Toward the Synthesis of New Zeolite Structures in the Presence of Cesium: Zeolite MMU-1

Nanosized small-pore zeolites RHO and MMU-1 (a new Cs-Na modification of an EDI-type zeolite structure) were prepared in the presence of cesium hydroxide by changing the molar composition of the precursor zeolite solution. XRD, Raman, and FTIR results indicated that Cs had a structure-directing effect and controlled the formation of the respective zeolite phases. The presence of water played a crucial role, and when the amount of water was under a certain level, the zeolite cancrinite was formed. Solvent-free syntheses were attempted, and the results confirmed the conclusions drawn based on the water-mediated syntheses, namely that both Cs and water determine the nature of the product formed. The structure of the new zeolite MMU-1 was resolved by Rietveld refinement. MMU-1 was found to exhibit tetragonal symmetry, space group P42$_{1}$c (No. 114), and its framework is composed of four-membered and eight-membered TO$_{4}$-rings. The results reported suggest that there is the potential to synthesize other novel small-pore zeolite structures in the presence of cesium hydroxide by modifications of the chemical composition of the zeolite precursor solution

Polarons in Rock-Forming Minerals: Physical Implications

The existence of thermally-activated quasiparticles in amphiboles is an important issue, as amphiboles are among the main hydrous complex silicate minerals in the Earth's lithosphere. The amphibole structure consists of stripes of 6-membered TO4-rings sandwiching MO6 octahedral slabs. To elucidate the atomistic origin of the anomalous rock conductivity in subduction-wedge regions, we studied several Fe-containing amphiboles with diverse chemistry by using in situ, temperature-dependent, polarised Raman spectroscopy. The occurrence of resonance Raman scattering at high temperatures unambiguously reveal temperature-activated small polarons arising from the coupling between polar optical phonons and electron transitions within Fe2+O6 octahedra, independently of the amphibole chemical composition. The FeO6-related polarons coexist with delocalised H+; that is, at elevated temperatures Fe-bearing amphiboles are conductive and exhibit two types of charge carriers: electronic polarons with highly anisotropic mobility and H+ cations. The results from density-functional-theory calculations on the electron band structure for a selected amphibole compound with a relatively simple composition are in full agreement with experimental data. The polaron activation temperature, mobility, and polaron-dipole magnitude and alignment can be controlled by varying the mineral composition, which makes amphiboles attractive "geo-stripes" that can serve as mineral-inspired technology to design thermally-stable smart materials with anisotropic properties

Toward the Synthesis of New Zeolite Structures in the Presence of Cesium: Zeolite MMU-1

Nanosized small-pore zeolites RHO and MMU-1 (a new Cs-Na modification of an EDI-type zeolite structure) were prepared in the presence of cesium hydroxide by changing the molar composition of the precursor zeolite solution. XRD, Raman, and FTIR results indicated that Cs had a structure-directing effect and controlled the formation of the respective zeolite phases. The presence of water played a crucial role, and when the amount of water was under a certain level, the zeolite cancrinite was formed. Solvent-free syntheses were attempted, and the results confirmed the conclusions drawn based on the water-mediated syntheses, namely that both Cs and water determine the nature of the product formed. The structure of the new zeolite MMU-1 was resolved by Rietveld refinement. MMU-1 was found to exhibit tetragonal symmetry, space group P421c (No. 114), and its framework is composed of four-membered and eight-membered TO4-rings. The results reported suggest that there is the potential to synthesize other novel small-pore zeolite structures in the presence of cesium hydroxide by modifications of the chemical composition of the zeolite precursor solution

The effect of the A-Site cation on the structural transformations in ABi4Ti4O15 (A¼ Ba, Sr): Raman scattering studies

The effect of the type of A-site cation on the structural transformations of ABi4Ti4O15 (A¼ Ba, Sr) was studied by a comparative analysis of the Raman scattering of BaBi4Ti4O15 and SrBi4Ti4O15 in the temperature range 120–850 K. The results are also compared with those previously reported on PbBi4Ti4O15. Similar to PbBi4Ti4O15, BaBi4- Ti4O15, and SrBi4Ti4O15 exhibit an additional structural transformation at Ta < Tc and Ta decreases with the increase in the ionic radius of the A-site cation. This structural alteration consists of rearrangements of the A-site cations, which for A¼ Ba is accompanied by readjustment of the octahedral tilting and a change in the BO6 geometry. The results presented here confirm that the development of spontaneous polarization in four-layer Aurivillius-type ferroelectrics is triggered by the rigid-layer phonon mode, i.e., vibrations of the Bi2O2 planes relative to the perovskite-like blocks, rather than by cationic vibrations only within the perovskites blocks

Using the elastic properties of zircon-garnet host-inclusion pairs for thermobarometry of the ultrahigh-pressure Dora-Maira whiteschists: problems and perspectives

