Phase Transition Pressure Research Articles

Structural and elastic behavior of aluminum pnictides with temperature effect

A realistic interaction potential model (RIPM) has been formulated to theoretically predict the pressure-induced phase transition, elastic properties and thermophysical properties of AlAs and AlSb, including temperature effect (300 K). This model exhibits a better agreement with the available experimental rather than theoretical data for obtained calculations of phase transition pressures and volume collapses. We have achieved elastic moduli, anisotropy factor, Poisson’s ratio, Kleinman parameter, on the basis of the calculated elastic constants. Apparently, this is the first time when thermophysical properties of these compounds are explored at temperature effect by using a single model. Our results are justified by available measured and other reported data which support the validity of our model.

Improved SMTB-Q model applied to oxygen migration and pressure phase transitions in UO2

The second-moment tight-binding variable-charge (SMTB-Q) interatomic potentials have been implemented in the molecular dynamics (MD) code LAMMPS in order to study the static and dynamical properties of uranium dioxide UO2. With respect to a previous work on UO2 the SMTB-Q model has been slightly modified in introducing a splitting energy of the U 5f orbitals. This improvement results in a better description of the electronic structure of UO2 namely the gap estimation which is now close to the experimental value (~2 eV). The structural and mechanical properties along with the cohesive energy of bulk UO2 are in good agreement with the experimental data. The ionic charges on uranium and oxygen are respectively equal to 2.86 and −1.43, very close to the Bader charges derived from ab initio calculations. The migration energies and the diffusion coefficient calculated respectively for oxygen vacancy (VO) and oxygen interstitials (IO) in under and over stoichiometry compare well with ab initio calculations and experimental data. The oxygen diffusivity is consistent at high temperature when additional Frenkel thermally formed swamps the effect of single IO and VO defects with recent prediction from EAM semi-empirical potentials.Additionally, a study on phase transitions between high pressure polymorphs of UO2 has been performed and has shown the good transferability of the SMBT-Q potential over different coordination. It is found that the UO2 phases stability order under tensile and compressive stresses, compared with stable fluorite phase at 0 GPa, are respected.

Low temperature heat capacity measurements of β-Si3N4 and γ-Si3N4: Determination of the equilibrium phase boundary between β-Si3N4 and γ-Si3N4

Isobaric heat capacities of β-Si3N4 and γ-Si3N4 were measured at temperatures between 1.8 and 309.9 K with a thermal relaxation method. The measured heat capacities of γ-Si3N4 are smaller than those of β-Si3N4 in this temperature range. Using these data, we determined the standard entropies of β-Si3N4 and γ-Si3N4 to be 62.30 J·mol−1 K−1 and 51.79 J·mol−1 K−1, respectively. The equilibrium phase boundary between β-Si3N4 and γ-Si3N4 was calculated using these values and thermodynamic parameters reported in previous studies. The obtained equilibrium phase transition pressure at 2000 K is 11.4 GPa. It is lower than the experimental pressures at which γ-Si3N4 was synthesized in previous studies. The calculated Clapeyron slope at this temperature is 0.6 MPa K−1, which is consistent with those of theoretical studies.

Characterization of the micro-contaminants from the inner-body of Kraak porcelain excavated from the \u201cNan\u2019ao I\u201d shipwreck, the South China Sea

In this paper, optical microscopy, SEM–EDS, Raman and FT-IR analyses were used to investigate different types of corrosion product between layers of glaze and body of Kraak porcelain objects that were excavated from “Nan’ao I” shipwreck, the South China Sea. Several contaminants including NaCl, CaSO4·2H2O, FeOOH and FeS2, were found in many of the pores and cracks of the porcelain objects from Jingdezhen Kiln and Zhangzhou Kiln, even after conservation. While compared with the sample of Jingdezhen Kiln, the contaminants in the sample of Zhangzhou Kilns much more severely. The research pointed out that these contaminants were formed due to the marine environment influence and cross-contamination with other metal relics located in the shipwreck, and more contaminants formed in porous structure, hence reduce the porcelain quality. As regards to conservation, the fragility of ceramic objects from underwater contexts are likely to be caused by various kinds of insoluble ferrous salts and its phase transition pressure. Previous studies on this kind of remains tend to focus on the macro-insoluble salts on the enamel surface. In this paper, preliminary investigations were performed for the first time on different kinds of inner micro-corrosion product in China.

