Pressure-induced Phase Changes Research Articles

Pressure and Composition Effects on a Common Nanoparticle Ligand-Solvent Pair.

The effect of pressure on the properties of nanoparticles is a growing area of investigation. These measurements are typically performed in a colloidal suspension; however, pressure-induced changes in the interactions between the nanoparticle surface and the solvent are often neglected. Here, we report vibrational spectroscopy of a common nanoparticle ligand, 1-dodecanethiol, and a common solvent, toluene, under pressure. We find that the pressure-induced phase change of the 1-dodecanethiol is altered by the presence of toluene and that change depends on the concentration of the free ligand in the solution. At near-equal concentrations, phase segregation is observed and the dodecanethiol crystallizes independently from the toluene. On the other hand, at unequal concentrations, concerted phase transitions are observed in the dodecanethiol and toluene, and a disordered conformation of dodecanethiol is maintained under much higher pressures. These results shed light on the pressure-induced changes in intermolecular interactions between nanoparticle ligands and solvents, which must be considered in the design of high-pressure investigations of colloidal nanoparticles.

Observation of non-superconducting phase changes in nitrogen doped lutetium hydrides

The recent report of near-ambient superconductivity and associated color changes in pressurized nitrogen doped lutetium hydride has triggered worldwide interest and raised major questions about the nature and underlying physics of these latest claims. Here we report synthesis and characterization of high-purity nitrogen doped lutetium hydride LuH2±xNy. We find that pressure conditions have notable effects on Lu-N and Lu-NH chemical bonding and the color changes likely stem from pressure-induced electron redistribution of nitrogen/vacancies and interaction with the LuH2 framework. No superconducting transition is found in all the phases at temperatures 1.8-300 K and pressures 0-38 GPa. Instead, we identify a notable temperature-induced resistance anomaly of electronic origin in LuH2±xNy, which is most pronounced in the pink phase and may have been erroneously interpreted as a sign of superconducting transition. This work establishes key benchmarks for nitrogen doped lutetium hydrides, allowing an in-depth understanding of its novel pressure-induced phase changes.

Structural phase transition of BiSb and formation of Weyl semimetallic phase under pressure: calculations and experiments

BiSb was found to transform into a Weyl semimetal at ∼4 GPa. Detailed pressure-induced structural phase transitions and changes in electrical transport properties are explored.

Effect of Low Pressure on Tetragonal to Cubic Phase Transition of Methylammonium Lead Iodide Perovskite.

The attractive optoelectronic properties of MAPbI3 (MA = CH3NH3), one of the most common halide perovskites, can be complicated by its tetragonal → cubic structural phase transition just above room temperature. We show that decreasing the ambient pressure can move that phase transition by ∼40 °C (at ∼10-3 mbar). Our report also includes control experiments, which show that desorption of water or oxygen can be excluded as possible causes for the change in phase transition temperature. On the basis of diffraction data, we postulate that an optimum volume is required to initiate a T → C phase transition. The pressure-induced phase change in effect stabilizes the tetragonal phase for work around room temperature, even if some natural heating occurs.

Pressure-Induced Phase Changes in Cesium Lead Bromide Perovskite Nanocrystals with and without Ruddlesden–Popper Faults

Lead halide perovskites have a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. Despite several reports on high-pressure...

Asymmetric double-core fiber for hydraulic pressure sensing

Abstract We propose a novel asymmetric double-core fiber (ADCF) which provides an ADCF-based hydraulic pressure sensing system without using optical spectrum measurement based on a costly optical spectrum analyzer. The ADCF is designed with asymmetric double fiber cores and two side air holes, which ensure the asymmetric pressure-induced phase change for the guiding light in the two fiber cores. Based on the finite element method, optical properties of the ADCF under hydraulic pressure are investigated considering the fiber parameters such as the refractive index, the distance of two holes or two fiber cores, the size of two fiber cores and the fiber length. A hydraulic pressure sensing system based on the ADCF and a low-cost linear array charge coupled device (CCD) is proposed, where the hydraulic pressure demodulation is achieved by measuring the movement of the spatial interference pattern of the light from the double fiber cores of the ADCF. Numerical simulation shows that the hydraulic pressure sensor based on a 10-cm ADCF and a 2.867-cm CCD has a sensing range from 0 to 900 MPa and a sensitivity of 0.3 mm/MPa. In this paper, a low-cost CCD is used instead of a costly spectrometer, which greatly reduces the cost of the sensing system. The proposal for the ADCF and the solution for hydraulic pressure sensing are of great significance to promote the wide application of the hydraulic pressure fiber sensor.

