Jeremy K. Cockcroft

η2-Benzyne and η1-benzylidene complexes of niobium with ancillary imido ligands

Rare η2-benzyne and benzylidene complexes of niobium have been synthesised from [Nb(C5R5)(NR′)Cl2](R = H, Me; R′= 2,6-Pri2C6H3)via intermediate diphenyl and dibenzyl species and their molecular structures have been determined: these complexes are analogues of well-established zirconocene derivatives.

Orientational disorder of C60 in Li2CsC60

The x-ray diffraction of the nonsuperconducting ternary fulleride Li2CsC60 reveals at room temperature a face-centered-cubic (Fm3m) disordered structure that persists to a temperature of 13 Kelvin. The crystal structure is best modeled as containing quasispherical [radius of 3.556(4) angstroms] C603- ions, in sharp contrast to their orientational state in superconducting face-centered-cubic K3C60 (merohedral disorder) and primitive cubic Na2CsC60 (orientational order). The orientational disorder of the carbon atoms on the C603- sphere was analyzed with symmetry-adapted spherical-harmonic functions. Excess atomic density is evident in the <111> directions, indicating strong bonding Li+—C interactions, not encountered before in any of the superconducting alkali fullerides. The intercalate-carbon interactions and the orientational state of the fullerenes have evidently affected the superconducting pair-binding mechanism in this material.

Crystal and Molecular Structures of Rhenium Heptafluoride

Rhenium heptafluoride, ReF7, is one of only two stable binary compounds MX7 with a heptacoordinated metal atom M and halide atom X. Its low-temperature crystal structure, as determined by high-resolution powder neutron diffraction, reveals the molecular structure, which has been the subject of speculation and debate for many years. Here it is shown within experimental error that at 1.5 kelvin the lowest energy configuration of ReF7 has symmetry Cs (m) and is a distorted pentagonal bipyramid. The deviation of the two axial Re-F bonds from collinear and the puckering of the ring of equatorial fluorine atoms are similar to what has been postulated as one of the conformations of the pseudorotational motion observed at higher temperatures.

THE STRUCTURE OF SOLID CARBON TETRAFLUORIDE

The structure of phase II of CF4 has been refined in the monoclinic space group C2/n (No. 15) from high-resolution powder neutron diffraction data. At 1.5 K the lattice parameters are a = 8.4098(2) angstrom, b = 4.3022(1) angstrom, c = 8.3506(2) angstrom, and beta = 119.588(1)-degrees, Z = 4. An error in the table of coordinates in the original report of the structure, which was improperly corrected in a subsequent paper, has been rectified. The refined structure of phase II is in excellent agreement with recent theoretical calculations of the crystal static energy and molecular dynamics simulations. The unit cell of the orientationally disordered phase I has been indexed as rhombohedral, a = 11.577(9) angstrom, alpha = 89.49(1)-degrees at 80 K. It is likely that this phase is isostructural with phase Ib of CCl4.

THE HINDRANCE POTENTIAL AND COUPLING OF THE NH3 QUANTUM ROTORS IN CO(NH3)6(PF6)2

The first example of transitions between almost free rotational levels of NH3 groups in the solid state is shown by the inelastic neutron scattering spectrum of Co(NH3)6(PF6)2, and can be interpreted in terms of a very weak sixfold potential. Additional satellite peaks are visible around the 0→1 rotational feature at high spectral resolution and it is shown that the temperature dependence of these satellites demonstrates that they originate from a coupling between NH3 rotors.

Rapid whole-rock mineral analysis and composition mapping by synchrotron X-ray diffraction

We show that 25–140 keV X-rays from high-brilliance synchrotron sources can penetrate through 25 mm of intact rock. Powder diffraction patterns are obtained rapidly by energy-dispersive detection. Data acquisition time is reduced by a large factor (say 102–103) compared with standard laboratory powder diffraction methods. Data are presented on sedimentary rock cores and mineral standards. Full-pattern fitting is used for quantitative modal analysis of the composition. Using acquisition times of only 20 s for each pattern, we show the feasibility of line traverse (conveyor-belt) X-ray diffraction analysis and compositional tomography with sub-millimeter resolution.

The structure of solid dichlorodifluoromethane CF2Cl2 by powder neutron diffraction

The crystal structure of solid dichlorodifluoromethane has been solved from powder neutron diffraction data. A single solid phase exists between the melting point and 1.5 K. The structure is orthorhombic, Fddl, Ζ =8 , with α =10.1676(3) A, b = 14.9638(4) Â, and c = 5.1004(2) A at 1.5 K. The molecules form columns with their dipole moments aligned ferroelectrically along the c direction.

The structure of solid tribromofluoromethane CFBr3 by powder neutron diffraction

The crystal structure of solid tribromofluoromethane, CFBr3, has been solved ab-initio from powder neutron diffraction data using direct methods. There is a single solid phase between 1.5 K and the melting point at 198.5 K. The structure is orthorhombic, space group Pnma, Z = 4, with a = 8.4931(2) angstrom, b = 9.6439(2) angstrom, and c = 6.2272(2) angstrom at 1.5 K. The crystal structure at 190 K is also reported.

The crystal structure of CBrF3 by high-resolution powder neutron diffraction

The crystal structure of solid bromotrifluoromethane, CBrF3, has been solved ab initio using direct methods and refined from high-resolution powder neutron diffraction data collected at 1·5 K. The structure is monoclinic, space group P21/a, Z = 4 with a = 8·1460(2) A, b = 5·8533(1) A, c = 7·9616(2) A and β = 111·722(2)°. Final R factors for 404 reflections in the range 10·00–155·45° 2θ are R wp = 12·4%, R exp = 3·6% and R I = 6·6%. The structure is composed of layers of packed molecules lying parallel with the ab-plane.

Electrical resistivity and magnetic ordering of gadolinium and terbium bromide deuterides, LnBrDx (23 < x ⩽ 1)

Abstract The dependence of the electrical resistivity of the layered cluster compounds LnBrDx (Ln ≡ Gd, Tb) on the D concentration has been determined. Both systems show anomalies of the resistivity for x > 0.75 indicative of magnetic phase transitions near 50 and 20 K respectively. The magnetic structures of TbBrDx with x = 0.81 and x = 0.88 have been solved using neutron diffraction. The terbium planes order ferromagnetically and are coupled antiferromagnetically to the adjacent planes of the same bilayer. The ordered components of the terbium magnetic moments are 4.3 μB for x = 0.88 and 2.6 μB for x = 0.81, while from susceptibility measurements in the paramagnetic temperature regime a value of 9.6 μB is derived. TbBrD0.69 reveals no long-range order.