Low-pressure Phase Research Articles

Superconductivity and strong anharmonicity in novel Nb–S phases

In this work we explore the phase diagram of the binary Nb–S system from ambient pressures up to 250 GPa using ab initio evolutionary crystal structure prediction. We find several new stable compositions and phases, especially in the high-pressure regime, and investigate their electronic, vibrational, and superconducting properties. Our calculations show that all materials, besides the low-pressure phases of pure sulfur, are metals with low electron–phonon (ep) coupling strengths and critical superconducting temperatures below 15 K. Furthermore, we investigate the effects of phonon anharmonicity on lattice dynamics, ep interactions, and superconductivity for the novel high-pressure phase of Nb2S, demonstrating that the inclusion of anharmonicity stabilizes the lattice and enhances the ep interaction.

Crystal and electronic structure engineering of tin monoxide by external pressure

Although tin monoxide (SnO) is an interesting compound due to its p-type conductivity, a widespread application of SnO has been limited by its narrow band gap of 0.7 eV. In this work, we theoretically investigate the structural and electronic properties of several SnO phases under high pressures through employing van der Waals (vdW) functionals. Our calculations reveal that a metastable SnO (β-SnO), which possesses space group P21/c and a wide band gap of 1.9 eV, is more stable than α-SnO at pressures higher than 80 GPa. Moreover, a stable (space group P2/c) and a metastable (space group Pnma) phases of SnO appear at pressures higher than 120 GPa. Energy and topological analyses show that P2/c-SnO has a high possibility to directly transform to β-SnO at around 120 GPa. Our work also reveals that β-SnO is a necessary intermediate state between high-pressure phase Pnma-SnO and low-pressure phase α-SnO for the phase transition path Pnma-SnO →β-SnO → α-SnO. Two phase transition analyses indicate that there is a high possibility to synthesize β-SnO under high-pressure conditions and have it remain stable under normal pressure. Finally, our study reveals that the conductive property of β-SnO can be engineered in a low-pressure range (0–9 GPa) through a semiconductor-to-metal transition, while maintaining transparency in the visible light range.

Microstructure and strengthening mechanisms in FCC-structured single-phase TiC–CoCrFeCuNiAl0.3 HEACs with deformation twinning

Abstract This study aimed to investigate dislocation structures in CuNiCoFeCrAl0.3 alloy particles using a transmission electron microscope (TEM). A non-equiatomic CoCrFeCuNiAl0.3 high-entropy alloy composite (HEAC) reinforced with TiC5vol% nanoparticles was produced by mechanical alloying and spark plasma sintering. X-ray and TEM microanalyses confirmed the predominance of a nanotwinned single-phase face-centered cubic (FCC) solid solution with TiC nanoparticles. Further findings revealed that twin structure formation in FCC high-entropy alloys (HEAs) by the powder metallurgy technology was dependent on dislocations and the stratified structure formed in particles during mechanical alloying, low stacking-fault energy γSF, pressure, and hardness phase. The TiC5vol%–CoCrFeCuNiAl0.3 HEAC had the following excellent comprehensive mechanical properties: yield strength, 1582 MPa; fracture strength, 2185 MPa; and plastic strain, 23.60%. The dislocation glide and semi-twin expansion in HEAs provided excellent plasticity, while TiC particles and transgranular twins contributed to the material strength.

Calculation for High Pressure Behaviour of Potential Solar Cell Materials Cu2FeSnS4 and Cu2MnSnS4

Exploring alternatives to the Cu2ZnSnS4 kesterite solar cell absorber, we have calculated first principle enthalpies of different plausible structural models (kesterite, stannite, P4¯ and GeSb type) for Cu2FeSnS4 and Cu2MnSnS4 to identify low and high pressure phases. Due to the magnetic nature of Fe and Mn atoms we included a ferromagnetic (FM) and anti-ferromagnetic (AM) phase for each structural model. For Cu2FeSnS4 we predict the following transitions: P4¯ (AM) →16.3GPa GeSb type (AM) →23.0GPa GeSb type (FM). At the first transition the electronic structure changes from semi-conducting to metallic and remains metallic throughout the second transition. For Cu2MnSnS4, we predict a direct AM (kesterite) to FM (GeSb-type) transitions at somewhat lower pressure (12.1 GPa). The GeSb-type structure also shows metallic behaviour.

