Phase Transition Pressure Research Articles

Crystal structure and properties of iodine monofluoride compounds at high pressure

The pressure-induced structural phase transitions of halogen compound IF under high pressure were studied by using the unbiased CALYPSO structure prediction method. The phase transition sequence of IF under high pressure is determined to be P1 → P21/c-I → P21/c-II → I4/mmm, and the corresponding phase transition pressures are 3 GPa, 16.5 GPa and 46 GPa, respectively. The physical properties of each predicted phase were thus fully studied, and it was found that the P1 phase was unstable which is in excellent agreement with the experiment result. Furthermore, our exploration of the high pressure phases for IF will provide fundamental insights for further exploration the structural phase transition of other halogen compounds under high pressure.

BaZrO3 stability under pressure: The role of nonlocal exchange and correlation

The ground-stale structure of BaZrO3 is experimentally known to be cubic down to absolute zero. However, there exist several measured properties and experimental characterizations that earlier computational works have failed to accurately describe and explain within this cubic symmetry. Among these properties and observations are the dielectric constant and the parallel mean-squared relative displacement value that tracks the fluctuations in distance for Ba-O atom pairs. Previous density-functional theory (DFT) studies have resolved the issue by assuming that BaZrO(3 )undergoes a phase transition from cubic to tetragonal I4/mcm symmetry, possibly while forming a glasslike state that reflects cubic symmetry on average. In this paper, we show that the set of experimental results can indeed be satisfactorily explained by DFT entirely within the cubic symmetry. We find that past theory limitations arose from the choice of exchange-correlation-functional approximations and that the inclusion of Fock exchange in hybrids significantly improves the DFT performance. We also find that the inclusion of nonlocal correlation effects is beneficial. We conclude by making a prediction for the phase-transition pressure for the transition from cubic to tetragonal symmetry at zero kelvin.

The yielding behavior of SiC under high pressure and temperature conditions

A large volume cubic anvil press integrated with synchrotron energy-dispersive x-ray diffraction was employed to study the yielding behavior of powdered beta silicon carbide (SiC) under high pressure and high temperature conditions up to 7.4 GPa and 1400 °C. During compression and heating, the x-ray pattern was collected at each pressure–temperature point, and, then, via assessing the peak width of the x-ray diffraction pattern, the strains/stresses developed inside the sample under varied pressure–temperature conditions were determined. From the constitutive response of the sample as a function of pressure and temperature, we did not observe the yielding occurrence in SiC at cold compression. In contrast, high temperature induces a yielding at 1100 °C with a constant loading pressure of ∼7.4 GPa. By comparison, we found that this material is the most stable, compared with the other three strong ones (diamond, moissanite, and alfa silicon nitride), in terms of the yielding under high pressure and temperature conditions. Along with its much higher pressure and temperature requirements for phase transition and decomposition, SiC is a competent material for the development of novel tools/devices to be used in the harshly extreme working environment, such as deep drilling, high-speed cutting, and aerospace engineering.

Theoretical investigations on the structural stability, structural and physical properties, and bonding feature for RuX (X = Si, Ge, Sn) with B20 and B2 phases

A systematic first-principles study on the structural features and stabilities, bonding features and related physical properties of ruthenium compounds RuX (X = Si, Ge, Sn) with B20 and B2 configurations including the elastic, electronic and thermodynamic properties and Vickers hardness has been carried out by means of the pseudopotential plane-waves method. Based on the fourth-order Birch–Murnaghan equation of state, the phase transition pressures of RuSi and RuGe from B20- to B2-type were determined at 2.7 GPa and 9.0 GPa, respectively. Nevertheless, there is no structural phase transition in RuSn. The calculated formation enthalpies and elastic constants confirm that all six considered polymorphs are thermodynamically and mechanically stable. Additionally, the hardness value of B20-type RuX is higher than that of homologues B2-type RuX, which can be attributed to the stronger Ru-X covalent bond in B20 configuration than that in B2 structure. The analyses of DOS, ELF and Bader charges indicate that the chemical bonding in six crystals comprises a combination of covalent and ionic bonds with the sequence of the iconicity of RuSi > RuSn > RuGe for the same configuration RuX. The current theoretical findings will most likely promote further experimental and theoretical studies on Ru-based compounds.