Shock-induced phase transition of g-C3N4 to a new C3N4 phase

In this study, phase transition of graphitic carbon nitride (g-C3N4) was investigated using the shockwave compression technique. Firstly, the shock Hugoniot data of g-C3N4 were obtained using a bore propellant gun and a light gas gun under impact velocities of 1.208–4.982 km/s, revealing one phase transition pressure of g-C3N4 at 22.4 GPa. Then, a series of shock recovery experiments was carried out with a pressure range of 17.0–62.1 GPa. The recovered samples were characterized by various techniques, including X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The measured d-values of the recovered samples were compared with those from the previous reported results, revealing a new carbon nitride phase synthesized by the shockwave compression technique. The new phase is indexed as a triclinic cell with a = 0.481 nm, b = 0.353 nm, c = 0.285 nm, α = 67.52°, β = 100.75°, γ = 106.47°, and Vcell = 0.043 nm3. EDX and XPS spectra reveal the existence of C and N elements with an atomic ratio of 0.754, also confirming the presence of a new C3N4 phase obtained via a g-C3N4 phase transition induced by shockwave compression with a pressure range of 29.3–62.1 GPa. These sample results are in good agreement with the shock Hugoniot data.

Revealing the Origin of Luminescence Center in 0D Cs4PbBr6 Perovskite

Zero dimensional perovskite Cs4PbBr6 has attracted considerable attention recently not only because of its highly efficient green photoluminescence (PL), but also its two highly debated opposing mechanisms of the luminescence: embedded CsPbBr3 nanocrystals versus intrinsic Br vacancy states. After a brief discussion on the root cause of the controversy, we provide sensitive but non-invasive methods that can not only directly correlate luminescence with the underlying structure, but also distinguish point defects from embedded nanostructures. We first synthesized both emissive and non-emissive Cs4PbBr6 crystals, obtained the complete Raman spectrum of Cs4PbBr6 and assigned all Raman bands based on density functional theory simulations. We then used correlated Raman-PL as a passive structure-property method to identify the difference between emissive and non-emissive Cs4PbBr6 crystals and revealed the existence of CsPbBr3 nanocrystals in emissive Cs4PbBr6. We finally employed a diamond anvil cell to probe the response of luminescence centers to hydrostatic pressure. The observations of fast red-shifting, diminishing and eventual disappearance of both green emission and Raman below Cs4PbBr6 phase transition pressure of ~3 GPa is compatible with CsPbBr3 nanocrystal inclusions as green PL emitters and cannot be explained by Br vacancies. The resolution of this long-lasting controversy paves the way for further device applications of low dimensional perovskites, and our comprehensive optical technique integrating structure-property with dynamic pressure response is generic and can be applied to other emerging optical materials to understand the nature of their luminescent centers.

Parallel background subtraction in diamond anvil cells for high pressure X-ray data analysis

ABSTRACTWe present a simple and effective approach to improve X-ray data collected under extreme conditions of pressure and temperature. The X-ray data of the sample and the amorphous, liquid, or crystalline pressure transmitting media (PTM) which surround the sample are collected separately at each pressure and temperature, allowing a satisfactory background subtraction. Using this method, we are able to identify weak diffraction peaks of the low-Z elements Li and Na and amorphous silica under pressure and at cryogenic temperatures. In addition to exploration of phase diagrams of low-Z materials, this method can also be applied to recognize new phases of other unknown materials such as binary hydrides in the high pressure and high-temperature synthesis, and to allow deterministic identification of the onset pressure of structural phase transitions and the presence of mixed phases.

Finely split phase transitions of rotating and accelerating black holes

We investigate the thermodynamic phase behaviour of rotating and slowly accelerating AdS black holes. While we find some similarities with the non-rotating charged counterparts, such as the peculiar phenomena of "snapping swallow tails" we also find subtle but significant distinctions that can be attributed to a qualitatively modified parameter space of the solution. Consequently the zeroth order phase transition now occurs over a range of pressures, mini-entropic black holes no longer exist in the regime of slow acceleration, and the "no black hole region" emerges continuously---from a zero temperature extremal black hole. The formerly equal transition pressure now experiences a fine splitting, as the emergence of a no black hole region and the zeroth-order phase transition appear at different pressures, different also from the termination pressure of the first order phase transition. This has the further effect of admitting reentrant phase transitions that can be achieved in two ways, either by varying the temperature at fixed pressure or varying the pressure at fixed temperature.

