323,801 research outputs found
Tracce di quartieri. Il legame sociale nella città che cambia
No full texttrasformazioni sociali nei quartieri suburbani o nelle porzioni di città medi
Elastic behavior and high pressure-induced phase transition in chabazite: new data from a sample from Nova Scotia
No full textRecently, the high pressure (HP) behavior of a natural chabazite from Vallerano [1] (VALL), and on the synthetic
phases SAPO-34 [1] and ALPO-34 [2], was investigated in the frame of a wider project aimed at understanding
the role of the framework/extraframework content on the compressibility of CHA-type porous materials.
In this work, further structural information is obtained studying the response to HP of another natural chabazite
sample from Nova Scotia (Canada) (NS) (s.g. R-3mR [3]), characterized by a different chemical composition with
respect to VALL. The study was performed by means of in situ synchrotron X-ray powder diffraction (XRPD) and
silicone oil as non-penetrating P-transmitting medium. XRPD experiments were performed in DAC at the BM01
beamline at ESRF (Grenoble, France) with a fixed =0.7355 Å. Powder patterns were collected from Pambup to 8.6
GPa and upon decompression. All the features of the Pamb pattern and the unit cell parameters are well recovered
upon P release. Below 2.1 GPa, a and cell parameters slightly decrease and increase respectively with a resulting
volume reduction of 3.6 %. Above 2.1 GPa, a transition to a triclinic P-1 pseudo-rhombohedral phase is observed.
The rhombohedral to triclinic phase transition is accompanied by an abrupt decrease in the unit cell parameters and
in the unit cell volume (V=-4.0%). Between 2.5 and 5.9 GPa, the triclinic/pseudo-rhombohedral cell parameters
decrease regularly and the unit cell volume variation (V=-3.0%) indicates a lower compressibility with respect to
that observed before the transition. In the highest P regime (5.9-7.2 GPa), a further slope change, with an increase
in compressibility, is observed. As a whole, V between Pamb and 7.2 GPa is -12.6%. The elastic parameters,
calculated with a second order BM-EoS, are V0 = 826 (1) Å3, K0 = 54(3) GPa and V0 = 784(2) Å3, K0 = 91(5)
GPa, for the rhombohedral and triclinic phase, respectively.
Preliminary results from Rietveld refinements up to about 1 GPa, suggest that the deformation mechanism acting
in the low-P regime is a cooperative tilting of the tetrahedra belonging to the double 6-ring – resulting in a decrease
of its thickness – accompanied by a simultaneous di-trigonalization of the two 6-rings. A similar mechanism was
previously observed during compression of levyne [4].
The HP-induced cell volume contraction of NS (12.6 %) is higher than that of VALL (10.3%) in the same P range.
This is congruent with the lower content in large extraframework potassium cations of NS, which contribute to
sustain the porous structure in VALL.
[1] L. Leardini, S. Quartieri, G. Vezzalini, Micropor. Mesopor. Mater. 127 (2010) 219-227
[2] L. Leardini, S. Quartieri, A. Martucci, G. Vezzalini, V. Dmitriev, Z. Kristallogr. (2012), doi:
10.1524/zkri.2012.1477.
[3] A. F. Gualtieri, E. Passaglia, Eur. J. Mineral. 18 (2006) 351–359.
