Explore the vast range of titles available.

The elastic behaviour and the structural evolution of microporous materials compressed hydrostatically in a pressure-transmitting fluid are drastically affected by the potential crystal–fluid interaction, with a penetration of new molecules through the zeolitic cavities in response to applied pressure. In this manuscript, the principal mechanisms that govern the P -behaviour of zeolites with and without crystal–fluid interaction are described, on the basis of previous experimental findings and computational modelling studies. When no crystal–fluid interaction occurs, the effects of pressure are mainly accommodated by tilting of (quasi-rigid) tetrahedra around O atoms that behave as hinges. Tilting of tetrahedra is the dominant mechanism at low-mid P -regime, whereas distortion and compression of tetrahedra represent the mechanisms which usually dominate the mid-high P regime. One of the most common deformation mechanisms in zeolitic framework is the increase of channels ellipticity. The deformation mechanisms are dictated by the topological configuration of the tetrahedral framework; however, the compressibility of the cavities is controlled by the nature and bonding configuration of the ionic and molecular content, resulting in different unit-cell volume compressibility in isotypic structures. The experimental results pertaining to compression in “penetrating” fluids, and thus with crystal–fluid interaction, showed that not all the zeolites experience a P -induced intrusion of new monoatomic species or molecules from the P -transmitting fluids. For example, zeolites with well-stuffed channels at room conditions (e.g. natural zeolites) tend to hinder the penetration of new species through the zeolitic cavities. Several variables govern the sorption phenomena at high pressure, among those: the “free diameters” of the framework cavities, the chemical nature and the configuration of the extra-framework population, the partial pressure of the penetrating molecule in the fluid (if mixed with other non-penetrating molecules), the rate of P -increase, the surface/volume ratio of the crystallites under investigations and the temperature at which the experiment is conducted. An overview of the intrusion phenomena of monoatomic species (e.g. He, Ar, Kr), small (e.g. H 2 O, CO 2 ) and complex molecules, along with the P -induced polymerization phenomena (e.g. C 2 H 2 , C 2 H 4 , C 2 H 6 O, C 2 H 6 O 2 , BNH 6 , electrolytic MgCl 2 ·21H 2 O solution) is provided, with a discussion of potential technological and geological implications of these experimental findings.

A Cs-bearing polyphase aggregate with composition (in wt%): 76(1) CsAlSi5O12  + 7(1) CsAlSi2O6  + 17(1) amorphous , was obtained from a clinoptilolite-rich epiclastic rock after a beneficiation process of the starting material (aimed to increase the fraction of zeolite to 90 wt%), cation exchange and then thermal treatment. CsAlSi 5 O 12 is an open-framework compound with CAS topology; CsAlSi 2 O 6 is a pollucite-like material with ANA topology. The thermal stability of this polyphase material was investigated by in situ high- T X-ray powder diffraction, the combined P – T effects by a series of runs with a single-stage piston cylinder apparatus, and its chemical stability following the “availability test” (“AVA test”) protocol. A series of additional investigations were performed by WDS–electron microprobe analysis in order to describe the P – T -induced modification of the material texture, and to chemically characterize the starting material and the run products. The “AVA tests” of the polyphase aggregate show an extremely modest release of Cs + : 0.05 mg/g. In response to applied temperature and at room P , CsAlSi 5 O 12 experiences an unquenchable and displacive Ama 2-to- Amam phase transition at about 770 K, and the Amam polymorph is stable in its crystalline form up to 1600 K; a crystalline-to-amorphous phase transition occurs between 1600 and 1650 K. In response to the applied P  = 0.5 GPa, the crystalline-to-amorphous transition of CsAlSi 5 O 12 occurs between 1670 and 1770 K. This leads to a positive Clapeyron slope (i.e., d P /d T  > 0) of the crystalline-to-amorphous transition. When the polyphase aggregate is subjected at P  = 0.5 GPa and T  > 1770 K, CsAlSi 5 O 12 melts and only CsAlSi 2 O 6 (pollucite-like; dominant) and Cs-rich glass (subordinate) are observed in the quenched sample. Based on its thermo-elastic behavior, P – T phase stability fields, and Cs + retention capacity, CsAlSi 5 O 12 is a possible candidate for use in the immobilization of radioactive isotopes of Cs, or as potential solid hosts for 137 Cs γ-radiation source in sterilization applications. More in general, even the CsAlSi 5 O 12 -rich aggregate obtained by a clinoptilolite-rich epiclastic rock appears to be suitable for this type of utilizations.

