Reversible Amorphization Research Articles

Computational research on memory effects in AlPO4-5 nanoporous material

Reversible amorphization and memory effects of both dense and open frameworks have received a great attention due to their prospective industrial applications. In this paper, the results of a computational study related to phase transition and memory effects in AlPO4-5 nanoporous material at high external pressure is presented. The behavior of the AlPO4-5 unit cell at high external pressures was studied by energy minimization techniques using classical potentials. A combination of interatomic potentials was used to describe the crystalline structure of the aluminophosphate. According to simulation's result a decrease of crystalline order is observed at a pressure about 3.5GPa. The behavior of the simulated infrared spectra of compressed structures is an unambiguous evidence of structural disorder. Also, an abrupt change in the slope of the unit cell volume vs. pressure curve was obtained. At P≤3.5GPa the process was found reversible. Contrary to what has been reported in other aluminosilicate systems the final crystalline state of AlPO4-5 at the highest simulated pressure was not amorphous. According to our knowledge this is the first evidence of a reversible first-order crystal-crystal phase transition in AlPO- family materials. This result could be important in future industrial and catalytic applications of these materials.

Pressure-induced reversible amorphization in superconducting compound FeSe0.5Te0.5

We report pressure-induced amorphization phenomenon in a superconducting compound FeSe0.5Te0.5 to a pressure of 27 GPa in a diamond anvil cell. The synchrotron x-ray diffraction studies reveal that the ambient pressure tetragonal phase (space group P4/nmm) transforms to an amorphous phase at 11.5 GPa during compression and reverts back to the tetragonal phase during decompression at 2.8 GPa. The measured equation of state for the tetragonal phase and the amorphous phase is presented. The disordering of Fe(SeTe)4 tetrahedra under compression is attributed to a kinetic hindrance to a stable phase and is likely to impact its superconducting properties under high pressures.

A study of the pressure-induced reversible amorphization of Xe containing-LTA zeolites by energy minimization technique

A previous computational study on dehydrated LTA zeolites [J. Gulín-González, G.B. Suffritti, Micropor. Mesopor. Mater . 69 (2004) 127] suggested the reversible amorphization of calcined LTA samples at pressures below 6 GPa, in agreement with the experimental results. However, the effect of guest molecules on the amorphization process was not evaluated in that study. In this paper, the potential reversible amorphization of Xe containing-LTA samples under high external pressures is studied via energy minimization calculations. The results of the simulations confirmed the pressure-induced amorphization of LTA zeolites at pressures about 2–4 GPa for all the studied samples. Besides, the simulations stressed the importance of the structural topology, particularly of D4R secondary units, for the recovering of the crystalline order. According to our calculations the exchangeable cations (Na + and Li +) play a crucial role in the process of amorphization. Our results suggest that the reversible amorphization is essentially independent on the presence of Xe molecules in the range of the pressures studied ( P ⩽ 6 GPa).

Pressure-induced phase transitions in cobalt-filled multiwalled carbon nanotubes

High-pressure structural properties of cobalt nanowires encapsulated in multiwalled carbon nanotubes have been investigated up to a pressure of $\ensuremath{\sim}38\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature. Our x-ray diffraction measurements show that cobalt, which exists in the face-centered-cubic (fcc) phase at ambient conditions in the carbon nanotubes, transforms irreversibly to a hexagonal close-packed (hcp) structure at $\ensuremath{\sim}9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. In comparison with the bulk Co, the compressibility of the fcc phase in nanowires is found to be similar to that of the high-pressure fcc phase [Phys. Rev. Lett. 84, 4132 (2000)] and the hcp phase is slightly less compressible than the bulk. Multiwalled carbon nanotubes that encapsulate the cobalt nanowires do not undergo any other structural transformation with pressure except partial reversible amorphization beyond $9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.

Reducibility of fast oxide-ion conductors La2−xRxMo2−yWyO9(R = Nd, Gd)

The substitutional range and cell parameter evolution of fast oxide-ion conductors La2−xRxMo2−yWyO9 (R = Nd, Gd) are investigated. In the whole series, the cubic β-La2Mo2O9 structural type is stabilized at room temperature. The effects on reducibility of both single and double substitutions are presented. Lanthanum substitution by rare earth appeared to be responsible for an increase in the reducibility and a strong but reversible amorphization under dilute hydrogen. On the contrary, the favourable role of tungsten on the compound stability under reducing conditions is evidenced: it depletes oxygen loss while making the La2Mo2O9 structural type less affected by it.

Why do zeolites with LTA structure undergo reversible amorphization under pressure?