Dependence of the thermoluminescent glow-peaks of Mexican zircon of different particle size on the exposure to high gamma radiation doses

The effect of gamma irradiation on the low-pressure phase of ZrSiO4 (zircon) was studied by the thermoluminescence technique and the analysis of the intensity in the TL glow-curves. Specifically, in this approach, the GlowFit computer program was used to solve the complex TL glow-curves produced in zircon after gamma-irradiation. Dose levels of 5, 10, and 30 Mega Gray (MGy) were applied to zircon samples exhibiting particle size of 100, 200, and 300 mesh. The TL results were associated with the main changes in lattice planes observed in the XRD spectra of irradiated samples. Furthermore, the surface damages due to gamma irradiation were evaluated by Scanning Electron Microscopy (SEM) and Atomica Force Microscopy (AFM). The effect of gamma irradiation is verified in the shallow layers of all the zircon samples, but no dependence of the superficial damage was observed regarding the dose of gamma. The size of the zircon particulate was also evaluated as a result of the surface damage in order to elucidate any relationship with the exposure of the material to the radiation.

Simulation of phase transitions and strength evolution in quartzite in shock experiments

Quartzite is a basic rock constituent. It has a complicated phase diagram where besides a low-pressure phase (α-quartz), two high-pressure solid ones are present – coesite and stishovite. Also present are the hightemperature cristobalite and tridymite phases with very small stability regions. In shock experiments, the transition to coesite almost never realizes and the transition from α-quartz to stishovite proceeds in a strongly nonequilibrium regime characterized by metastability and a long transition time. Quartzite is polycrystalline quartz with small amounts of impurities. The paper describes experiments with samples of Pervouralsk quartzite with mass fractions of SiO2 above 96%.

Structural, vibrational and electronic properties of α'-Ga2S3 under compression.

We report a joint experimental and theoretical study of the low-pressure phase of α'-Ga2S3 under compression. Theoretical ab initio calculations have been compared to X-ray diffraction and Raman scattering measurements under high pressure carried out up to 17.5 and 16.1 GPa, respectively. In addition, we report Raman scattering measurements of α'-Ga2S3 at high temperature that have allowed us to study its anharmonic properties. To understand better the compression of this compound, we have evaluated the topological properties of the electron density, the electron localization function, and the electronic properties as a function of pressure. As a result, we shed light on the role of the Ga-S bonds, the van der Waals interactions inside the channels of the crystalline structure, and the single and double lone electron pairs of the sulphur atoms in the anisotropic compression of α'-Ga2S3. We found that the structural channels are responsible for the anisotropic properties of α'-Ga2S3 and the A'(6) phonon, known as the breathing mode and associated with these channels, exhibits the highest anharmonic behaviour. Finally, we report calculations of the electronic band structure of α'-Ga2S3 at different pressures and find a nonlinear pressure behaviour of the direct band gap and a pressure-induced direct-to-indirect band gap crossover that is similar to the behaviour previously reported in other ordered-vacancy compounds, including β-Ga2Se3. The importance of the single and, more specially, the double lone electron pairs of sulphur in the pressure dependence of the topmost valence band of α'-Ga2S3 is stressed.

Pressure-Induced Phase Transition in Mn(Ta,Nb)2O6: An Experimental Investigation and First-Principle Study.

The high-pressure and high-temperature behaviors of manganotantalite Mn(Ta,Nb)2O6 have been investigated by single-crystal X-ray diffraction and Raman spectroscopy combined with diamond anvil cell technique, as well as first-principle calculations. A pressure-induced reversible phase transition of manganotantalite occurs at 9.5 GPa and room temperature, accompanied by a large volume collapse (∼7.0%) and drastic color change from brownish-yellow to red. The space groups of low-pressure (LP) and high-pressure (HP) phases are the same (Pbcn), but the coordination numbers of Mn increase from six to eight and Ta increase from six to seven, respectively. The band gap becomes narrow from 2.37 to 1.59 eV. We determined the P-T phase diagram of manganotantalite with a positive Clapeyron slope of dP/dT = 0.0073 GPa/K. The P-V data were fitted to a second-order Birch-Murnaghan equation of state with B0 = 149(4) GPa for the LP phase and B0 = 188(3) GPa for the HP phase. The isothermal Grüneisen parameters were determined to be 0.23∼2.03 of the LP phase and 0.59∼0.86 of the HP phase for Raman modes. High-pressure behaviors of Mn(Ta,Nb)2O6 indicate that this kind of material is a potential effective pressure sensor to monitor pressure change or warn pressure abnormality.