Phase transitions and elastic properties of InN1–xBix under high pressure

ABSTRACTPressure-induced structural phase transition and elastic property of InN1−xBix were investigated by using first-principles calculations. All the structural parameters of InN1−xBix almost exhibit a monotonic decrease with an increasing pressure, except for InN0.9062Bi0.0938. Phase transition pressures from wurtzite (WZ) to rock-salt (RS) are about 11.9, 10.9 and 11.7 GPa for InN, InN0.9687Bi0.0313, and InN0.9062Bi0.0938, respectively. Zinc-blende (ZB) structure is a more stable phase for InN0.9375Bi0.0625 alloy under the general environment and transforms to RS structure when pressure is at 14.7 GPa. Elastic constants C11 and C33 increase with the increase of pressure for three different phases. C12 and C13 also change in WZ and ZB phases while keeping unchanged in RS phase with the increase of pressure, and C44 also has the same trend as C12 and C13 have in RS structures. Our calculations present an investigation of the overall elastic properties of pressure-induced InN1−xBix alloy.

Structural Phase Transition, Optical and Electrical Property Evolutions of Thiospinel AgIn5S8 under High Pressure.

Thiospinel AgIn5S8 as a visible-light-active semiconductor has been frequently used as a photoabsorber in solar cells, optoelectronics devices, and photoelectrochemical cells. Similar to temperature, pressure is an efficient external stimulus for both crystalline structural and electronic modulations to improve properties. Herein, we present the pressure tuning effect on AgIn5S8 up to 40 GPa. A pressure-driven phase transition from the ambient cubic spinel structure to an orthorhombic structure is observed around 10 GPa as evidenced from the in situ high pressure synchrotron X-ray diffraction results. The high pressure phase of AgIn5S8 adopts the defective LiVO2-type structure with all the Ag+/In3+ cations sitting in the octahedrally coordinated environments. Both the electric transport and photocurrent measurements show dramatic changes along with the phase transition around 10 GPa, and the high pressure phase of AgIn5S8 exhibits greatly improved conductivity but decreased responses to visible light illumination. Surprisingly, the in situ UV-vis measurements reveal the onset pressure point of bandgap evolution around 7.5 GPa, far below the structural phase transition pressure around 10 GPa, which indicates the early initiated local structural change in the pressure range 7.5-10 GPa. An in situ Raman technique is used to confirm the coordination environment changes of AgIn5S8 under compression, the results of which reveal the coexistence of both the ambient and the high pressure structure features of AgIn5S8 in the pressure range 7.5-10 GPa. This work provides a demonstration on how external pressure affects the crystal structure, electronic structure, and optical properties of chalcogenide semiconductors and sheds light on the structure design of better optoelectrical materials under ambient conditions.

High pressure structural phase transition and elastic properties of ZnSexTe1−x semiconducting compounds

This paper discusses the high pressure-dependent crystal structure phase transformation and elastic behavior of ZnSe[Formula: see text]Te[Formula: see text] (x = 0.0, 0.2, 0.55, 0.81, 0.93 and 1.0) by speculating effective interionic interaction potential (EIoIP), that contains long-range type Coulomb force, short-range overlap repulsion of the Hafemeister and Flygare types and the van der Waals (vdW) interaction. The assessed values of structural phase transition pressure and the considerable volume discontinuity associated with the pressure–volume phase diagram support the structural phase transformation from zinc blende (B3) type to rock-salt (B1) type crystal structure. The equation of state curves (relationship between V(p)/V(0) and pressure in the pressure–volume phase diagram) for both the zinc blende (B3) and rock-salt (B1) structures is in reasonably good agreement with that of experimental observations. Moreover, systematic variations of the second-order elastic constants (SOEC) with pressure have been found to follow systematic trends showed by the other systems with B3 type to B1 type pressure-dependent structural phase transition.