In this Letter, we report the unusual behavior of zeolites with LTA structure (a prototype of synthetic zeolite) under high external pressure. We found that these materials undergo pressure-induced amorphization. Interestingly, the pressure-induced amorphization is reversible for some of these materials. Apparently, the amorphization occurs via the weakening of the double four membered rings in zeolite lattice. We found that the structural memory of the glasses and the stability of the LTA zeolites depend strongly on the nature of the charge balancing cations present in the framework.

Mechanism of pressure-induced amorphization

Abstract Several crystalline substances have been found to be transformed into the amorphous state under compressed condition at kinetically low temperature. Dynamical lattice-instability due to elastic deformation by shear and stress induces the reversible amorphization, some of which produces memory glass. On the other hand the irreversible modes are attributed to the plastic deformation by the nucleation of high-pressure form in the parent lattice but thermal energy is not kinetically high enough to provide the large crystallite size coherent to the X-ray radiation. They can be defined as X-ray amorphous. These reversible and irreversible transformations arise from the hindrance to sufficient atomic mobility. These pressure-induced amorphizations are the precursor phenomena of the phase transformation to high-pressure polymorphs. Successive structure changes of the pressure-induced amorphization are investigated under various pressure and temperature by X-ray diffractometry, EXAFS and Raman spectroscopy. The amorphization has been also simulated by the molecular dynamics.

Pressure-induced intercalation and amorphization in the spin-Peierls compound CuGeO3.

High pressure Raman scattering and optical microscopic studies on ${\mathrm{CuGeO}}_{3}$ have been carried out in the diamond cell at room temperature. A pressure-induced reversible transition presumably due to intercalation of alcohol is observed at 7 GPa with 4:1 methanol ethanol as pressure medium. In this transition the sample dimension along the $b$ axis visibly decreases by about 20%, preserving the single crystal nature. Such behavior is rather unique and unusual. Further, a pressure-induced reversible amorphization transition occurs near 15 GPa. Intercalation with ${\mathrm{H}}_{2}\mathrm{O}$ is also indicated near 9 GPa with no change in the sample dimension.

圧力誘起非晶質相転移機構

In the last decade the pressure-induced amorphization of several crystals such as SiO2, Ca (OH) 2, H2O, GeO2, α-AlPO4 etc. have been found under compression at room temperature. The pressure-induced amorphization was previously regarded as the thermodynamically metastable phase, which corresponds the super cooled liquid phase at room temperature. The amorphization is a precursor phenomenon of the phase transition to the high pressure form. There are two different types of the reversible and irreversible amorphization. Dynamical lattice-instability due to shear and stress induces the reversible amorphization, which produces memory glass. On the other hand the irreversible mode is attributed to the nucleation of highpressure form in the parent lattice but thermal energy is not high enough to provide the large crystallite size coherent to the X-ray wave length. The successive structure change of the pressure-induced amorphizaiton was investigated under various pressure and temperature by X-ray diffraction study, XAFS and Raman spectroscopy with diamond anvil pressure cell.

Pressure-induced band gap reduction, orientational ordering and reversible amorphization in single crystals of C70: Photoluminescence and Raman studies

Abstract Photoluminescence and Raman scattering experiments have been carried out on single crystals of C70 up to 31 GPa to investigate the effect of pressure on the optical band gap, vibrational modes and stability of the molecule. The photoluminescence band shifts to lower energies and the pressure dependence of the band maxima yields the hydrostatic deformation potential to be 2.15 eV. The slope changes in the pressure dependence of peak positions and linewidths of the Raman modes associated with the intramolecular vibrations at 1 GPa mark the known face-centred cubic→orhombohedral orientational ordering transition. The reversible amorphization in C70 at P> 20 GPa has been compared with the irreversible amorphization in C60 at P > 22 GPa in terms of carbon-carbon distance between the neighbouring molecules at the threshold transition pressures, in conjunction with the interplay between the intermolecular and intramolecular interactions.

The role of non-deformable units in pressure-induced reversible amorphization of clathrasils

PRESSURE-induced amorphization of solids has been much studied since it was first observed in 19841. It was found recently2,3 that some materials can be amorphized reversibly under pressure, reverting back to the original crystalline structure and orientation when the pressure is decreased. It has been suggested4 that the presence of non-deformable units is essential for this reversibility, these units acting as templates around which the original structure is reformed. Here we investigate this idea by comparing the effect of pressure on two clathrasils—silica solids with open, microporous structures—with and without guest molecules inside the pores. We have studied pressure-induced amorphization of dodecasil-3C, which has cage-like voids, and dodecasil-3R, which has two-dimensional channels. For both materials, our experiments and simulations show that amorphization is fully or partly reversible only when guest molecules are present, suggesting that these do indeed act as rigid 'organizing centres' for the reversible transformation.