PVT and Thermal-Pressure Coefficient Measurements and Derived Thermodynamic Properties of 2-Propanol in the Critical and Supercritical Regions

PVT properties and the thermal-pressure coefficient, $$\gamma_{\text{V}} = \left( {\partial P/\partial T} \right)_{\text{V}}$$ , of 2-propanol have been simultaneously measured in the near- and supercritical regions using high-temperature and high-pressure nearly constant-volume piezo-calorimeter. Measurements were made along 9 liquid and 3 vapor isochores between (234.3 and 697.69) kg·m−3 and at temperatures from (317.17 to 522.36) K at pressures up to 6.05 MPa. For each measured isochore ( $$\rho$$ ), the values of phase transition temperature ( $$T_{S}$$ ) and pressure ( $$P_{S}$$ ) at the liquid–gas phase equilibrium curve have been determined using the isochoric $$P - T$$ break point and thermal-pressure coefficient abruptness techniques. The measured saturated liquid ( $$\rho^{\prime}_{S}$$ ) and vapor ( $$\rho^{\prime\prime}_{S}$$ ) densities near the critical point and thermal-pressure coefficient abruptness disappearance were used to estimate the values of the critical parameters ( $$T_{C}$$ = 508.72 K, $$P_{C}$$ = 4.840 MPa, and $$\rho_{C}$$ = 268.82 kg·m−3) of 2-propanol. The expanded uncertainty of the pressure ( $$P$$ ), density ( $$\rho$$ ), and thermal-pressure coefficient ( $$\gamma_{\text{V}}$$ ) measurements at the 95 % confidence level with a coverage factor of k = 2 are estimated to be 0.05 %, 0.16 %, and 1.5 %, respectively. Scaling-type critical anomaly of the asymptotic behavior of thermal-pressure coefficient in the immediate vicinity of the critical point was experimentally observed. The measured pressures ( $$PVT$$ ) and thermal-pressure coefficients ( $$\gamma_{\text{V}} VT$$ ) have been used to calculate of the other key thermodynamic properties such as internal pressure ( $$P_{\text{int}}$$ ), enthalpy ( $$\Delta H_{vap}$$ ) and entropy ( $$\Delta S_{vap}$$ ) of vaporization, saturation ( $$C_{\text{sat}}$$ ), isobaric ( $$C_{\text{P}}$$ ), and isochoric ( $$C_{\text{V}}$$ ) heat capacities.

Potential Interaction of Noble Gas Atoms and Anionic Electrons in Ca2N

Noble gas (NG) is often used as a hydrostatic pressure-transmitting medium in high-pressure experiments. However, the NG atoms under compression are found to be readily trapped in the voids of some ionic compounds. Electrides usually have large open frameworks and show strong hydrogen and oxygen affinities. To investigate the interaction of light NG with electrides under pressure, we perform the structural investigations of Ca2N in a diamond anvil cell under different NG (He, Ne, and Ar) conditions, assisted by density functional theory calculations. Experimental results find that in comparison with nonhydrostatic pressure, transition paths of Ca2N change with different pressure media and phase transition pressure is reduced significantly, and NG is chemically inert for electrides in the studied pressure range. Theoretical analysis indicates that the anionic electrons in Ca2N electride show strong repulsion with NG, preventing NG atoms from entering electrides. Such repulsion is due to the almost neutral character of noble atoms in electrides, which makes it difficult for them to interact with anionic electrons. Moreover, in the predicted metastable Ca2N–NG, the neutral features of NG atoms result in the reservation of the intrinsic electride character. Our results reveal that the NGs can be used as protective gases for electrides under ambient conditions or hydrostatic pressure-transmitting media for the studies of high-pressure electrides within 50 GPa.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

Shape Dependence of Pressure-Induced Phase Transition in CdS Semiconductor Nanocrystals.

Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high-pressure research on nanomaterials has been widely conducted, their shape-dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. Here we have employed in situ synchrotron wide-angle X-ray scattering and transmission electron microscopy (TEM) to investigate the high-pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape: rod-shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface-to-volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. Additionally, TEM images of spherical particles exhibited sintering-induced morphology change after high-pressure compression. Calculations of the bulk modulus reveal that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high-pressure properties. Our study provides important insights into understanding the phase-structure-property relationship, guiding future design and synthesis of nanoparticles for promising applications.

Localization of Annexin A6 in Matrix Vesicles During Physiological Mineralization.

Annexin A6 (AnxA6) is the largest member of the annexin family of proteins present in matrix vesicles (MVs). MVs are a special class of extracellular vesicles that serve as a nucleation site during cartilage, bone, and mantle dentin mineralization. In this study, we assessed the localization of AnxA6 in the MV membrane bilayer using native MVs and MV biomimetics. Biochemical analyses revealed that AnxA6 in MVs can be divided into three distinct groups. The first group corresponds to Ca2+-bound AnxA6 interacting with the inner leaflet of the MV membrane. The second group corresponds to AnxA6 localized on the surface of the outer leaflet. The third group corresponds to AnxA6 inserted in the membrane’s hydrophobic bilayer and co-localized with cholesterol (Chol). Using monolayers and proteoliposomes composed of either dipalmitoylphosphatidylcholine (DPPC) to mimic the outer leaflet of the MV membrane bilayer or a 9:1 DPPC:dipalmitoylphosphatidylserine (DPPS) mixture to mimic the inner leaflet, with and without Ca2+, we confirmed that, in agreement with the biochemical data, AnxA6 interacted differently with the MV membrane. Thermodynamic analyses based on the measurement of surface pressure exclusion (πexc), enthalpy (ΔH), and phase transition cooperativity (Δt1/2) showed that AnxA6 interacted with DPPC and 9:1 DPPC:DPPS systems and that this interaction increased in the presence of Chol. The selective recruitment of AnxA6 by Chol was observed in MVs as probed by the addition of methyl-β-cyclodextrin (MβCD). AnxA6-lipid interaction was also Ca2+-dependent, as evidenced by the increase in πexc in negatively charged 9:1 DPPC:DPPS monolayers and the decrease in ΔH in 9:1 DPPC:DPPS proteoliposomes caused by the addition of AnxA6 in the presence of Ca2+ compared to DPPC zwitterionic bilayers. The interaction of AnxA6 with DPPC and 9:1 DPPC:DPPS systems was distinct even in the absence of Ca2+ as observed by the larger change in Δt1/2 in 9:1 DPPC:DPPS vesicles as compared to DPPC vesicles. Protrusions on the surface of DPPC proteoliposomes observed by atomic force microscopy suggested that oligomeric AnxA6 interacted with the vesicle membrane. Further work is needed to delineate possible functions of AnxA6 at its different localizations and ways of interaction with lipids.

Symmetry-Adapted Finite Strain Landau Theory Applied to KMnF3

In recent years, finite strain Landau theory has been gradually developed as both a conceptual as well as a quantitative framework to study high pressure phase transitions of the group-subgroup type. In the current paper, we introduce a new version of this approach which is based on symmetry-adapted finite strains. This results in a substantial simplification of the original formulation. Moreover, it allows for replacing the clumsy use of truncated Taylor expansions by a convenient functional parametrization. Both the weaknesses of the traditional Landau approach based on infinitesimal strains as well as the major improvements made possible by our new parametrization are illustrated in great detail in an application to the ambient temperature high pressure transition of the perovskite KMnF 3 .

