cu

Abstract New single-crystal X-ray diffraction data have been obtained for the θ-phase of aluminum-copper (Al2Cu). Results of the refinement are presented. The structure is found to be in accordance with the one determined by J. B. Friauf (J. Amer. Chem. Soc. 49, 3107–3114 (1927)) . The compound crystallizes in the tetragonal space group I4 mcm (No. 140) with a unit cell of dimensions a = 6.067(1) and c = 4.877(1) A. There are four formula-units per unit cell. A full-matrix least-squares refinement with 237 observed reflections and 8 parameters converged to RF = 0.032 (wR = 0.037). In addition to more accurate values for the lattice parameters and the atomic coordinates, anisotropic temperature factors are obtained.

STRUCTURAL PHASE-TRANSITIONS IN C-70

Cubic as well as hexagonal single crystals of C70 have been grown and investigated by electron diffraction, electron microscopy and x-ray diffraction. Several phase transitions have been detected and crystallographic models are proposed. Hexagonal crystals, stable at room temperature with c/a = 1.63 will undergo two transitions, upon cooling. First the c/a ratio will increase to 1.82 owing to c-axis alignment of the molecules; at a lower temperature the molecules will orientationally order, resulting in a monoclinic structure.

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 2HTaS2. 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 2HTaS2. 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.

REFINEMENT OF THE CRYSTAL-STRUCTURE OF HEXAGONAL AL2CULI

Abstract The crystal structure of the T1 phase of Al-Cu-Li has been determined by means of single crystal X-raydiffraction. The compound crystallizes in the hexagonal space group P6/mmm (No. 191) with a unit cell of dimensions a = b = 4.954(3) A and c = 9.327(4) A The least-squares refinement on F with isotropic temperature parameters for all atoms converged at RF = 0.087 (wR = 0.095), for 371 observed reflections and eight parameters. Although a global agreement is observed with the structure proposed by J. C. Huang and A. J. Ardell (Mater. Sci. Technol. 3, 176 (1987)), some significant differences are found. These differences are interpreted in terms of a more likely coordination of Li and Al atoms.

Ab initio structure determination of two polymorphs of cyclopentadienylrubidium in a single powder pattern

The structures of two polymorphic phases of solid RbCsH 5 have been solved ab initio by high-resolution powder X-ray diffraction measured in a single powder pattern. The compound crystallizes in space group Pbcm (phase I), Z--8, with unit-cell parameters a = 9.3396(1), b = 10.9666(1), c = 10.5490(1)A, and in space group Pnma, Z = 4 (phase II), with unitcell parameters a = 10.7990 (2), b = 8.6923 (2), c 5.7061(2)A. Both phases show polymeric zigzag chains, so-called bent-polydecker sandwich structures, with an .average Rb--Cpz (Cp-ring-centroid) distance of 2.97 A. Phase II contains a single chain along the a axis, whereas phase I contains two similar but crystallographically independent chains perpendicular to each other along the b axis and along the c axis, leading to a more complex coordination sphere around the Rb atoms. The chains are bent with Cpz--Rb--Cpz angles ranging from 123.5 to 136.5 ° .

The structure of LiCu2O2 with mixed-valence copper from twin-crystal data

The structure of LiCu2O2 was solved using two sets of X-ray diffraction twin-crystal data. Extended twinning creates virtual tetragonal symmetry. The compound crystallizes in Pnma (62) with unit-cell parameters a = 5.72 A, b = 2.86 A and c = 12.4 A with a certain homogeneity range. The structure consists of LiCuIIO2 layers interleaved by layers of CuI connected to oxygen in an almost linear coordination. Lithium and copper(II) have five oxygen neighbours in pyramidal arrangements that run as parallel bands through edge connections. Electron diffraction was used for characterizing the twinning.

X-RAY CRYSTAL STRUCTURE DETERMINATION OF THE TRICLINIC MISFIT LAYER COMPOUND (SNS)1.20TIS2

(SnS)1.20TiS2 is a misfit layer compound built of alternately double layers of SnS with distorted rocksalt-type structure and sandwiches of TiS2 slightly distorted compared with those of 1T-TiS2. For comparison with other misfit layer compounds the triclinic subsystems are described in centred unit cells: for the SnS part, a1=5.683(1)AA, b1=5.832(1)AA, c1=11.680(5)AA, alpha 1=95.85(3) degrees , beta 1=94.78(3) degrees , gamma 1=90.03(2) degrees , the space group is C1 and Z=4; for the TiS2 part, a2=3.412(1) AA, b2=5.835(1) AA, c2=23.289(3) AA, alpha 2=95.86(1) degrees , beta 2=90.30(1) degrees , gamma 2=90.01(1) degrees , the space group is F1 and Z=4. In real space, a1 and a2 as well as b1 and b2 are parallel while the c axes diverge. In reciprocal space both sublattices have the (b*,c*) plane in common. Refinements were performed of the SnS part using 1351 independent reflections (RF=0.087) and of the TiS2 part using 714 reflections (RF=0.080). Each Sn atom is coordinated to five S atoms of the SnS double layer with SnS distances of 2.610(3), 2.866(4), 2.873(4), 2.911(4) and 2.976(4) AA, and at larger distances to S of TiS2. The Ti-S distances of the distorted TiS6 octahedra with symmetry 1 are 2.417(2), 2.429(1) and 2.428(1) AA. Sn atoms are between rows of sulphur along a of S of TiS2. The structural relationship with other misfit layer compounds is discussed.