Equation of state and high-pressure phase behaviour of SrCO<sub>3</sub>

Abstract. The high-pressure phase transition of strontianite (SrCO3) was investigated at ambient temperature by means of powder and single-crystal X-ray diffraction. The samples were compressed in a diamond anvil cell to a maximum pressure of 49 GPa. Structure refinements confirm the existence of SrCO3 in the low pressure aragonite-type phase Pmcn (62) up to about 26 GPa. Above this pressure, SrCO3 transforms into a high-pressure phase with post-aragonite crystal structure Pmmn (59). Fitting the volume extracted from the compression data to the third-order Birch–Murnaghan equation of state for the low-pressure phase of SrCO3 yields K0=62.7(6) GPa and K0′=3.2(1), and for the high-pressure phase this yields K0=103(10) GPa and K0′=2.3(6). The unit cell parameters change non-uniformly, with the c axis being 4 times more compressible than the a and b axes. Our results unequivocally show the existence of a Pmmn structure in SrCO3 above 26 GPa and provide important structural parameters for this phase.

Crystal structure of Ca(Fe<sub>0.4</sub>Si<sub>0.6</sub>)O<sub>2.8</sub> oxygen-deficient perovskite

The crystal structure of Ca(Fe3+0.4Si0.6)O2.8 oxygen-deficient perovskite phase synthesized at 12 GPa and 1400 °C was studied using synchrotron powder X-ray diffraction. The phase is isostructural to low-pressure phase of Ca(Al0.4Si0.6)O2.8. The structure was refined by the Rietveld method and is consists of a perovskite-like triple-layer of corner-shared (Fe3+,Si)O6 octahedra and a double-layer of SiO4 tetrahedra those are stacked alternatively in the [111] direction of ideal cubic perovskite. Small degree of Fe3+/Si disorder was detected between two octahedral sites. The structure is compared with other oxygen-deficient perovskites.

Effects of cycle periods and pressure amplitudes of alternating pressure on sacral skin blood flow responses

BackgroundThere are no guidelines on selecting alternating pressure (AP) configurations on increasing sacral skin blood flow (SBF). AimThe specific aims were to compare different cycle periods and pressure amplitudes of AP on sacral SBF responses in healthy people to establish the efficacy and safety of the protocols. MethodsTwo studies were tested, including the cycle period study (8 2.5-min vs 4 5-min protocols) and the pressure amplitude study (75/5 vs 65/15 mmHg protocols). Sacral SBF was measured using laser Doppler flowmetry (LDF) in 20 participants. AP loads were randomly applied using an indenter through the rigid LDF probe. Each protocol included a 10-min baseline, 20-min AP and 10-min recovery periods. A 30-min washout period was provided. The SBF response was normalized to the baseline SBF of each condition of each participant. ResultsFor the cycle period study, the 4 5-min cycle protocol partially restored more SBF than the 8 2.5-min cycle protocol at the low-pressure phase (0.87 ± 0.04 vs 0.71 ± 0.03, p < 0.05) and at the high-pressure phase (0.25 ± 0.03 vs 0.19 ± 0.03, p < 0.05). For the pressure amplitude study, the 75/5 mmHg protocol partially restored more sacral SBF than the 65/15 mmHg protocol at the low-pressure phase (0.87 ± 0.1 vs 0.25 ± 0.03, p < 0.05) but not at the high-pressure phase (0.23 ± 0.02 vs 0.21 ± 0.02, non-significant). ConclusionThis study demonstrated that 1) a cycle period of 5 min was better than 2.5 min and 2) a pressure amplitude of 75/5 mmHg was better than 65/15 mmHg. The finding provides insights for selecting the AP configurations for increasing SBF.

PH-Dependent Water Clusters in Photoacid Solution: Real-Time Observation by ToF-SIMS at a Submicropore Confined Liquid-Vacuum Interface.

Water clusters are ubiquitously formed in aqueous solutions by hydrogen bonding, which is quite sensitive to various environment factors such as temperature, pressure, electrolytes, and pH. Investigation of how the environment has impact on water structure is important for further understanding of the nature of water and the interactions between water and solutes. In this work, pH-dependent water structure changes were studied by monitoring the changes for the size distribution of protonated water clusters by in-situ liquid ToF-SIMS. In combination with a light illumination system, in-situ liquid ToF-SIMS was used to real-time measure the changes of a light-activated organic photoacid under different light illumination conditions. Thus, the proton transfer and pH-mediated water cluster changes were analyzed in real-time. It was found that higher concentration of free protons could lead to a strengthened local hydrogen bonding network as well as relatively larger protonated water clusters in both organic acid and inorganic acid. Besides, the accumulation of protons at the liquid-vacuum interface under light illumination was observed owing to the affinity of organic molecules to the low-pressure gas phase. The application of in-situ liquid ToF-SIMS analysis in combination with in-situ light illumination system opened up an avenue to real-time investigate light-activated reactions. Besides, the results regarding water structure changes in acidic solutions showed important insights in related atmospheric and physiochemical processes.