BAND STRUCTURE, METALLIZATION AND STRUCTURAL PHASE TRANSITION OF NABR UNDER HIGHPRESSURE

The metallization and the phase transition of the alkali bromide sodium bromide (NaBr) is investigated through its band structure. The equilibrium lattice constant, bulk modulus, pressure derivative of bulk modulus and the phase transition pressure at which the compound undergo structural phase transition from NaCl (B1) to the CsCl (B2) structure is predicted from the total energy calculations. The ground state properties and band gap values are compared with the experimental and other theoretical results. At normal pressure NaBr is a direct band gap insulator. When the pressure is increased there is enhanced overlapping between the wave functions of the neighboring atoms. As a result the widths of the valence and empty conduction bands increase. These changes lead to the narrowing and indirect closing of band gap in NaBr (metallization). It is also confirmed that the metallization and structural phase transition do not occur simultaneously in ionic compounds.

A novel approach to the structural distortions of U/Th snub-disphenoids and their control on zircon → reidite type phase transitions of U1−xThxSiO4

Coffinite (USiO4) and thorite (ThSiO4) are conspicuous radiogenic silicates in the geonomy. They form U1−xThxSiO4 (uranothorite) solid solutions in zircon-type phase. Investigating the phase-evolution of these minerals is of utmost significance in realizing their applicability in the front-as well as at the back-end of nuclear industries. We carried out a systematic study of zircon- to reidite-type (tetragonal I41/amd to I41/a) structural transitions of U1−xThxSiO4 solid solution, and investigated their mechanical behaviour. We found a unique behaviour of transition pressure with the change in U-Th concentration in the solid solution. The phase transition pressure (p t) is found to be minimum for x = 0.5. We develop the necessary formalism and present an efficient method to estimate the longitudinal and angular distortions of U/ThO8-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: δ (longitudinal distortions) and σ2 (angular distortions) to quantify the polyhedral distortions. A detailed analysis of U/ThO8 snub-disphenoidal distortions is presented to address such variation of p t with U and Th concentration. We argue that our approach is independent of polyhedral volume and can be used for any AB8 (A: cation, B: anion) type snub-disphenoidal system.

Structural changes in lipid mesophases due to intercalation of dendritic polymer nanoparticles: Swollen lamellae, suppressed curvature, and augmented structural disorder

Understanding interactions between nanoparticles and model membranes is relevant to functional nano-composites and the fundamentals of nanotoxicity. In this study, the effect of polyamidoamine (PAMAM) dendrimers as model nanoparticles (NP) on the mesophase behaviour of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) has been investigated using high-pressure small-angle X-ray scattering (HP-SAXS). The pressure-temperature (p−T) diagrams for POPE mesophases in excess water were obtained in the absence and presence of G2 and G4 polyamidoamine (PAMAM) dendrimers (29 Å and 45 Å in diameter, respectively) at varying NP-lipid number ratio (ν = 0.0002–0.02) over the pressure range p = 1–3000 bar and temperature range T = 20–80 °C. The p−T phase diagram of POPE exhibited the Lβ, Lα and HII phases. Complete analysis of the phase diagrams, including the relative area pervaded by different phases, phase transition temperatures (Tt) and pressures (pt), the lattice parameters (d-spacing), the pressure-dependence of d-spacing (Δd/Δp), and the structural ordering in the mesophase as gauged by the Scherrer coherence length (L) permitted insights into the size- and concentration-dependent interactions between the dendrimers and the model membrane system. The addition of dendrimers changed the phase transition pressure and temperature and resulted in the emergence of highly swollen lamellar phases, dubbed Lβ-den and Lα-den. G4 PAMAM dendrimers at the highest concentration ν = 0.02 suppressed the formation of the HII phase within the temperature range studied, whereas the addition of G2 PAMAM dendrimers at the same concentration promoted an extended mixed lamellar region in which Lα and Lβ phases coexisted. Statement of SignificanceUsing high pressure small angle X-ray scattering in the pressure range 1–3000 bar and temperature range 20–60 °C, we have studied interactions between PAMAM dendrimers (as model nanoparticles) and POPE lipid mesophases (as model membranes). We report the pressure-temperature phase diagrams for the dendrimer-lipid mesophases for the first time. We find that the dendrimers alter the phase transition temperatures (Tt) and pressures (pt), the lattice parameters (d-spacing), and the structural order in the mesophase. We interpret these unprecedented results in terms of the fluidity of the lipid membranes and the interactions between the dendrimers and the membranes. Our findings are of fundamental relevance to the field of nanotoxicity and functional nanomaterials that integrate nanoparticles and organized lipid structures.