Photoabsorption markers of pressure-induced phase changes in small mercury clusters. A case study on Hg8.

Photoabsorption spectra of Hg8 have been calculated at various cluster temperatures and external pressures. A diatomics-in-molecules method has been used to model cluster electronic structure and classical isothermal–isobaric Monte Carlo simulations have been employed for sampling representative cluster configurations. Contributions of different structural isomers of the Hg8 cluster have been analyzed and related to structural transitions in the cluster, particularly those induced by an increased pressure.

Influence of Metal Substitution on the Pressure-Induced Phase Change in Flexible Zeolitic Imidazolate Frameworks.

Metal-organic frameworks that display step-shaped adsorption profiles arising from discrete pressure-induced phase changes are promising materials for applications in both high-capacity gas storage and energy-efficient gas separations. The thorough investigation of such materials through chemical diversification, gas adsorption measurements, and in situ structural characterization is therefore crucial for broadening their utility. We examine a series of isoreticular, flexible zeolitic imidazolate frameworks (ZIFs) of the type M(bim)2 (SOD; M = Zn (ZIF-7), Co (ZIF-9), Cd (CdIF-13); bim- = benzimidazolate), and elucidate the effects of metal substitution on the pressure-responsive phase changes and the resulting CO2 and CH4 step positions, pre-step uptakes, and step capacities. Using ZIF-7 as a benchmark, we reexamine the poorly understood structural transition responsible for its adsorption steps and, through high-pressure adsorption measurements, verify that it displays a step in its CH4 adsorption isotherms. The ZIF-9 material is shown to undergo an analogous phase change, yielding adsorption steps for CO2 and CH4 with similar profiles and capacities to ZIF-7, but with shifted threshold pressures. Further, the Cd2+ analogue CdIF-13 is reported here for the first time, and shown to display adsorption behavior distinct from both ZIF-7 and ZIF-9, with negligible pre-step adsorption, a ∼50% increase in CO2 and CH4 capacity, and dramatically higher threshold adsorption pressures. Remarkably, a single-crystal-to-single-crystal phase change to a pore-gated phase is also achieved with CdIF-13, providing insight into the phase change that yields step-shaped adsorption in these flexible ZIFs. Finally, we show that the endothermic phase change of these frameworks provides intrinsic heat management during gas adsorption.

Pressure tuning of the thermal conductivity of gallium arsenide from first-principles calculations

In this paper, the variation of the lattice thermal conductivity of GaAs under pressure within the range of 0-20 GPa at room temperature is investigated by combining first-principles calculations with an iterative solution of the phonon Boltzmann transport equation. Firstly, we calculated the lattice thermal conductivity of GaAs at 12 GPa, which increases by more than 37% in comparison with that under atmospheric pressure. The detailed analysis of phonon heat transport properties shows that the pressure contributes to increased phonon group velocity coupled with decreased phonon relaxation time, resulting in the pressure-induced nonlinear increase of the thermal conductivity of zinc blende GaAs. Besides, not only the structure but also the phonon heat transport properties of GaAs transform from isotropic to anisotropic beyond the phase transition pressure. This study provides a quantitative understanding of the thermal conductivity of GaAs considering pressure-induced phase transitions and highlights the importance of pressure in tuning lattice thermal conductivity, especially in pressure-induced phase change materials.

Reversible Pressure-Controlled Depolymerization of a Copper(II)-Containing Coordination Polymer.

A unique pressure-induced Cu-N bond breaking/bond forming reaction is reported. The variation of pressure on a single crystal of a one-dimensional copper- (II)-containing coordination polymer (Cu2 L2 (1-methylpiperazine)2 ]n , where H2 L is 1,1'-(1,3-phenylene)-bis(4,4-dimethylpentane-1,3-dione)), was monitored using single crystal X-ray diffraction with the aid of a diamond anvil cell. At a very low elevated pressure (≈0.05 GPa) a remarkable reversible phase change was observed. The phase change results in the depolymerization of the material through the cleavage and formation of axial Cu-N bonds as well as "ring flips" of individual axially coordinated 1-methylpiperazine ligands. Overall, the pressure-induced phase change is associated with a surprising (and non-intuitive) shift in structure-from a 1-dimensional coordination polymer to a discrete dinuclear complex.