High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO 3 were investigated by analytical in situ high-pressure high-temperature experiments on oriented single-crystal samples. All experiments at non-ambient conditions were carried out by means of Raman scattering, X-ray, and synchrotron diffraction techniques using diamond-anvil cells in the pressure range up to 6.5 GPa. The composite-gasket resistive heating technique was applied for all high-pressure investigations at temperatures up to 550 K. High-pressure Raman spectra reveal distinguishable characteristic spectral differences located in the wave number range of external modes with the occurrence of band splitting and shoulders due to subtle symmetry changes. Constraints from in situ observations suggest a stability field of CaCO 3 -IIIb at relatively low temperatures adjacent to the calcite-II field. Isothermal compression of calcite provides the sequence from I to II, IIIb, and finally, III, with all transformations showing volume discontinuities. Re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to determine the transition sequence. Clausius–Clapeyron slopes of the phase boundary lines were determined as: Δ P /Δ T  = −2.79 ± 0.28 × 10 −3  GPa K −1 (I–II); +1.87 ± 0.31 × 10 −3  GPa K −1 (II/III); +4.01 ± 0.5 × 10 −3  GPa K −1 (II/IIIb); −33.9 ± 0.4 × 10 −3  GPa K −1 (IIIb/III). The triple point between phases II, IIIb, and III was determined by intersection and is located at 2.01(7) GPa/338(5) K. The pathway of transition from I over II to IIIb can be interpreted by displacement with small shear involved (by 2.9° on I/II and by 8.2° on II/IIIb). The former triad of calcite-I corresponds to the [20-1] direction in the P 2 1 / c unit cell of phase II and to [101] in the pseudomonoclinic C 1 ¯ setting of phase IIIb. Crystal structure investigations of triclinic CaCO 3 -III at non-ambient pressure–temperature conditions confirm the reported structure, and the small changes associated with the variation in P and T explain the broad stability of this structure with respect to variations in P and T . PVT equation of state parameters was determined from experimental data points in the range of 2.20–6.50 GPa at 298–405 K providing K T 0  = 87.5(5.1) GPa, ( δK T / δT) P  = −0.21(0.23) GPa K −1 , α 0  = 0.8(21.4) × 10 −5  K −1 , and α 1  = 1.0(3.7) × 10 −7  K −1 using a second-order Birch–Murnaghan equation of state formalism.

We consider T–shaped, two–dimensional quantum waveguides containing attractive or repulsive impurities with a smooth, realistic shape, and study how the resonance behavior of the total conductance depends upon the strength of the defect potential and the geometry of the device. The resonance parameters are determined locating the relevant S–matrix poles in the Riemann energy surface. The total scattering operator is obtained from the S–matrices of the various constituent segments of the device through the \\(\\)–product composition rule. This allows for a numerically stable evaluation of the scattering matrix and of the resonance parameters.

In situ high-pressure investigations on norsethite, BaMg(CO 3 ) 2 , have been performed in sequence of diamond-anvil cell experiments by means of single-crystal X-ray and synchrotron diffraction and Raman spectroscopy. Isothermal hydrostatic compression at room temperature yields a high-pressure phase transition at P c  ≈ 2.32 ± 0.04 GPa, which is weakly first order in character and reveals significant elastic softening of the high-pressure form of norsethite. X-ray structure determination reveals C 2 /c symmetry (Z = 4; a  = 8.6522(14) Å, b  = 4.9774(13) Å, c  = 11.1542(9) Å, β  = 104.928(8)°, V  = 464.20(12) Å 3 at 3.00 GPa), and the structure refinement ( R 1  = 0.0763) confirms a distorted, but topologically similar crystal structure of the so-called γ-norsethite, with Ba in 12-fold and Mg in octahedral coordination. The CO 3 groups were found to get tilted off the ab- plane direction by ~16.5°. Positional shifts, in particular of the Ba atoms and the three crystallographically independent oxygen sites, give a higher flexibility for atomic displacements, from which both the relatively higher compressibility and the remarkable softening originate. The corresponding bulk moduli are K 0  = 66.2 ± 2.3 GPa and d K/ d P  = 2.0 ± 1.8 for α-norsethite and K 0  = 41.9 ± 0.4 GPa and d K/ d P  = 6.1 ± 0.3 for γ-norsethite, displaying a pronounced directional anisotropy (α: β a −1  = 444(53) GPa, β c −1  = 76(2) GPa; γ: β a −1  = 5.1(1.3) × 10 3  GPa, β b −1  = 193(6) GPa β c −1  = 53.4(0.4) GPa). High-pressure Raman spectra show a significant splitting of several modes, which were used to identify the transformation in high-pressure high-temperature experiments in the range up to 4 GPa and 542 K. Based on the experimental series of data points determined by XRD and Raman measurements, the phase boundary of the α-to-γ-transition was determined with a Clausius–Clapeyron slope of 9.8(7) × 10 −3  GPa K −1 . An in situ measurement of the X-ray intensities was taken at 1.5 GPa and 411 K in order to identify the nature of the structural variation on increased temperatures corresponding to the previously reported transformation from α- to β-norsethite at 343 K and 1 bar. The investigations revealed, in contrast to all X-ray diffraction data recorded at 298 K, the disappearance of the superstructure reflections and the observed reflection conditions confirm the anticipated R 3 ¯ m space-group symmetry. The same superstructure reflections, which disappear as temperature increases, were found to gain in intensity due to the positional shift of the Ba atoms in the γ-phase.