Phase Transition of (Pr1-xNdx)2NiO4 Under Reducing Atmospheres

Over the past few years, the use of nickelates as the cathode material in solid oxide fuel cells (SOFCs), has attracted a growing attention due largely to their superior properties to some commercialized perovskite materials. However, the nickelate does go through phase transition at operation temperatures. To improve the stability of Pr2NiO4, Nd was used to substitute Pr to form (Pr1-xNdx)2NiO4 (PNNO) which exhibits an improved phase stability. The phase degradation was found to accelerate when the PNO cathode is under an operation. It is known that the apparent oxygen partial pressure at the cathode/electrolyte interface is lower than that in cathode gas stream. In order to investigate the relationship between the accelerated phase transition and oxygen partial pressure, PNNO oxides are annealed under different PO2 conditions. Multiple post-analyses are carried out on these samples to obtain a quantitative relationship between the phase composition and PO2 during the annealing process.

High-pressure phase transitions of forsterite from first-principles

From ab initio evolutionary algorithm, we predict a new high pressure phase transition of Mg2SiO4 (Forsterite) above 30 GPa. At this pressure, Forsterite changes into post-spinel structure with Pbam space group. This structure has attracted the interest of many previous works, but which have not been definitively conclusive. Forsterite is stabilized under further phase transition at 64.5 GPa. Above this pressure, the structure with Cmc21 space group becomes the most stable.

In silico study of α-γ phase transformation in hexahydro-1,3,5-trinitro-1,3,5-triazine

Molecular dynamics (MD) simulations are performed using the Smith and Bharadwaj (SB) molecular potential to study the response of α-RDX to hydrostatic compression up to 4 GPa between 300 K and 450 K. We observe a phase transformation to the high-pressure γ polymorph around 2.5 GPa for temperatures greater than or equal to 350 K. The α-γ phase transformation is captured through an abrupt decrease in the ‘b’ lattice parameter and volume while the ‘c’ lattice parameter shows an abrupt increase. The molecular conformation is shown to evolve from AAE to AAE-AAI during the phase transition, consistent with experimental observations. Abrupt changes are also observed in molecular and non-bonded energy terms of the SB potential. The phase at the end of hydrostatic loading is verified to be γ-RDX. The simulations demonstrate that SB potential can be effective in further elucidating the deformation mechanisms in RDX compressed beyond the phase transition pressure. We hypothesize that the difference in angle energy between the two phases plays an important role in the occurrence of α-γ phase transformation in RDX using the SB potential.

DFT Calculations of the Structural, Mechanical, and Electronic Properties of TiV Alloy Under High Pressure

A calculation program based on the density functional theory (DFT) is applied to study the structural, mechanical, and electronic properties of TiV alloys with symmetric structure under high pressure. We calculate the dimensionless ratio, elastic constants, shear modulus, Young’s modulus, bulk modulus, ductile-brittle transition, material anisotropy, and Poisson’s ratio as functions of applied pressure. Results suggest that the critical pressure of structural phase transition is 42.05 GPa for the TiV alloy, and structural phase transition occurs when the applied pressure exceeds 42.05 GPa. High pressure can improve resistance to volume change, as well as the ductility and atomic bonding, but the strongest resistances to elastic and shear deformation occur at P = 5 GPa for TiV alloy. Furthermore, the results of the density of states (DOS) indicate that the TiV alloy presents metallicity. High pressure disrupts the structural stability of the TiV alloy with symmetry, thereby inducing structural phase transition.

Structural, elastic and dynamical properties of $$\hbox { YP}_{1-x}\hbox {As}_{x}$$ alloys from first principles calculations

The structural, elastic and lattice vibration properties of the ternary alloys \({\mathrm{YP}}_{1-x}{\mathrm{As}}_{x}\) at various As concentrations (x) from 0 to 1, \(x=0\), 0.25, 0.5, 0.75 and 1, are presented. The calculations were performed using the density functional perturbation theory (DFPT) within the generalised gradient approximation (GGA) and employing virtual crystal approximation (VCA). We studied the effect of arsenic (As) composition on structural parameters, the phase transition pressure, the elastic constants, the optical and acoustic phonon frequencies at high symmetry points \(\Gamma \), X and L, the static and electronic dielectric constants, and the Born effective charge. It is established that all these properties follow a quadratic law in the concentration (x). There is a good agreement between our results and the available data for binary compounds YP and YAs. Our results are for \({\mathrm{YP}}_{1-x}{\mathrm{As}}_{x}\) predictions.

