Volume 216, Issue 5

The large bond valence sums found for O 2– en-capsulated by Pb 2+ ions are shown to result from the use of inappropriate bond valence parameters. New values of r o = 1.963 Å and b = 0.49 Å are recommended for Pb 2+ -O bonds.

Our knowledge of the structure of the Earth´s interior has been obtained by analysing seismic waves that travel in the Earth, and the reference Earth global models used by geophysicists are essentially seismological. Depth profiles of the seismic waves velocities reveal that the deep Earth is divided in several shells, separated by velocity and density discontinuities. The main discontinuity located at a depth of 2900 km corresponds to the transition between the mantle and the core. The Earth´s mantle can be further divided into the upper mantle and the lower mantle, with a transition zone characterised by two prominent increases in velocities observed at 410- and 660-km depths. This article will be focused on the mineral phases of the Earth´s mantle. The interpretation of seismological models in terms of chemical composition and temperature relies on the knowledge of the nature, structure and elastic properties of the candidate materials. We will describe to what extent recent advances in experimental mineral physics and X-ray diffraction have yielded essential knowledge on the structure and high-pressure high-temperature behaviour of pertinent materials, and major improvements in our understanding of the chemical and mineralogical composition of the Earth´s mantle.

The crystal structure of synthetic Rb 2 Sb 8 S 12 (S 2 ) · 2 H 2 O was determined from single crystal X-ray diffraction data: space group P 2 1 / n , Z = 2, a = 7.0750(17) Å, b = 25.403(8) Å, c = 8.0527(19) Å, β = 97.842(10)° ( R 1 = 4.88% for 1050 reflections with F o > 4 σ [F o ]). The structure is composed of slabs with formal composition [Sb 8 S 12 ] that are based on (110) PbS cut-outs of the PbS archetype, are interspaced by channels with Rb + and H 2 O, and separated by S 2 2- disulfide groups which also interconnect the slabs. Rb 2 Sb 8 S 12 (S 2 ) · 2 H 2 O is a N 1,2 = 1,2 member of the hutchinsonite family of merotypes.

The paraelastic-to-ferroelastic phase transition in the lithium vanadium oxide bronze ∊ ʹ-Li 0.527 V 2 O 5 was studied by low-temperature, high-resolution Guinier diffractometry. At the transition the symmetry changes from orthorhombic Pmmn to monoclinic P 2 1 m 11 at T c = 285(5) K. At all temperatures anisotropic broadening of the reflections occurs, indicating the presence of stacking faults in this layer structure. X-ray intensity calculations in terms of a one-dimensional model of stacking disorder were carried out in order to elucidate the structural changes induced by the phase transition, and to determine the critical behaviour of the order parameter Q . The symmetry relations between the Pmmn and P 2 1 m 11 phases allow the development of the spontaneous strain which is, in this system, proportional to the change in cos α . A bilinear coupling between e s and Q was found.

High temperature neutron investigations up to 1950 K of 3:2-sintered mullite show a smooth reversible behaviour which can be described by a rotation of more or less rigid TO 4 (T = Al(2),Si)-tetrahedra and accompanying distortions of the Al(1)O 6 -octahedra. Large atomic dispacement parameters along the c-direction, including static parts, match to respective structural modulations in the mullite structure. A comparison of a pristine and a preheated sample reveals an irreversible depopulation of the Al(2) atomic site in favour of the Al(*) site.

The crystal structure of a complex organic compound containing 27 independent non-hydrogen atoms in asymmetric unit has been solved from powder diffraction data collected at ESRF Grenoble. Structure model was found using PATSEE program. By Rietveld method the structure was completed and refined to final values of discrepancy factors R F and R wp equal to 14.6 and 13.2%, respectively. The space group is P 2 1 / c , unit cell parameters a = 11.0769(5), b = 22.387(1), c = 7.3885(3) Å, β = 91.923(4)°.

The crystal structure of sodium oxamate, NaC 2 O 3 NH 2 , has been determined from synchrotron powder diffraction data. First Guinier-Johannson photographs have been taken to derive the unit cell. Grid search and Rietveld refinement have been used to determine the structure. The crystal structure is monoclinic, P 2 1 / c , Z = 4, a = 3.5513(1) Å, b = 5.2561(2) Å, c = 20.4451(8) Å, β = 99.329(2)° and V = 376.58 Å 3 . The final R P value obtained with Rietveld refinement was 6.5 %

The characterisation of R 3 PAu(3-Hmba) [R = Cy ( 1 ) and Ph ( 2 ); 3-H 2 mba = 3-mercaptobenzoic acid] is reported. The gold atoms exist in linear arrays defined by P- and S-donor atoms. The subtle combination of steric bulk of the phosphine ligands and steric profile of the thiolate ligand is demonstrated to exert control over the appearance of hydrogen-bonded or Au...Au-mediated dimer motifs.