The elastic scattering process for the nuclear reactions induced by the Radioactive Ion Beams 7Be and 8B on a 208Pb target was measured for the first time in the energy range around the Coulomb barrier. Extensive theoretical calculations within the framework of the optical model were performed. An excellent agreement between experimental data and theoretical predictions was achieved for the reaction 7Be + 208Pb, while a comprehensive understanding of the reaction dynamics induced by the more exotic projectile 8B is still far to be reached. Predictions of the cross section for the breakup for both systems will also be given.

We present a brief review of the reaction mechanisms involved in collisions of weakly bound projectiles with tightly bound targets, at near-barrier energies. We discuss systematic behaviors of the data, with emphasis in fusion, breakup, nucleon transfer and elastic scattering. The dependence of the breakup cross section on the charge and mass of the target is discussed, and the influence of the breakup channel on complete fusion is investigated. For this purpose, we compare reduced fusion cross sections with a benchmark universal curve. The behaviors observed in the comparisons are explained in terms of polarization potentials and of nucleon transfer followed by breakup. The influence of the breakup process on elastic scattering is also discussed. Some apparent contradictions between results of different authors are explained and some perspectives of the field are presented.

We investigate the occurrence of bound states in the continuum (BICs) in serial structures of quantum dots coupled to an external waveguide, when some characteristic length of the system is changed. By resorting to a multichannel scattering-matrix approach, we show that BICs do actually occur in two-dimensional serial structures, and that they are a robust effect. When a BIC is produced in a two-dot system, BICs also occur for several coupled dots. We also show that the complex dependence of the conductance upon the geometry of the multi-dot system allows for a simple picture in terms of the resonance pole motion in the multi-sheeted Riemann energy surface. Finally, we show that in correspondence to zero-width states for the open system one has a multiplet of degenerate eigenenergies for the associated closed serial system, thereby generalizing results previously obtained for single dots and two-dot structures.

The high-temperature thermoelastic behavior of a natural cancrinite has been investigated by in situ single-crystal X-ray diffraction. The unit-cell volume variation as a function of temperature ( T ) exhibits a continuous trend up to 748 K (hydrous expansion regime). The unit-cell edges expansion clearly shows an anisotropic expansion scheme ( α a  <  α c ). At 748 K, a dehydration process takes place, and a series of unit-cell parameter measurements at constant temperature (748 K) for a period of 12 days indicate that the dehydration process continued for the entire period of time, until the cell parameters were found to be constant. After the dehydration process is completed, the structure expands almost linearly with increasing temperature up to 823 K, where a sudden broadening of the diffraction peaks, likely due to the impending decomposition, did not allow the collection of further data points. Even with a very limited temperature range for the anhydrous regime, we observed that the behavior of the two (i.e., hydrous and anhydrous) high-temperature structures is similar in terms of (1) volume thermal expansion coefficient and (2) thermoelastic anisotropy. The structure refinements based on the data collected at 303, 478 and 748 K (after the dehydration), respectively, showed a change in the mechanism of tilting of the quasi-rigid (Si,Al)O 4 tetrahedra, following the loss of H 2 O molecules, ascribable to the high-temperature Na + coordination environment within the cages.

We have studied the behavior of the S–matrix poles near threshold for quantum waveguides coupled to a cavity with a defect. We emphasize the occurrence of both dominant and shadow poles on the various sheets of the energy Riemann surface, and show that the changes of the total conductivity near threshold as the cavity's width changes can be explained in terms of dominant to shadow pole transitions.