A DFT study on effects of W solution on cemented carbides Co phase pressurized properties

Effects of W solution (3 at.%) on the structure, elastic properties, and structural phase transition of Co under pressure were investigated in this paper by first principles calculations. The lattice constants of pure Co and Co–W alloy, both with fcc and hcp structure, gradually decrease with the increasing pressure. In general, the bulk modulus(B), shear modulus(G) and Young's modulus(E) of pure Co and Co–W alloy increase approximately linearly with the increasing pressure. W solution brings almost negligible impact on B of fcc and hcp Co, however, effect of W solution on G and E can not be neglected. It is worth to mention that B, G, and E for fcc Co and Co–W alloy are always higher than those for hcp Co and Co–W alloy at the same pressure. According to the calculated results, W solution can improve the ductility and thermodynamic stability of Co, and increase the hcp to fcc phase transition pressure from 102 GPa to 130 GPa.

High-Pressure-Induced Transition from Ferromagnetic Semiconductor to Spin Gapless Semiconductor in Quaternary Heusler Alloy VFeScZ (Z = Sb, As, P)

In this paper, the structure and the electronic and magnetic properties of VFeScZ (Z = Sb, As, P) series alloys are systematically studied based on the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) calculation within the first-principles density functional theory. The results showed that VFeScSb and VFeScP are ferromagnetic semiconductors and VFeScAs exhibits half-metallic ferromagnetism under zero pressure. As the pressure increases, the narrow indirect gap of VFeScZ (Z = Sb, As, P) alloy gradually decreases, and gets close to zero, leading to spin gapless semiconductor (SGS) transition. The pressure phase transition point of VFeScSb, VFeScAs, and VFeScP alloy is 132 GPa, 58 GPa, and 32 GPa, respectively. As a result, the pressure effect provides an opportunity to tune the electronic properties of the alloys by external pressure. The present findings provide a technical method for us to actually use the Heusler alloy SGS.

First principles calculation of UO2 polymorphs and phase transitions under compressive and tensile loading

We present a study of the structural, phase transition, electronic, elastic and thermodynamic properties of UO2 in the fluorite-type (space group Fm3-m), cotunnite-type (space group Pnma), marcasite-type (space group Pnnm) and rutile-type (space group P42/mnm) phases based on density functional theory (DFT). By GGA + U approach, ground state properties of UO2 polymorphs including structural parameters, electronic structure and mechanical properties were obtained and compared with reported numerical and experimental results. The calculated phase transition pressures for the structure transition from fluorite-type to cotunnite-type, marcasite-type and rutile-type phases are 20.6 GPa, −9.2 GPa and −7.0 GPa respectively. The dependence of electronic properties on pressure implies that there will be narrowing of band gap for phase transitions under both compressive and tensile loading. The calculations of elastic constants indicate the fluorite and cotunnite-type structures are mechanically stable in the studied pressure range, but the marcasite-type and rutile-type phases are unstable at high pressure. All the phases of UO2 were predicted to be dynamically stable under ambient pressure by the phonon dispersions calculations. The heat capacities, bulk moduli, Gibbs free energies and thermal expansion coefficients of the four UO2 polymorphs as a function of temperature are given, and those of fluorite UO2 by GGA + U calculation yield good agreement with reported experimental data.

Lateral interaction of cholesterol with a semifluorinated amphiphile at the air–water interface

Lipid rafts consisting mainly of sphingomyelin and cholesterol (Ch) on biomembrane surfaces are deeply related to cellular processes such as protein trafficking and signal transduction. During the processes, the raft microdomains affect the fluidty of biological membranes, which is controlled to large extents by Ch. In this paper, we have investigated the interaction between Ch and a semiflurinated alcohol (F6H9OH) from the aspect of a fluidty control using surface chemistry. The two-component Langmuir monolayer at the air–water interface was characterized by the surface pressure (π)–molecular area (A) and surface potential (ΔV)−A isotherms. The compressibility modulus and excess Gibbs free energy of mixing were calculated from the π−A isotherms. And also the two-dimensional phase diagram was constructed on the basis of phase transition pressures and monolayer collapse pressures. Furthermore, the phase behavior of binary monolayers was visualized with fluorescence microscopy (in situ) and atomic force microscopy (ex situ). The result here indicates a possibility of fluidity control of Ch-related membranes by arranging the fluorination degree of the constituent lipids.