Fluorine-, yttrium- and lanthanide-rich cerianite-(Ce) from carbonatitic rocks of the Kerimasi volcano and surrounding explosion craters, Gregory Rift, northern Tanzania

Cerianite-(Ce), ideally CeO2, occurs as rounded grains up to 5 μm across in a block of highly altered calcite carbonatite lava from the Kerimasi volcano, and as euhedral crystals up to 200 μm across in carbonatite-derived eluvial deposits in the Kisete and Loluni explosion craters in the Gregory Rift, northern Tanzania. X-ray powder diffraction data (a = 5.434(5) Å) and Raman spectroscopy (minor vibration modes at 184 and 571 cm—1 in addition to a strong signal at 449 cm—1) suggest the presence of essential amounts of large cations and oxygen vacancies in the Kisete material. Microprobe analyses reveal that the mineral contains both light and heavy trivalent rare earth elements (REE) (7.9-15.5 wt.% LREE2O3 and 4.9-9.7 wt.% HREE2O3), and that it is enriched in yttrium (7.1 — 14.5 wt.% Y2O3) and fluorine (2.2—3.5 wt.%). Single-crystal structure refinement of the mineral confirms a fluorite-type structure with a cation—anion distance of 2.3471(6) Å. The cerianite-(Ce) is considered to be a late-stage secondary mineral in the carbonatitic rocks.

The defect structure of Y- and N-doped zirconia

The diffuse scattering of a single crystal of the quaternary system Zr–Y–O–N (Zr0.96Y0.04O1.64N0.223) has been recorded by neutron diffraction in different layers of the [1\bar{1}0] and [100] zones to study the short-range order originating from anion vacancies. A new model is proposed with uncorrelated anion vacancies and radial shifts of the surrounding ions towards or away from the vacancies. The parameters of the model were refined by fitting to the diffuse scattering intensities. The shortest Zr–anion distance is reduced, indicating covalent contributions.

Structure of AFeTi(PO4)3 (A=Ca,Cd) Nasicon phases from powder X-ray data

AFeTi(PO4)3 (A=Ca,Cd) materials were obtained by solid state reaction in air at 1000 °C. Structures of the two compounds were determined from X-ray diffraction data using Rietveld analysis. Both phases exhibit the Nasicon-type structure (R3¯c space group) with a statistical Fe(Ti) distribution within the framework. Their hexagonal cell parameters are a=8.518(1) Å, c=21.797(2) Å and a=8.534(1) Å, c=21.416(2) Å, for CaFeTi(PO4)3 and CdFeTi(PO4)3, respectively. Cd atoms occupy the M1 site in CdFeTi(PO4)3. From XRD data, it is difficult to distinguish without ambiguity between Ca2+ and Ti4+ ions in CaFeTi(PO4)3. Nevertheless from the cation–anion distance found after the structure determination, Ca2+ distribution within the M1 site of Nasicon structure are validated.

Beiträge zur Chemie der Schwefelhalogenide, 6 [1] Untersuchungen an Trichlorsulfonium(IV)-Salzen / Contributions to the Chemistry of Sulfur Halides, 6 [1] Investigations on Trichlorosulfonium(IV) Salts

The crystal structure of SCl3+AsF6- is reported.The mean S-Cl stretching frequency ν̄ of the SCl3+ cations, weighted according to the degree of degeneration, in the different SCl3+X- salts is directly proportional to the mean S-CI distance r̄scl, the force constant fSCl and the sulfur. . . anion distance (Cl3S+. . . X-).That SCI3+Cl- should be isostructural with PCl4-, the large variation in the S-CI stretching frequencies in the SCl3+X- salts is due to cation-anion interaction forces

23Na nuclear magnetic resonance linewidth studies of sodium tetraethylaluminate: complexes with hexamethylphosphoramide in benzene

The linewidth of the 23Na resonance in sodium tetraethylaluminate in benzene has been measured as a function of the ratio of [donor]: [Na+] using the strong complexing agent hexamethylphosphoramide. In this system changes in the sodium linewidth arise primarily from changes in the nuclear quadrupole relaxation at the sodium nucleus which in turn arise from changes in the electric field gradient. As the ratio of hexamethylphosphoramide to sodium ion is increased up to 3: 1 the 23Na linewidth progressively decreases due to an increase in cation–anion distance in the ion pair. After a ratio of 4:1 is reached, essentially no linewidth change is noted, indicating a stable solvation number of 4 for hexamethylphosphoramide. The results of this study indicate that this is a new and promising approach to the determination of solvation numbers of the sodium ion.

Cation and anion substitutions in the humite minerals

SummaryThe humites are structurally analogous to olivine wherein the replacement of four oxygen by four (F,OH) anions in the slightly distorted, hexagonal close-packed array is balanced by the replacement of one tetrahedrally coordinated Si by a tetrahedral void, according to the general formula Mg2xSix-1O4x-4(F,OH)4 where x = 3, 5, 7, 9. In humites the key structural units are not ‘olivine and sellaite (or brucite) layers’, as previously assumed, but are zigzag chains of edge-sharing octahedra, just as in olivines. It is shown that for humites and olivines alike the unit cell parameters a, b, and d001/n and the cell volume (normalized to one-half the mean anion-anion distance along the normal to (001)) vary linearly with the average radius of the octahedrally coordinated cation in the chain.Substitutions of (F,OH) for O and vacancies for Si have second-order effects on the unit cell parameters, causing a linear decrease of the normalized cell volume with increase in F/O ratio in the synthetic series forsterite-humite-norbergite. Comparison of the crystal structures of forsterite and norbergite shows that the polyhedral distortions in norbergite are smaller than in forsterite in accord with the decreased number of shared edges: the fluorines in norbergite are bonded to three Mg atoms whereas all anions in forsterite are bonded to three Mg and one Si atom.