Pressure-Induced Frustration–Frustration Process in 1-Butyl-3-methylimidazolium Hexafluorophosphate, a Room-Temperature Ionic Liquid

We have found that the room-temperature ionic liquid (RTIL) reveals outstanding pressure-induced phase changes from a liquid state to a crystal polymorph and finally to a glass form upon compression by up to 8 GPa. The RTIL is 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], which offers the opportunity to investigate a variety of fluctuations in one system and can be completely recovered without dissociation or polymerization, even after decompression. Similar to charge frustration, spin ice-like frustration, and geometric frustration in high potential spintronics/multiferroic materials, the RTIL frustrations are classified into charge (scalar), orientation (vector), and coordination number (topology). Degrees of freedom at each state of [C4mim][PF6] are described by charge balancing, molecular orientational order/disorder, molecular conformations of the C4mim(+) cation, and the coordination number. Here, we show a novel "conformation glass" induced by high pressure.

Shock, release and Taylor impact of the semicrystalline thermoplastic polytetrafluoroethylene

The high strain-rate response of polymers is a subject that has gathered interest over recent years due to their increasing engineering importance, particularly in load bearing applications subject to extremes of pressure and strain rate. The current work presents two specific sets of experiments interrogating the effect of dynamic, high-pressure loading in the regime of the phase II to phase III pressure-induced crystalline phase transition in polytetrafluoroethylene (PTFE). These are gas-gun driven plate- and Taylor impact. Together these experiments highlight several effects associated with the dynamic, pressure-induced phase transitions in PTFE. An elevated release wave speed shows evidence of a pressure-induced phase change at a stress commensurate with that observed statically. It is shown that convergence between analytic derivations of release wave speed and the data requires the phase II to III transition to occur. Taylor impact is an integrated test that highlights continuum behavior that has origin in mesoscale response. There is a rapid transition from ductile to brittle behavior observed that occurs at a pressure consistent with this phase transition.

Pressure-induced phase change in poly(tetrafluoroethylene) at modest impact velocities

Although poly(tetrafluoroethylene) (PTFE) is an unusually ductile polymer, it undergoes an abrupt ductile-brittle transition at modest impact velocities. No previous explanation for this behavior has been found after an extensive literature search. In this paper, we examine the role of a pressure-induced phase transition in PTFE in the dynamic failure of Taylor cylinder samples. There is a known phase transition in PTFE with a marked decrease in volume and compressibility that occurs at 0.5–0.65GPa at 21°C, with the transition pressure inversely related to temperature. Varying the temperature of the samples in the experiment revealed that the phase transition is probably involved in sample failure because the ductile/brittle transition velocity increased for decreasing temperature, despite the material fracture toughness decreasing. Additionally, Taylor tests were carried out on samples of poly(chlorotrifluoroethylene) PCTFE to investigate the behavior of a similar material to PTFE but without a pressure-induced phase transition. PCTFE did not exhibit an abrupt ductile to brittle transition.

Structural changes and thermal expansion behavior of ultrafine titanium powders during compaction and heating

This work represents an attempt to understand the nature of micron and attrition milled nano-sized titanium powders on two different aspects, i.e., pressure-induced phase change and thermal expansion. Contraction in the volume of unit cell in terms of decrease in interplaner spacing (d) has been observed in both powders and tends to restore upon annealing. At a given pressure, nano titanium shows a smaller decrease indrelative to micron titanium. The stress analysis of the compacts indicates higher value of residual stresses and deformations in micron powder than in nano powder. The dilatometric study reveals, first, the release of internal stresses and entrapped gases causes huge expansion in nanopowder compacts during heating. Secondly, there is no significant difference in the expansion coefficients of sintered micro- and nanocrystalline titanium samples.

High-pressure recrystallisation—a route to new polymorphs and solvates of acetamide and parabanic acid

Although the application of high pressure to organic compounds may favour thermodynamically the adoption of a new polymorphic form, for compounds with high melting points there is often a substantial kinetic barrier to be overcome before the molecules can rearrange. Hence pressure-induced phase changes are often very slow or may not occur at all. We have overcome this problem by growing single crystals from solution at high pressure. Using this technique we have prepared and structurally characterised for the first time a new polymorph of acetamide and a sesquihydrate of parabanic acid. We also report the results of a high-pressure powder neutron diffraction study on parabanic acid-d 2, which show that up to a pressure of 2.1 GPa direct compression does not induce a phase transition although several intermolecular interactions are substantially altered.