A novel high-pressure polymorph of TaSi2

We report a novel high-pressure cubic cF24 phase of TaSi2 which is stable above 185 GPa through first-principles swarm intelligence structure search method. Results on enthalpy difference show that the structural transition of TaSi2 from the ambient hexagonal C40 phase to cF24 phase is first order with a volume drop of 4.1%. Under compression, the formation of cubic cF24 phase originates from the dramatic polymerization of one dimensional helical Si chain in the low-pressure C40 phase creating peculiar 3D Si tetrahedrons framework embedded in diamond-like Ta sublattice. The emergence of 3D Si tetrahedrons framework in cF24 phase are suggested to be the main causes for its weaker metallic nature compared to the hexagonal C40 phase.

Resistive anisotropy of candidate excitonic insulator Ta2NiSe5 under pressure

We have measured anisotropic electrical resistivity of quasi-one-dimensional Ta2NiSe5 single crystals under pressure. While the in-plane anisotropy is almost pressure independent, the inter-plane one greatly decreases at P c ∼ 3 GPa accompanied by a pressure-induced semiconductor-semimetal transition. Nevertheless, a similar anomaly in the temperature dependence of in-plane anisotropy has been observed in each phase. This anomaly in the low-pressure phase below P c is regarded as an excitonic transition, and thus both of these anomalies are suppressed with increasing pressure. These results suggest that the suppression of the excitonic binding energy with pressure is caused by the increase of free carrier densities and the inter-plane conductivity.

Pressure- and temperature- driven phase transitions in pyridine

Studies of aromatic and heteroaromatic compounds under high pressure in recent years have attracted great attention to the possibility of the synthesis of amorphous hydrogenated materials with attractive technological properties. In this context, pyridine is of particular interest in terms of its rich polymorphism. We studied the liquid and solid phases of pyridine by dielectric spectroscopy and pulsed ultrasonic methods in the pressure and temperature range P <4.2 GPa and 78 <T <350 K. It is shown that up to 20% of the sample passes into the glassy state during crystallization from the melt to the low-pressure phase. Disordered phase has a high dielectric constant and a high absorption coefficient of ultrasound. All these features are absent in the high pressure phase.

Ironic Process Theory In Softball Pitching: How Knowing Information About An Opponent’s Strengths Affects Performance

INTRODUCTION: As athletic competition and college athletics continue to grow and flourish, there is an increased emphasis on game preparation. Collegiate softball pitchers are expected to handle an immense amount of pressure, perform with precision, and incur few errors. PURPOSE: Examine the Ironic Process Theory related to fast pitch softball pitching and to determine how knowing information about an opponent’s strengths affects experienced pitcher’s performance under pressure. METHODS: Experienced college softball pitchers (n = 12) were recruited as subjects. Each pitcher was randomly instructed through two 30 pitch phases (a high and low pressure phase) with two different conditions: black target only condition (BOTC) or black and red target condition (BRTC). Subjects were asked to aim and hit the black target and avoid the red target. The black target represented the weakness of the opponent and the red target represented the strength of the opponent. Performance pressure was measured before each phase using the Mental Readiness Form (MRF-3) (Krane, 1994). RESULTS: Pre-MRF-3 reached statistical significance across the between-subjects factor of pressure, [t(22) = 3.102, p = 0.005] with a mean difference of 4.75 (95% C.I. 1.57 to 7.92) indicating that the pressure situation induced an increase in perceived anxiety and stress. ANCOVA did not reach statistical significance on the main effects of black targets hit nor the interactions terms for black targets hit by MRF-3 and black targets hit by Pressure. This finding asserts that there was no difference in performance between BRTC and BOTC across pressure after adjusting for perceived anxiety. There was no statistically significant difference of red target hit between the high pressure and low pressure situations, d = 0.25 (95% C.I. -0.463 to 0.963), t(22) = 0.723, p = 0.963. DISCUSSION: Practically speaking, the pitchers in this study did perform more effectively in the high pressure situation. Although different from previous Ironic Theory research, it is important to note this increased ability for pitchers to hit a desired target while under pressure. Even if not statistically significant, this can help pitchers and coaches understand the link between pressure and performance more effectively, and add training components to improve in stress situations.