Dynamic behaviors of RDX single crystal under ramp compression

The dynamics of RDX single crystal under ramp wave loading is studied experimentally and numerically. The ramp wave loading experiments on RDX single crystal in the orientation of (210) and (100) within 15 GPa are carried out with the magnetic driven device CQ-4, which can provide a loading pressure waveform with a rising time of 450–600 ns. The particle velocity curves of the interface between RDX single crystal and LiF window are obtained with the photonic Doppler velocimetry (PDV). The velocity profiles show an obvious three-wave structure, indicating that the RDX undergoes physical processes such as elastic-plastic transition and α-to-<i>γ</i> phase transition in the loading section. The stress yield limits of different crystallographic orientations of RDX single crystal show obvious difference. The onset phase transition pressures in two crystallographic orientations are the same, which is between 3.5 GPa and 4.0 GPa. The pressure range of phase transition is between initial phase transition pressure and 5 GPa. The <i>γ</i> phase is stable from 5 GPa to 15 GPa. The Hayes multi-phase equation of state and non-equilibrium phase transition kinetic model are employed to simulate the experimental process, and the numerical results can well describe the experimental physical processes such as elastoplastic transformation and phase transformation in the loading section. The calculated results reveal that the correction of the bulk modulus with pressure is necessary under ramp wave compression.

Valid Geometric Solutions for Indentations with Algebraic Calculations

This paper analyzes the force vs depth loading curves of conical, pyramidal, wedged and for spherical indentations on a strict mathematical basis by explicit use of the indenter geometries rather than on still world-wide used iterated “contact depths” with elastic theory and violation of the energy law. The now correctly analyzed loading curves provide as yet undetectable phase-transition. For the spherical indentations, this includes an obvious correction for the varying depth/radius ratio, which had previously been disregarded. Only algebraic formulas are now used for the calculation of material’s properties without data-fittings, or simplifications, or false simulations. Penetration resistance differences of materials’ polymorphs provide precise intersection values as kink unsteadiness by equalization of linear regression lines from mathematically linearized loading curves. These intersections indicate phase transition onset values for depth and force. The precise and correct determination of phase-transition onsets allows for energy and phase-transition energy calculations. The unprecedented algebraic equations are most simply and mathematically reproducibly deduced. There are no restrictions for elastic and/or plastic behavior and no use of different formulas for different force ranges. The novel indentation formulas reveal unprecedented access to the onset, energy and transition energy of phase-transitions. This is now also achieved for spherical indentations. Their formula as deduced for plotting is reformulated for integrations. The distinction of applied work (Wapplied) and indentation work (Windent) allows now for comparing spherical with pyramidal indentation phase-transitions. Only low energy phase-transitions from pyramidal indentation may be missed in spherical indentations. The rather low penetration depths of sphere calottes calculate very close for cap and flat area values. This allows for the calculation of the indentation phase-transition onset pressure and thus the successful comparison with hydrostatic anvil pressurizing results. This is very helpful for their interpretations, as low energy phase-transitions are often missed under the anvil, and it further strengthens the unparalleled ease of the indentation techniques. Exemplification is reported for pyramidal, spherical, and hydrostatic anvil stressing by the numerical analysis of published germanium data. The previous widely accepted historical indentation theories and standards are challenged. Falsely simulated and even published so-called “experimental” indentation data from the literature can most easily be checked. They are mathematically unsound and their correction is urgently necessary for scientific reasons and daily safety with stressed materials. The motivation for this paper is the challenge of worldwide incorrect ISO 14577 standards for false and incomplete characterization of materials. The minimization of catastrophic failures e.g. in aviation requires the strengthening and the advancements of the mathematical truth by using our closed formulas that are based on undeniable geometric and algebraic calculation rules.

