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An in situ HT-HP single crystal X-ray diffraction study of armstrongite, a microporous zirconium silicate
No full textArmstrongite, CaZr[Si6O15]·2H2O, is a natural “zeolite-like” Zr-silicate with a heteropolyhedral framework consisting of SiO4 tetrahedra and ZrO6 octahedra that form cavities occupied by Ca-exchangeable cations (Mesto et al., 2014). The behavior at non ambient conditions of armstrongite from Khan Bogdo deposit (Gobi, Mongolia) was studied by in-situ High Temperature Single Crystal X-ray Diffraction (HT SCXRD), both in air and under dry conditions up to 500°C and 375 °C respectively, and by in-situ High Pressure Single Crystal X-ray Diffraction (HP SCXRD) using synchrotron X-ray diffraction data (collected up to 8.01 GPa), a diamond anvil cell and the mix methanol:ethanol:water as hydrostatic pressure-transmitting fluid.
On heating an abrupt discontinuity in the trend of the cell parameters and unit-cell volume occurs at T = 275°C in dry condition and at T = 450°C in air. The cell volume decreased by ~7.5%, compared to that measured at RT, and is compatible with the loss of the two water molecules. The dehydrated phase (solved and refined at 275°C only under dry conditions) exhibits the same space group (C2/m) as RT armstrongite, significantly shortened a and b cell dimensions, increased β angle, and smaller unit-cell volume (a = 13.406(3), b = 13.752(3), c = 7.811(2) Å, β = 110.22(3)°, V = 1351.3(5) Å3) with respect to the hydrated phase (a = 14.0135(7), b = 14.1234(6), c = 7.8388(4) Å, β = 109.401(4)°, V = 1463.4(1) Å3) at RT. The process is also accompanied by the distortion of the cavities as a consequence of Ca splitting and positional disorder of tetrahedral framework oxygens. The dehydration/rehydration process of armstrongite is completely reversible as also found from previous HT XRPD investigation (Schingaro et al., 2018).
HP SCXRD data show a first-order phase transition between 4.01(5) and 5.07(5) GPa. In the high-pressure polymorph, the unit-cell volume triplicates. The bulk compression of armstrongite is mainly accommodated through the tilting of both SiO4 tetrahedra and ZrO6 octahedra around the shared oxygen hinges. The high-P polymorph of armstrongite is found to be stiffer (KV0 increase of ~ 66%), and a remarkable change of the elastic anisotropic scheme occurs. No evidence of crystal-fluid interaction, with a selective sorption of molecules of the pressure-transmitting fluid through the cavities, was observed.
Mesto, E., Kaneva, E., Schingaro, E., Vladykin, N., Lacalamita, M. & Scordari, F. (2014): Armstrongite from Khan
Bogdo (Mongolia): crystal structure determination and implications for zeolite-like cation exchange properties. Am.
Mineral., 99, 2424-2432.
Schingaro, E., Lacalamita, M., Mesto, E. & Della Ventura, G. (2018): Thermal stability and dehydration of armstrongite,
a microporous zirconium silicate. Micropor. Mesopor. Mat., in press
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The betti base: absolute configuration and routes to a family of related chiral nonracemic bases
No full textA detailed protocol for the resolution of the Betti base [i.e. 1-(alpha-aminobenzyl)-2-naphthol] was investigated and the absolute configuration of the two isomers was established by means of an X-ray diffractometric study. A series of optically active derivatives was also prepared. The list includes the N-benzyl- and the N,N,O-trimethyl derivatives. The N,N-dimethyl derivative was prepared in racemic form by means of the Betti reaction and was resolved into the two enantiomers with an extremely easy and efficient procedure. The O-methyl derivative was prepared in a racemic form and subjected to resolution. The configuration of each base was determined by correlation with the configuration of the Betti amine. (C) 1998 Elsevier Science Ltd. All rights reserved
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe23+(H2O)4[O(SO4)2]
No full textThe iron sulfate FeOHSO4 studied was obtained as a dehydration product of metahohmannite Fe2(H2O)4[O(SO4)2] during a synchrotron real-time powder diffraction experiment. As quoted in the literature, FeOHSO4 has iron atoms octahedrally coordinated with two hydroxyl groups and four sulfate O atoms, while each hydroxyl group is bonded to two iron atoms. This compound is commonly described in the orthorhombic system with space group Pnma, lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å (aJ, bJ, and cJ are the Johansson lattice parameters), and Z = 4. However a preliminary Rietveld refinement of the pattern at about 220 °C using the structural model from the literature yielded a poor fit of the observed data and a final Rp value of about 23%. A careful analysis of the calculated powder diffraction pattern showed unexpected peaks, not observed in the experimental trace, for h = 2n + 1, while sharp reflections for h = 2n seemed to point to different lattice constants and space group. The recognition of the order-disorder character of the FeOHSO4 compound was the key to successfully interpreting the unexpected features of the experimental powder pattern and the misfit with respect to the calculated pattern. In fact, FeOHSO4 belongs to a family of OD structures formed by equivalent layers of symmetry Pbmm. Only two MDO (Maximum Degree of Order) polytypes are possible. MDO1 results from a regular alternation of stacking operators 21/2 and 2−1/2, and yields an orthorhombic structure with space group Pnma and lattice parameters aJ = 7.33, bJ = 6.42, and cJ = 7.14 Å. MDO2 results from the 21/2|21/2 |21/2... sequence of symmetry operators and yields a monoclinic structure with space group P21/c, aM = 7.33, bM = 7.14, cM = 7.39 Å, and β = 119.7°.
The analysis of one-dimensional stacking disorder was performed by fitting the observed XRPD pattern with a calculated intensity curve generated by DIFFaX. The disorder model was investigated by taking into account a probability matrix for the occurrence of OD layer sequences. The best fit (Rp = 0.009) to the observed powder pattern was obtained with a 61:39 ratio of monoclinic and orthorhombic polytypes for a fully disordered OD layers sequence
Phlogopite from the Ventaruolo subsystem volcanics (Mt Vulture, Italy): a multi-method study
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