High-pressure Raman scattering and x-ray diffraction studies of MgTa2O6

High-pressure Raman scattering and angle dispersive synchrotron x-ray diffraction studies have been carried out on magnesium tantalate (MgTa2O6) up to 41 GPa at room temperature. A pressure-induced phase transition at pressures above 10 GPa accompanied by softening of the internal ν11 (Eg) modes has been observed. Another phase transition took place at 20.2 GPa, which indicates that the edge shared TaO6 octahedra are distorted. The equation of state for MgTa2O6 in low pressure phase I was obtained by the second-order Birch–Murnaghan method, yielding a zero-pressure bulk modulus B0 of 205.8 ± 8.4 GPa. The pressure dependencies of the lattice vibration mode were extracted, and it is suggested that the instability of the trirutile structure at high pressure is related to the strong deformation of the TaO6 octahedra. Decompression measurements suggest that the pressure-induced transformation is irreversible.

Pressure-induced instability of the fergusonite phase of EuNbO4 studied by in situ Raman spectroscopy, x-ray diffraction, and photoluminescence spectroscopy

In this article, we present high-pressure experimental investigations on EuNbO4, an interesting technologically important material, using synchrotron based x-ray powder diffraction, Raman spectroscopy, and europium photoluminescence measurements up to 39.2, 31.6, and 32.4 GPa, respectively. All three techniques show the stability of the ambient monoclinic phase until 20 GPa. Beyond that, a pressure-induced structural phase transition takes place with the coexistence of two phases over a wide pressure range. The structure of the high-pressure phase has been determined as orthorhombic (space group: Imma) with a volume discontinuity of nearly 9% at the transition indicating the nature of transition to be first order. The high-pressure phase remains stable up to ∼40 GPa. The equation of state data for both phases have been determined, and the modulus of incompressibility indicates that the high-pressure phase with bulk modulus as 246(29) GPa is less compressible compared to the low-pressure phase having bulk modulus as 138(4) GPa. The transition involves a change in the coordination polyhedron around europium, increasing the coordination number from 8 in the fergusonite to 12 in the orthorhombic phase, while the coordination around niobium remains 4 in both the phases. All the 18 expected Raman modes in the fergusonite structure have been observed, and their variation as a function of pressure has been reported. The mode Grüneisen parameters for the observed Raman frequencies for monoclinic and orthorhombic phases have been determined. Europium photoluminescence spectra show drastic changes near the transition pressure, which point toward changes in the local environment around the europium ion.

Quantitative evaluation of enhanced Er luminescence in GaAs-based two-dimensional photonic crystal nanocavities

We evaluate the enhancement of Er luminescence coupled to two-dimensional (2D) photonic crystal (PC) nanocavities by means of photoluminescence measurements and numerical simulations. L3 PC nanocavities are utilized for characterization and evaluation, with GaAs:Er,O grown by low-pressure organometallic vapor phase epitaxy as the active layer. Optical characterization at room temperature demonstrates a 5.8-fold enhancement of Er luminescence due to coupling to the cavity mode of the 2D-PC nanocavities. This enhancement of Er luminescence is supported by a finite-difference time domain simulation where an enhancement of 4.1 times is found, which is in reasonable agreement with the observed experimental results. These results pave the way toward understanding the interaction between cavity modes in PC nanocavities and luminescence from rare-earth elements.

Prediction of a novel high-pressure phase of hydrogen peroxide

The binary H-O system almost exclusively exists in the form of water ice with stoichiometry of ${\mathrm{H}}_{2}\mathrm{O}$ in a wide range of pressure (\ensuremath{\sim}5 TPa) that is one of the most abundant substances in the solar system. Hydrogen peroxide (${\mathrm{H}}_{2}{\mathrm{O}}_{2}$) is only metastable at ambient condition. We herein report a stable ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ phase at high pressure identified by first-principles calculations in combination with a swarm-intelligence structure search. The predicted ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ compound is formed by stacking of an intriguing planar ${\mathrm{H}}_{2}{\mathrm{O}}_{2}$ molecule different from the metastable three-dimensional isomer at ambient condition. The planarity of hydrogen peroxide molecules is expected to be caused by the steric repulsion between molecules at high pressure, which is responsible for the stability of the crystal. The phase is energetically stable in the pressure range of \ensuremath{\sim}423--600 GPa. It is thus a low-pressure phase of binary H-O system with H:O stoichiometry other than 2:1 and promises to be accessible in static compression experiment.