Numerical prediction of structural stability and thermodynamic properties for MgF<sub>2</sub> with fluorite- type structure under high pressure

MgF<sub>2</sub> is an important member of alkaline-earth fluorides and has a wide range of applications in industry. Meanwhile, MgF<sub>2</sub> occurs naturally as a mineral sellaite. Compared with the study of its electronic structure and optical properties, the researches of the behavior under high pressure of MgF<sub>2</sub>, especially the thermodynamic properties are still limited. The high-pressure melting, volume thermal expansion coefficient, and thermoelastic parameter of the Earth’s lower mantle mineral, like MgF<sub>2</sub>, are of interest and importance for understanding the physical nature of the functional material and for recognizing the structural compositions, dynamics, evolution and origin of the earth. Using the first-principles calculations based on density functional theory, the thermodynamic, mechanical, and dynamic stability of the fluorite-type structure for MgF<sub>2</sub> are systematically studied. The calculations indicate that the fluorite-type structure is a high-pressure phase and it is stable at least up to 135 GPa. According to the principle of equal enthalpies, the phase transition pressure of MgF<sub>2</sub> crystal from stable rutile structure to high pressure fluorite structure is determined to be 19.26 GPa and 18.15 GPa based on the the generalized gradient approximation and local density approximation calculations, respectively. The high-temperature structural stability of MgF<sub>2</sub> with the fluorite-type structure is investigated and confirmed by using the classical molecular dynamics (MD) simulations by taking into account the molar volume and total energy change behavior in a temperature range from 300 to 6000 K. On the basis of previous research, the volume thermal expansion coefficient, isothermal bulk modulus, and thermoelastic parameter of MgF<sub>2</sub> with the CaF<sub>2</sub>-type fluorite structure are predicted systematically in a temperature range from 300 to 1500 K and in a pressure range from 0 to 135 GPa with the help of the generalized gradient approximation of the revised Perdew-Burke-Ernzerhof form combined with quasiharmonic Debye model calculations and the molecular dynamics method combined with reliable interatomic potentials. An important discovery is that the thermoelastic parameter of this material under low temperature and low pressure is not a constant as assumed usually in previous studies of the equation of states, but it approaches to a constant under both high temperature and high pressure.

Judging the phase transition pressure of the unknown parent phase if the resulting state is known.

In high-pressure phase transition experiments, the crystal structure of the intermediate phase in some phase transitions is difficult to successfully measure due to the limitations of the experimental conditions. The absence of crystal structure data for the intermediate phase also makes it difficult to calculate the pressure point from the intermediate phase to the new phase by the traditional thermodynamic criterion in theoretical simulations. The Conch Criterion is employed by us to successfully verify the phase transition points by observing the reverse shifts of the DOS (electron density of states) curves for the new phase of Cu2S, PbS, PbSe and PbTe, which breaks through the constraints of the traditional criterion and realizes tracing the source of the phase transition in theoretical calculations.

Molecular dynamics simulation of shock-induced isostructural phase transition in single crystal Ce

