Elastic instability in alpha -quartz under pressure

A new high-pressure structure of SiO2directly converted fromα-quartz under nonhydrostatic compression

High-pressure behavior of SiO₂is one of the prototypical subjects in several research areas including condensed matter physics, inorganic chemistry, mineralogy, materials science, and crystallography. Therefore, numerous studies have been performed on the structure evolution of SiO₂under pressure. Here, we show a new structure directly converted fromα-quartz under uniaxial compression. Ourab initiocalculations elucidate a simple transition pathway fromα-quartz to the Fe₂P-type phase, and an intermediate state with the Li₂ZrF₆-type structure appears in this structure conversion. Some interesting properties are found on this intermediate state. (1) The Li₂ZrF₆-type phase is metastable probably due to a volumetric unbalance between the Li and Zr sites but becomes more energetically stable thanα-quartz over ∼12 GPa. (2) It is vibrationally stable at 0 GPa, suggesting that this phase can be recovered down to ambient condition once synthesized. (3) The crystal structures of Li₂ZrF₆-type SiO₂and phase D, one of dense magnesium hydrous silicates, are found identical, suggesting the stabilization of their solid solution under high-P,Tcondition.

Extending the single-crystal quartz pressure gauge up to hydrostatic pressure of 19 GPa

In situhigh-pressure diffraction experiments on single-crystal α-quartz under quasi-hydrostatic conditions up to 19 GPa were performed with diamond-anvil cells. Isotropic pressures were calibrated through the ruby-luminescence technique. A 4:1 methanol–ethanol mixture and the densified noble gases helium and neon were used as pressure media. The compression data revealed no significant influence of the pressure medium at room temperature on the high-pressure behavior of α-quartz. In order to describe its compressibility for use as a pressure standard, a fourth-order Birch–Murnaghan equation of state (EoS) with parametersKT0 = 37.0 (3) GPa,KT0′ = 6.7 (2) andKT0′′ = −0.73 (8) GPa−1was applied to fit the data set of 99 individual data points. The fit of the axial compressibilities yieldsMT0 = 104.5 (8) GPa,MT0′ = 13.7 (4),MT0′′ = −1.04 (11) GPa−1(aaxis) andMT0 = 141 (3) GPa,MT0′ = 21 (2),MT0′′ = 8.4 (6) GPa−1(caxis), confirming the previously reported anisotropy. Assuming an estimated standard deviation of 0.0001% in the quartz volume, an uncertainty of 0.013 GPa can be expected using the new set of EoS parameters to determine the pressure.

Pressure induced elastic softening in framework aluminosilicate- albite (NaAlSi3O8)

Albite (NaAlSi₃O₈) is an aluminosilicate mineral. Its crystal structure consists of 3-D framework of Al and Si tetrahedral units. We have used Density Functional Theory to investigate the high-pressure behavior of the crystal structure and how it affects the elasticity of albite. Our results indicate elastic softening between 6–8 GPa. This is observed in all the individual elastic stiffness components. Our analysis indicates that the softening is due to the response of the three-dimensional tetrahedral framework, in particular by the pressure dependent changes in the tetrahedral tilts. At pressure <6 GPa, the PAW-GGA can be described by a Birch-Murnaghan equation of state with  = 687.4 ų,  = 51.7 GPa, and  = 4.7. The shear modulus and its pressure derivative are  = 33.7 GPa, and  = 2.9. At 1 bar, the azimuthal compressional and shear wave anisotropy  = 42.8%, and  = 50.1%. We also investigate the densification of albite to a mixture of jadeite and quartz. The transformation is likely to cause a discontinuity in density, compressional, and shear wave velocity across the crust and mantle. This could partially account for the Mohorovicic discontinuity in thickened continental crustal regions.

