R. B. Von Dreele

Symmetry and cation displacements in hollandites : structure refinements of hollandite cryptomelane and priderite

Precise single-crystal X-ray structure refinements of three hollandite-type minerals have allowed a detailed study of the hoUandite structure to be made. The minerals hollandite [(Bao.75Pbo.16Nao.loKo.04)(Mn,Fe, A1)s(O,OH)16], cryptomelane [(Ko.94Nao.25Sro.13Bao.lo)(Mn,Fe,A1)s(O,OH)16], and priderite [(Ko.9oBa0.35) (Ti,Fe,Mg)sO16] were refined to residuals of R = 0.0165 (599 observations, 48 parameters), R = 0.0299 (623 observations, 53 parameters), and R = 0.0096 (316 observations, 29 parameters) respectively. The first two structures are monoclinic (I2/m) and priderite is tetragonal (I4/m). The symmetry of hollandite compounds depends on the ratio of the average ionic radius of the octahedral cations to that of the tunnel cations. Structures in which this ratio is >0.48 distort, reducing the tunnel volume, and thereby lowering the symmetry from tetragonal to monoclinic. The position occupied by a tunnel cation is determined primarily by the size of the cation. Relatively small cations, such as Ba 2+ in priderite and Pb 2+ in hollandite, displace from the special position, 2(a), to more stable sites that are at the sum of the ionic radii from the nearest O atoms. This study also indicates that the reduced form of Mn in hoUandite and cryptomelane is Mn3+; bond lengths calculated from the refinements suggest that Mn 3+ is more easily accommodated in the structures than the larger Mn 2+.

Isolation and structural elucidation of 3,6-dioxo-hexahydro-pyrrolo [1,2-a]-pyrazine from the echinodermLuidia clathrata

Es wird über die Isolierung und Charakterisierung der Titelverbindung (Gly-l-Pro Lactam, I) aus dem SeesternLuidia clathrata berichtet. Die Kristallstruktur dieses Dioxopiperazinderivats wurde ermittelt. Der Piperazinring liegt in der Wannen-Konformation vor.

STRUCTURAL PHASE TRANSITIONS AND CRYSTAL CHEMISTRY OF HFV2 AT LOW TEMPERATURES

We report a high-resolution synchrotron X-ray powder diffraction study on HfV(2), hafnium divanadium, at low temperatures. In this work we show, for the first time, a complete sequence of structural phase transitions of HfV(2) from cubic (Fd3m) to tetragonal (I4(1)/amd) to orthorhombic (Imma) in succession as temperature decreases. Peak splitting and extra diffraction peaks owing to lattice distortion can be clearly distinguished for the low-symmetry phases. The atomic positions and lattice parameters were obtained by Rietveld refinement. The bond lengths and angles of the HfV(2) crystal structure at the low-symmetry phases were correctly determined from the structure refinement. The face-centered cubic (Fd3m) unit cell (Z = 24) transforms to a body-centered tetragonal (I4(1)/amd) phase with a 45 degrees rotation relative to the cubic cell and with a reduced number of atoms (Z = 12) in the unit cell at a temperature of T = 112 K. The orthorhombic phase occurs at T = 102 K and it keeps the body-centered symmetry (Imma) and Z = 12 in the unit cell. The refinement results indicate that there may be a small amount of untransformed cubic phase left over in the lower symmetry phases. The abnormal thermal contraction of both tetragonal phase and orthorhombic phase marks the significance of structural change in HfV(2).

The crystal and molecular structure of N-acetylactinobolin: the α-helix in a small peptide

A crystal structure analysis of N-acetylactinobolin has revealed a molecular conformation which includes an N-acetyl-L-alanyl side chain in the ~t-helix conformation ((,0=-83.6 °, ~,=-24-1 °) and is controlled by molecular packing and intermolecular hydrogen bonding. The conformation of the isocoumarin ring system is in accord with the interpretation of NMR studies in both water and dimethyl sulfoxide. The structure was solved by direct methods of analysis of data from a crystal with space group P2,2t2, and a=8.708 (1), b=8.851 (1), c=21.718 (4)/~, Z = 4 and density 0c,,c= 1.364 g cm -3. Anisotropic least-squares refinement converged to a conventional residual of R= 0-064 for 2427 independent observed reflections recorded with Mo Ke radiation on an automatic four-circle diffractometer.

The structure of germanium niobium oxide, an inherently non-stoichiometric 'block' structure

Germanium niobium oxide, reported as Ge02. 9Nb20 5, is inherently nonstoichiometric since it appears to be isostructural with P 20 5. 9Nb20 5. To fit this structure, there must be vacant oxygen sites or some sites accommodating ‘interstitial’ metal atoms, in relatively high concentration, and the mode of incorporating a stoichiometric excess of cations should cast some light on other niobium oxide type structures which have a reported range of composition. The structure of germanium niobium oxide has been determined by a combination of three methods: lattice imaging electron microscopy, to establish that the non-stoichiometry was not attributable to extended defects; neutron diffraction, using the powder method and profile analysis, for particular evidence about the anion sublattice and distribution of cations; and X-ray diffraction, for an ab initio refined structure. It has been proved that the anion lattice is essentially complete, and that the cation excess is accommodated by inserting cations into a set of sites, with distorted octahedral coordination, in the square tunnels formed by the junctions between columnar elements of structure. Occupation of these octahedral sites precludes the occupation of adjacent tetrahedral cation sites, proper to the type structure. In consequence, there are constraints on the way that the two kinds of tunnel site can be occupied to produce the observed stoichiometric excess of cations. The resulting model can be generalized to interpret the metal-excess composition ranges found for other niobium oxide structures.

Synthesis, characterization and properties of the new ionic intercalation compound (NH+4)0.22TiS0.22−2☆

Abstract The new ionic intercalation compound (NH+4)0.22TiS0.22−2 has been synthesized by vacuum deintercalation of ammoniated TiS2, characterized by thermogravimetric analysis and powder X-ray diffraction, and examined by SQUID magnetometry (indirectly), differential scanning calorimetry, incoherent inelastic neutron scattering, and nuclear magnetic resonance. Although the diffraction pattern of this compound resembles a stage-II structure in which every other van der Waals gap is occupied by NH+4 it cannot be indexed completely on this basis. Magnetic and calorimetric measurements show that one electron is transferred to the host TiS2 conduction band per NH+4 cation and that the deintercalation enthalpy of NH+4 is 22 kcal/mol NH+4, respectively. Neutron scattering has provided evidence for an NH+4 torsional fundamental mode at 215 cm−1. The proton NMR linewidth and spin-lattice relaxation time are independent of temperature between 100 and 540 K, and the linewidth of 2.6 ± 0.3 G is in good agreement with that calculated for isotropic reorientation of the NH+4 cations.