Cerium (Ce), a rare earth metal, undergoes a significant (14%−17%) and discontinuous volume shrinkage when subjected to ~0.7 GPa compression at ambient temperature: there happens a first-order isostructural phase transition from <i>γ</i>-Ce phase to <i>α</i>-Ce phase (these two phases are both face-centered-cubic (fcc) phase). Because of the <i>α</i>→ <i>γ</i> transition in Ce under shock compression, the shock front in cerium exhibits a 3-wave configuration: elastic precursor, plastic shock wave in <i>γ</i>-Ce, and phase transition wave corresponding to the <i>γ </i>→ <i>α</i> transition according to the experimental observation. In this paper, a recently developed embedded-atom-method (EAM) potential for fcc Ce is employed in the large-scale molecular dynamics simulations of shock loading onto single crystal Ce to study its dynamic behavior, especially the shock-induced <i>α</i>→ <i>γ</i> phase transition, and the orientation dependence with [001], [011] and [111] shock loading. The simulation results show single-wave or multi-wave configuration for shock wave profiles. Under the shock loading along the [001] or [011] crystallographic orientation, the shock wave possesses a 2-wave structure: an elastic precursor and a phase transition wave, while under shock loading along the [111] crystallographic orientation, the obtained shock wave shows a 3-wave profile as observed experimentally. Thus the shock wave structure is obviously dependent on loading orientation. The Hugoniot data obtained in MD simulation show good agreement with the experimental results. The shock loading MD simulation shows lower phase transition pressure than hydrostatic loading, indicating an accelerant role of the deviatoric stress played in the shock induced <i>γ </i>→ <i>α</i> phase transition in Ce. The local lattice structure before and after shocked are recognized with polyhedral template matching and confirmed with radial distribution functions. Under the [011] and [111] loading, the lattice structure maintains the fcc before and after the shocks, and experiences a collapse during the last shock (the second shock for the [011] loading and the third shock for the [111] loading). The lattice structure also maintains fcc before and after the first shock for the [001] loading, while after the second shock the structure type is considered to be body-centered-tetragonal (bct) which is a meta-stable structure resulting from the used EAM potential for Ce. The fcc lattice rotation after shock is observed in the [011] and [111] loading after the phase transition, while no re-orientation occurs in the [001] loading.

Real and Fitted Spherical Indentations

Spherical indentations that rely on original date are analyzed with the physically correct mathematical formula and its integration that take into account the radius over depth changes upon penetration. Linear plots, phase-transition onsets, energies, and pressures are algebraically obtained for germanium, zinc-oxide and gallium-nitride. There are low pressure phase-transitions that correspond to, or are not resolved by hydrostatic anvil onset pressures. This enables the attribution of polymorph structures, by comparing with known structures from pulsed laser deposition or molecular beam epitaxy and twinning. The spherical indentation is the easiest way for the synthesis and further characterization of polymorphs, now available in pure form under diamond calotte and in contact with their corresponding less dense polymorph. The unprecedented results and new possibilities require loading curves from experimental data. These are now easily distinguished from data that are “fitted” to make them concur with widely used unphysical Johnson’s formula for spheres (“P = (4/3)h3/2R1/2E∗”) not taking care of the R/h variation. Its challenge is indispensable, because its use involves “fitting equations” for making the data concur. These faked reports (no “experimental” data) provide dangerous false moduli and theories. The fitted spherical indentation reports with radii ranging from 4 to 250 μm are identified for PDMS, GaAs, Al, Si, SiC, MgO, and Steel. The detailed analysis reveals characteristic features.

High pressure phase transitions and physical properties of Li2MgH4; implications for hydrogen storage

A systematic calculation has been performed in order to study phase transitions and hydrogen storage properties of ternary hydride Li2MgH4 under pressure. The structural, elastic, electronic and vibrational properties of Li2MgH4 are collected by means of density functional theory. There are three phases identified; Pbam at 0 GPa,Pnma at 5 GPa and Pna21 at 65 GPa Pbam and Pnma phases of Li2MgH4 are found to be mechanically and dynamically stable. Ductility of the phases are determined based on Pugh's criteria. It is found that Li2MgH4 becomes ductile at 5 GPa, otherwise it is a brittle material. Electronic band structures and corresponding partial density of states of phases are also obtained. All phases at 0 GPa, 5 GPa and 65 GPa have wide band gaps, indicating that Li2MgH4 is an insulator at all pressures. The phonon dispersion curves of Pbam and Pnma phases have no imaginary frequency indicating that both phases of Li2MgH4 are dynamically stable. The gravimetric hydrogen density of Li2MgH4 is calculated as 10.52 wt %, which is a great rate along with the hydrogen desorption temperature of 670 K.