Unveiling the crystal structure of ‘antimonic acid’: short- and long-range perspectives

The elusive crystal structure of the socalled “antimonic acid” has been investigated by means of robust and state-of-the-art techniques. The synergic results of solidstate magicangle spinning nuclear magnetic resonance spectroscopy and a combined Rietveld refinement from synchrotron X-ray and neutron powder diffraction data reveal that this compound contains two types of protons, in a pyrochloretype structure of stoichiometric formula (H3O)1.20(7)H0.77(9)Sb2O6. Some protons belong to heavily delocalized H3O+ subunits, while some H+ are directly bonded to the oxygen atoms of the covalent framework of the pyrochlore structure, with O − H distances close to 1 Å. A proton diffusion mechanism is proposed relying on percolation pathways determined by bondvalence energy landscape analysis. Xray absorption spectroscopy results corroborate the structural data around Sb5+ ions at shortrange order. Thermogravimetric analysis and differential scanning calorimetry endorsed the conclusions on the water content within antimonic acid. Additional 0.7 water molecules per formula were assessed as moisture water by thermal analysis.

Crystal-chemistry and short-range order of fluoro-edenite and fluoro-pargasite: a combined X-ray diffraction and FTIR spectroscopic approach

This study addresses the crystal chemistry of a set of five samples of F-rich amphiboles from the Franklin marble (USA), using a combination of microchemical (Electron microprobe analysis (EMPA)), single-crystal refinement (SREF) and Fourier transform infrared (FTIR) spectroscopy methods. The EMPA data show that three samples fall into the compositional field of fluoro-edenite (Hawthorne et al., 2012), whereas two samples are enriched in high-charged C cations and – although very close to theCR3+boundary – must be classified as fluoro-pargasite. Magnesium is by far the dominant C cation, Ca is the dominant B cation (withBNa in the range 0.00−0.05 a.p.f.u., atoms per formula unit) and Na is the dominant A cation, withA☐ (vacancy) in the range 0.07−0.21 a.p.f.u.;WF is in the range 1.18−1.46 a.p.f.u. SREF data show that: TAl is completely ordered at the T(1) site; theM(1) site is occupied only by divalent cations (Mg and Fe2+);CAl is disordered between theM(2) andM(3) sites;ANa is ordered at the A(m) site, as expected in F-rich compositions. The FTIR spectra show a triplet of intense and sharp components at ~3690, 3675 and 3660 cm−1, which are assigned to the amphibole and the systematic presence of two very broad absorptions at 3560 and 3430 cm−1. These latter are assigned, on the basis of polarized measurements and FPA (focal plane array) imaging, to chlorite-type inclusions within the amphibole matrix. Up to eight components can be fitted to the spectra; band assignment based on previous literature on similar compositions shows thatCAl is disordered over theM(2) andM(3) sites, thus supporting the SREF conclusions based on the <M−O> bond distance analysis. The measured frequencies of all components are typical of O−H groups pointing towards Si−O(7)−Al tetrahedral linkages, thus allowing characterization of the SRO (shortrange- order) ofTAl in the double chain. Accordingly, the spectra show that in the fluoro-edenite/ pargasite structure, the T cations, Si and Al, are ordered in such a way that Si−O(7)−Si linkages regularly alternate with Si−O(7)−Al linkages along the double chain.

TEM Study of Short-Range-Order in Zirconolite Induced by High Temperature Ion Irradiation

Zirconolite (CaZrTi207) is an important phase proposed for high level nuclear waste immobilization. Zirconolite was irradiated by 1 MeV Kr+ at various temperatures. At room temperature, zirconolite became amorphous after a dose of 7x1014 ions/cm2.1 Amorphization dose increased with temperature due to thermal annealing. The critical temperature, above which amorphization does not occur, was estimated to be 654 K. During the low temperature irradiation (<654 K), concurrent with amorphization, zirconolite transformed from a monoclinic structure to the cubic pyrochlore structure and then to the fluorite substructure. The structural change is due to the disordering between cations and between oxygen and oxygen vacancies.After an irradiation at 673 K to a dose of 3.6x1015 ions/cm, the zirconolite samples remained crystalline. The diffraction pattern consists of strong maxima from the fluorite structure and diffuse maxima surrounding the Bragg positions of the pyrochlore superlattice (FIG. 1). Diffuse scattering patterns have been reported in other phases, and were generally attributed to the shortrange- order (SRO) domains.

Influence of Plural Scattering on the Image Quality of Thick Amorphous Objects in Transmission Electron Microscopy

The ultimate goal of electron microscopy is to elucidate the three-dimensional atomic structure of arbitrary objects. Objects are generally classified with respect to their inherent structural symmetries. The extreme cases of total order and total disorder are perfect crystals and entirely amorphous objects. In the former case the atoms are arranged in a periodic order; in the latter they are distributed at random. Neither perfect crystals nor completely amorphous solid objects exist in reality. The finite size of the atoms gives rise in every solid amorphous object to a certain shortrange order that rules out complete disorder. The amorphous carbon foils commonly used as supporting films are an example of this behavior.

Structure of thallium amalgams and their reaction with oxygen

The stoichiometry was determined for the oxide formed by bubbling oxygen through thallium amalgam until the wt.% thallium in the amalgam was reduced by about 2%. The composition of the oxide depended upon the wt.% Tl in the amalgam. In the most dilute amalgams the oxide approached the formula Tl2O and as the wt.% Tl increased the oxide became richer in thallium(III). Between 20 and 29 wt.% Tl the oxide approximated Tl3O2 and for alloys richer in thallium the oxide became richer in thallium(III). When the wt.% Tl in the alloy was 41.0 the ratio of thallium to oxygen in the oxide was 1.19. It is suggested that the experimental results indicate the existence of shortrange order in the liquid.