Modelling and numerical simulation of convection driven high pressure induced phase changes

High pressure induced freezing and thawing processes are investigated by means of modelling and numerical simulation. The enthalpy–porosity model is reformulated in terms of conservation equations of mass, momentum and heat for a compressible medium. It can be shown that convective fluid motion has a major influence on the formation of the ice front. Furthermore, due to a vanishing density anomaly under high pressure, natural convection exhibits a different behaviour than under low pressure thus the formation of the ice front under high pressure obeys different mechanisms than that under ambient pressure.

Spherical indentation of compound semiconductors

Abstract Details of indentation-induced mechanical deformation of GaAs, InP and GaN have been studied. In particular, the origin of the discontinuities in the load-penetration curves during loading (so-called ‘pop-in’ events) was examined. Cross-sectional transmission electron microscopy (XTEM) samples of indents were prepared using focused-ion-beam milling. Atomic force microscopy (AFM) was used to examine the surface deformation after indentation. In all materials, slip appeared to be the prime mechanism of plastic deformation, and, in contrast with Si, no evidence of pressure-induced phase changes was found. Slip along the {111} planes is clearly observed by XTEM and AFM in both GaAs and InP following indentation above the ‘pop-in’ threshold. At high loads, subsurface median cracking is also revealed in these materials. This cracking appeared to be nucleated at the intersection of the slip planes. This suggests that dislocation pile-up at the slip band intersection and the consequential shear stress build-up cause the nucleation of a microcrack. In contrast, although slip is observed in GaN (predominantly along the basal planes parallel to the surface), no cracking or film delamination has been found. The difference between the crystallographic structures and dislocation densities of wurtzite GaN and cubic GaAs and InP can account for the different deformation modes.

Spherical indentation of compound semiconductors

Abstract Details of indentation-induced mechanical deformation of GaAs, InP and GaN have been studied. In particular, the origin of the discontinuities in the load-penetration curves during loading (so-called ‘pop-in’ events) was examined. Cross-sectional transmission electron microscopy (XTEM) samples of indents were prepared using focused-ion-beam milling. Atomic force microscopy (AFM) was used to examine the surface deformation after indentation. In all materials, slip appeared to be the prime mechanism of plastic deformation, and, in contrast with Si, no evidence of pressure-induced phase changes was found. Slip along the {111} planes is clearly observed by XTEM and AFM in both GaAs and InP following indentation above the ‘pop-in’ threshold. At high loads, subsurface median cracking is also revealed in these materials. This cracking appeared to be nucleated at the intersection of the slip planes. This suggests that dislocation pile-up at the slip band intersection and the consequential shear stress bu...

Neutral to Ionic Pressure-Induced Phase Transition in the Mixed Stack Charge-Transfer Crystal ClMePD-DMeDCNQI

In the present work we report a spectroscopic investigation of the pressure-induced neutral to ionic (NI) phase transition in the mixed stack charge-transfer crystal 2-chloro-5-methyl-p-phenylenediamine 2,5-dimethyl-dicyanoquinonediimine, ClMePD-DMeDCNQI. The pressure induced phase change is studied by calculating, from spectroscopic data, the degree of ionicity, 𝜌 , which undergoes a continuous change as a function of pressure, reaching a value of about 0.65 at 3 GPa. The NI crossing reveals unique properties with respect to previously studied cases on similar compounds.

Pressure-induced phase changes of argon hydrate and methane hydrate at room temperature

High-pressure experiments of argon hydrate and methane hydrate were performed using a diamond anvil cell in a pressure range of 0.2 to 10.0GPa at room temperature. In-situ X-ray diffractometry and optical microscopy revealed that two high pressure structures of argon hydrate, a primitive tetragonal structure and a body-centered orthorhombic structure, existed under pressures of up to 6.5GPa. The structural analysis showed that the tetragonal structure was composed solely of 14-hedra accommodating two argon atoms, and that the body-centered orthorhombic structure belonged to a“filled-ice”structure, i.e., a new type of structure in a water-guest system. As for methane hydrate, three high-pressure structures, a hexagonal structure, a primitive orthorhombic structure, and a body-centered orthorhombic structure, were found. The structural analysis indicated that the hexagonal structure was a modified structure of a hexagonal one reported at ambient pressure, and that the latter orthorhombic structure was the“filled-ice”structure.