Universal elastic-hardening-driven mechanical instability in α-quartz and quartz homeotypes under pressure

As a fundamental property of pressure-induced amorphization (PIA) in ice and ice-like materials (notably α-quartz), the occurrence of mechanical instability can be related to violation of Born criteria for elasticity. The most outstanding elastic feature of α-quartz before PIA has been experimentally reported to be the linear softening of shear modulus C₄₄, which was proposed to trigger the transition through Born criteria B₃. However, by using density-functional theory, we surprisingly found that both C₄₄ and C₆₆ in α-quartz exhibit strong nonlinearity under compression and the Born criteria B₃ vanishes dominated by stiffening of C₁₄, instead of by decreasing of C₄₄. Further studies of archetypal quartz homeotypes (GeO₂ and AlPO₄) repeatedly reproduced the same elastic-hardening-driven mechanical instability, suggesting a universal feature of this family of crystals and challenging the long-standing idea that negative pressure derivatives of individual elastic moduli can be interpreted as the precursor effect to an intrinsic structural instability preceding PIA. The implications of this elastic anomaly in relation to the dispersive softening of the lowest acoustic branch and the possible transformation mechanism were also discussed.

Pressure effects on the vibrational properties of α-Bi(2)O(3): an experimental and theoretical study

We report an experimental and theoretical high-pressure study of the vibrational properties of synthetic monoclinic bismuth oxide (α-Bi(2)O(3): ), also known as mineral bismite. The comparison of Raman scattering measurements and theoretical lattice-dynamics ab initio calculations is key to understanding the complex vibrational properties of bismite. On one hand, calculations help in the symmetry assignment of phonons and to discover the phonon interactions taking place in this low-symmetry compound, which shows considerable phonon anticrossings; and, on the other hand, measurements help to validate the accuracy of first-principles calculations relating to this compound. We have also studied the pressure-induced amorphization (PIA) of synthetic bismite occurring around 20 GPa and showed that it is reversible below 25 GPa. Furthermore, a partial temperature-induced recrystallization (TIR) of the amorphous sample can be observed above 20 GPa upon heating to 200°C, thus evidencing that PIA at room temperature occurs because of the inability of the α phase to undergo a phase transition to a high-pressure phase. Raman scattering measurements of the TIR sample at room temperature during pressure release have been performed. The interpretation of these results in the light of ab initio calculations of the candidate phases at high pressures has allowed us to tentatively attribute the TIR phase to the recently found high-pressure hexagonal HPC phase and to discuss its lattice dynamics.

Structural study of α-Bi2O3 under pressure

An experimental and theoretical study of the structural properties of monoclinic bismuth oxide (α-Bi2O3) under high pressures is here reported. Both synthetic and mineral bismite powder samples have been compressed up to 45 GPa and their equations of state have been determined with angle-dispersive x-ray diffraction measurements. Experimental results have been also compared with theoretical calculations which suggest the possibility of several phase transitions below 10 GPa. However, experiments reveal only a pressure-induced amorphization between 15 and 25 GPa, depending on sample quality and deviatoric stresses. The amorphous phase has been followed up to 45 GPa and its nature discussed.

The effects of stoichiometry on the mechanical properties of icosahedral boron carbide under loading

The effects of stoichiometry on the atomic structure and the related mechanical properties of boron carbide (B(4)C) have been studied using density functional theory and quantum molecular dynamics simulations. Computational cells of boron carbide containing up to 960 atoms and spanning compositions ranging from 6.7% to 26.7% carbon were used to determine the effects of stoichiometry on the atomic structure, elastic properties, and stress-strain response as a function of hydrostatic, uniaxial, and shear loading paths. It was found that different stoichiometries, as well as variable atomic arrangements within a fixed stoichiometry, can have a significant impact on the yield stress of boron carbide when compressed uniaxially (by as much as 70% in some cases); the significantly reduced strength of boron carbide under shear loading is also demonstrated.

Mechanism of pressure-induced amorphization

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.

The effects of pressure, temperature and composition on the crystal structures of α-quartz homeotypes

α-Quartz and its homeotypes are of great importance for both materials and Earth sciences. The properties of these materials depend strongly on their crystal structures and particularly the intertetrahedral bridging angle and the tetrahedral tilt angle. These angles are highly dependent on composition and the external parameters pressure and temperature. The behavior of the eleven known α-quartz homeotypes, along with examples of α-quartz-type solid solutions, are compared. The distortion in α-quartz-type structures decreases as a function of temperature and increases as a function of pressure. Thermal stability depends on initial structural distortion and on the electronic configuration of the cation. Pressure stability also depends on the former and on cation size. Transitions to new crystalline and/or amorphous forms, often with increased cation coordination number, are commonly observed at high-pressure. The combined use of high-pressure and high-temperature can be used to synthesize novel α-quartz homeotypes in compounds with small cations.