The ultrahigh-pressure (UHP) whiteschists of the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps) provide a natural laboratory in which to compare results from classical pressure (P)–temperature (T) determinations through thermodynamic modelling with the emerging field of elastic thermobarometry. Phase equilibria and chemical composition of three garnet megablasts coupled with Zr-in-rutile thermometry of inclusions constrain garnet growth within a narrow P–T range at 3–3.5 GPa and 675–720 °C. On the other hand, the zircon-in-garnet host-inclusion system combined with Zr-in-rutile thermometry would suggest inclusion entrapment conditions below 1.5 GPa and 650 °C that are inconsistent with the thermodynamic modelling and the occurrence of coesite as inclusion in the garnet rims. The observed distribution of inclusion pressures cannot be explained by either zircon metamictization, or by the presence of fluids in the inclusions. Comparison of the measured inclusion strains with numerical simulations shows that post-entrapment plastic relaxation of garnet from metamorphic peak conditions down to 0.5 GPa and 600–650 °C, on the retrograde path, best explains the measured inclusion pressures and their disagreement with the results of phase equilibria modelling. This study suggests that the zircon-garnet couple is more reliable at relatively low temperatures (&lt; 600 °C), where entrapment conditions are well preserved but chemical equilibration might be sluggish. On the other hand, thermodynamic modelling appears to be better suited for higher temperatures where rock-scale equilibrium can be achieved more easily but the local plasticity of the host-inclusion system might prevent the preservation of the signal of peak metamorphic conditions in the stress state of inclusions. Currently, we cannot define a precise threshold temperature for resetting of inclusion pressures. However, the application of both chemical and elastic thermobarometry allows a more detailed interpretation of metamorphic P–T paths.The ultrahigh-pressure (UHP) whiteschists of the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps) provide a natural laboratory in which to compare results from classical pressure (P)-temperature (T) determinations through thermodynamic modelling with the emerging field of elastic thermobarometry. Phase equilibria and chemical composition of three garnet megablasts coupled with Zr-in-rutile thermometry of inclusions constrain garnet growth within a narrow P-T range at 3-3.5 GPa and 675-720 degrees C. On the other hand, the zircon-in-garnet host-inclusion system combined with Zr-in-rutile thermometry would suggest inclusion entrapment conditions below 1.5 GPa and 650 degrees C that are inconsistent with the thermodynamic modelling and the occurrence of coesite as inclusion in the garnet rims. The observed distribution of inclusion pressures cannot be explained by either zircon metamictization, or by the presence of fluids in the inclusions. Comparison of the measured inclusion strains with numerical simulations shows that post-entrapment plastic relaxation of garnet from metamorphic peak conditions down to 0.5 GPa and 600-650 degrees C, on the retrograde path, best explains the measured inclusion pressures and their disagreement with the results of phase equilibria modelling. This study suggests that the zircon-garnet couple is more reliable at relatively low temperatures (&lt; 600 degrees C), where entrapment conditions are well preserved but chemical equilibration might be sluggish. On the other hand, thermodynamic modelling appears to be better suited for higher temperatures where rock-scale equilibrium can be achieved more easily but the local plasticity of the host-inclusion system might prevent the preservation of the signal of peak metamorphic conditions in the stress state of inclusions. Currently, we cannot define a precise threshold temperature for resetting of inclusion pressures. However, the application of both chemical and elastic thermobarometry allows a more detailed interpretation of metamorphic P-T paths

The true structural relationship between zircon and scheelite structure types, and a new polymorph of zircon

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Raman scattering study of the effect of A- and B-site substitution on the room-temperature structure of ABi4Ti4O15

Aurivillius-type materials exhibit promising ferroelectric and multiferroic properties that can be tailored via chemistry variations in the perovskite block. Hence, it is important to clarify the relations composition-structure, also on a local-scale level. The aim of this contribution is to give further insights into the effect of A- and B-site cations to the room-temperature local structure of Aurivillius four-layered ABi4Ti4O15 (A = Sr, Pb, Ba) and Pb1-xBi4+xTi4-xMn x O15 (x = 0, 0.2, 0.4) by Raman scattering. The effect of A-site cation to the local structure of perovskite block was identified by the phonon mode near 750 and 870 cm-1 arising from BO6 stretching. A-site Ba2+, having the largest ionic radius among the considered elements, significantly stiffens the TiO6 octahedra, as derived from the fact that the TiO6 stretching modes have the highest wavenumber for BaBi4Ti4O15, i.e. the Ti-O bond strength is strongest for this compound. The replacement of Ti4+ by Mn3+ cation at the B- site also influences the B-O bond. The comparison of the phonon modes near 700 and 870 cm-1 in Pb1-xBi4+xTi4-xMn x O15 with x = 0, 0.2, and 0.4 shows that the lowest wavenumber, which is due to the elongation of Ti-O bonds is observed for x = 0.4

Silicalite-1 macrostructures – preparation and structural features

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