[4] G. D. Gatta, P. Comodi, P. F. Zanazzi, T. F. Ballaran, Am. Miner. 90 (2005) 645-652
Dachiardite from Hokiya-Dake: Evidence of a new topology
No full textA refinement of the crystal structure of dachiardite from Hokiya-dak, Japan, shows the presence (16% modal abundance) of domains of the hypothetical structure called "modified" dachiardite by S. Merlino (1975, 1976) assocd. with domains of "normal" dachiardite. As in the Elba dachiardite, 2 acentric framework configurations are present with the same frequency in the Hokiya-dake dachiardite, so that the statistical symmetry C2/m is maintained. This feature is interpreted as a tendency (common to all zeolites) to avoid T-O-T angles of 180° in the topol. C2/m symmetry of dachiardite. As with the related structures of mordenite and epistilbite, a remarkable Al-enrichment is found in the tetrahydrons of the 4-membered ring
Crystal chemistry of stilbites: structure refinements of one normal and four chemically anomalous samples
No full textStructural refinements were performed on 4 stilbites with anomalous chem. compns. as regards the monovalent/divalent cation ratio, and a normal stilbite. The 4 anomalous samples showed, moreover, a monoclinic angle lower than that expected from an empirical correlation between monovalent cation content and β angle. In all samples the 2 cationic sites characteristic of stilbite, were found; in only 1 sample was a new cationic site localized. One site is fully occupied by Ca alone, or, with some Na, whereas the other sites are fully or partially occupied by monovalent cations only. In the 5 stilbites, different nos. of water mol. sites were localized. The frameworks, however, seem to be unaffected by the no. and compn. of extra framework sites. The small β angle of the anomalous samples was only partially explained by structural data
Modello di laser a CO2 di grande potenza
No full textSi presenta il modello numerico unidimensionale di un laser continuo a CO2 d’alta potenza con preionizzazione a fascio elettronico. Il codice simula il comportamento di una miscela di He:N2:CO2:CO in rapido flusso (≈100 m/s) in un condotto in cui viene addotta energia tramite scarica elettrica e prodotti elettroni secondari per mezzo di un fascio di elettroni energetici (≈60 keV). La densità degli elettroni prodotti nella camera di scarica è valutata facendo uso della teoria di Bethe secondo la formulazione di Rorhlich e Carlson. La densità degli elettroni della scarica elettrica è calcolata nella ipotesi di neutralità di carica locale tenendo conto dei processi di perdita per ricombinazione ione-elettrone ed attaccamento elettrone-molecola. La cinetica vibrazionale è descritta tramite un set di rate-equations che tengono conto della variazione, lungo la direzione di flusso della miscela, dell’energia immagazzinata in ciascun modo di vibrazione. L’insieme di equazioni cinetiche è accoppiato tramite l’equazione dell’entalpia al set di equazioni fluidodinamiche. Si presentano alcuni risultati forniti dal codice numerico quali ad esempio il coefficiente di guadagno per piccolo segnale, il flusso di saturazione, etc., e si confrontano con i dati sperimentali disponibili
Crystal structure of a potassium-rich natural gmelinite and comparison with the other refined gmelinite samples
No full textA complete crystallochem. characterization of a gmelinite sample from Fara Vicentina (Vicenza, Italy) is given. The mineral is anomalous for the prevalence of K (2.72 atoms per unit cell) among the exchangeable cations. The lattice consts. are a = 13.621(3) and c = 10.254(1) Å. Its crystal structure was refined down to Rw = 5.92% and compared with the structures reported for Na-, Ca- and K-rich gmelinites in the literature; the sample studied is characterized by a very low occupancy (3%) of 1 (C2) of the 2 exchangeable cation sites and by water mols. distributed among 8 partially occupied sites. A correlation involving the C2 site and the geometry of the tetrahedral framework was found between the bond strengths: with decrease in the bond strength, the shape of the 8-ring of the gmelinite cage becomes more circular, the double 6-ring becomes more hexagonal and, consequently, the c parameter increases and the a parameter decreases. Post-crystn. cation exchanges explain the present chem. compn. of the mineral
“ImPACT” FIRB Futuro in Ricerca: Pressure-induced supermolecular organization of water and ethanol in all-silica zeolites.
No full textIn situ LS-XRPD experiments in high pressure using the pressure transmitting media methanol:ethanol:water 16:3:1 and ethanol:water 3:1. Penetration of water and ethanol molecules and formation of water and ethanol chains
Tetrahedral ferric iron in phlogopite: Xanes and Mossbauer compared to single crystal X-ray data
No full textThis paper reports the results of a combined methodological study performed on tetraferriphlogopite from the Araxa carbonatite. We compare Mossbauer spectroscopy and X-ray absorption near-edge spectroscopy (XANES) results to previously reported single-crystal X-ray and electron microprobe analysis data (Cruciani & Zanazzi, 1994). The combined techniques provide evidence for the presence of a remarkable amount of tetrahedrally coordinated Fe3+ in this sample, and show that high OH content in tetraferriphlogopite is compatible with the large Fe2O3/FeO ratio inferred from the chemical and spectroscopic data
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