Amorphization induced by pressure: results for zeolites and general implications

We report an ab initio study of pressure-induced amorphization (PIA) in zeolites, which are model systems for this phenomenon. We confirm the occurrence of recently reported low-density amorphous phases that preserve the crystalline topology, and explain the role of the zeolite composition regarding PIA. Our results support the correctness of existing models for the basic PIA mechanism, but suggest that energetic, rather than kinetic, factors determine the irreversibility of the transition.

Phonon dispersion of ice under pressure

We report measurements of the phonon dispersion of ice Ih under hydrostatic pressure up to 0.5 GPa, at 140 K, using inelastic neutron scattering. They reveal a pronounced softening of various low-energy modes, in particular, those of the transverse acoustic phonon branch in the [100] direction and polarization in the hexagonal plane. We demonstrate with the aid of a lattice dynamical model that these anomalous features in the phonon dispersion are at the origin of the negative thermal expansion (NTE) coefficient in ice below 60 K. Moreover, extrapolation to higher pressures shows that the mode frequencies responsible for the NTE approach zero at approximately 2.5 GPa, which explains the known pressure-induced amorphization (PIA) in ice. These results give the first clear experimental evidence that PIA in ice is due to a lattice instability, i.e., mechanical melting.

AMORPHOUS WATER

After providing some background material to establish the interest content of this subject, we summarize the many different ways in which water can be prepared in the amorphous state, making clear that there seems to be more than one distinct amorphous state to be considered. We then give some space to structural and spectroscopic characterization of the distinct states, recognizing that whereas there seems to be unambiguously two distinct states, there may be in fact be more, the additional states mimicking the structures of the higher-density crystalline polymorphs. The low-frequency vibrational properties of the amorphous solid states are then examined in some detail because of the gathering evidence that glassy water, while difficult to form directly from the liquid like other glasses, may have some unusual and almost ideal glassy features, manifested by unusually low states of disorder. This notion is pursued in the following section dealing with thermodynamic and relaxational properties, where the uniquely low excess entropy of the vitreous state of water is confirmed by three different estimates. The fact that the most nearly ideal glass known has no properly established glass transition temperature is highlighted, using known dielectric loss data for amorphous solid water (ASW) and relevant molecular glasses. Finally, the polyamorphism of glassy water, and the kinetic aspects of transformation from one form to the other, are reviewed.

Anomalous pressure-induced transition(s) in ice XI

The effects of pressure on the structure of ice XI-an ordered form of the phase of ice Ih, which is known to amorphize under pressure-are investigated theoretically using density-functional theory. We find that pressure induces a mechanical instability, which is initiated by the softening of an acoustic phonon occurring at an incommensurate wavelength, followed by the collapse of the entire acoustic band and by the violation of the Born stability criteria. It is argued that phonon collapse may be a quite general feature of pressure-induced amorphization. The implications of our findings for the amorphization of ice Ih are also discussed.

First-order amorphous-amorphous transformation in silica

Molecular simulations predict that a first-order amorphous-amorphous transformation occurs in SiO2 under pressure, analogous to the first-order amorphous-amorphous transformation known to occur in H2O. At low temperatures the first-order transformation is kinetically hindered, and an amorphous-amorphous transformation occurs instead by gradual spinodal decomposition at higher pressures. We suggest that previous experiments have observed the spinodal decomposition pathway in SiO2 and that the predicted first-order transformation will be observed in experiments carried out at higher temperatures.

High-pressure elasticity of alpha-quartz: instability and ferroelastic transition

The single-crystal elastic moduli of alpha-quartz were measured to above 20 GPa in a diamond-anvil cell by Brillouin spectroscopy. The behavior of the elastic moduli indicates that the high-pressure phase transition in quartz is ferroelastic in nature and is driven by softening of C44 through one of the Born stability criteria. The trends in elastic moduli confirm theoretical predictions, but there are important differences, particularly with respect to the magnitudes of the B(i). The quartz I-II transition occurs prior to complete softening of the mode and amorphization.