Pressure-induced Amorphization Research Articles

The impact of alcohol and ammonium fluoride on pressure-induced amorphization of cubic structure I clathrate hydrates.

We have investigated pressure-induced amorphization (PIA) of an alcohol clathrate hydrate (CH) of cubic structure type I (sI) in the presence of NH4F utilizing dilatometry and x-ray powder diffraction. PIA occurs at 0.98GPa at 77K, which is at a much lower pressure than for other CHs of the same structure type. The amorphized CH also shows remarkable resistance against crystallization upon decompression. While amorphized sI CHs could not be recovered previously at all, this is possible in the present case. By contrast to other CHs, the recovery of the amorphized CHs to ambient pressure does not even require a high-pressure annealing step, where recovery without any loss of amorphicity is possible at 120K and below. Furthermore, PIA is accessible upon compression at unusually high temperatures of up to 140K, where it reaches the highest degree of amorphicity. Molecular dynamics simulations confirm that polar alcoholic guests, as opposed to non-polar guests, induce cage deformation at lower pressure. The substitution of NH4F into the host-lattice stabilizes the collapsed state more than the crystalline state, thereby enhancing the collapse kinetics and lowering the pressure of collapse.

The self-healing and robust photostability of (PEA)2PbI4 perovskite via pressure-induced amorphization and recrystallization

The photostability of hybrid organic-inorganic perovskites (HOIPs) is an important criterion due to unavoidability of light in photoelectric elements of solar cells, light emitting diodes (LEDs) or photo detectors. This research aims to explore the photostability of a compressed two-dimensional (2D) (PEA)2PbI4 crystal. The structural and compositional responses of the sample to laser illumination under pressure were recorded via in-situ photoluminescence (PL) spectroscopy and morphological analysis methods. The enhancement in laser endurance limit from 3 to 11 mW and long-duration photostablity were achieved along with structural self-healing in the recrystallized (PEA)2PbI4 sample. The robust stability of the material was ascribed to pressure-induced suppression of the nonradiative decay and grain boundary minimization in the rebirthed crystal. A similar result was also obtained in compressed (BA)2PbI4 and MAPbI3 crystals. Therefore, high pressure offers an effective and green method to improve the photostability of HOIP crystals and has a profound significance in the widespread application of HOIP-based photoelectric devices.

Size-Dependent High-Pressure Behavior of Pure and Eu3+-Doped Y2O3 Nanoparticles: Insights from Experimental and Theoretical Investigations.

We report a joint high-pressure experimental and theoretical study of the structural, vibrational, and photoluminescent properties of pure and Eu3+-doped cubic Y2O3 nanoparticles with two very different average particle sizes. We compare the results of synchrotron X-ray diffraction, Raman scattering, and photoluminescence measurements in nanoparticles with ab initio density-functional simulations in bulk material with the aim to understand the influence of the average particle size on the properties of pure and doped Y2O3 nanoparticles under compression. We observe that the high-pressure phase behavior of Y2O3 nanoparticles depends on the average particle size, but in a different way to that previously reported. Nanoparticles with an average particle size of ~37 nm show the same pressure-induced phase transition sequence on upstroke and downstroke as the bulk sample; however, nanoparticles with an average particle size of ~6 nm undergo an irreversible pressure-induced amorphization above 16 GPa that is completed above 24 GPa. On downstroke, 6 nm nanoparticles likely consist of an amorphous phase.

Manipulating the phase stability of a halide perovskite, CH3NH3PbI3 by high-pressure cycling

Methylammonium lead iodide perovskite, CH3NH3PbI3 (MAPbI3) is a highly promising photovoltaic material, whose structural and electronic properties are very sensitive to applied stresses. Here, we show that the phase stability of a MAPbI3 crystal can be controlled by structural defects and strains. We synthesized a single crystal of MAPbI3 and measured its electrical properties across two successive structural phase transitions under applied pressure up to 4–9 GPa under multiple high-pressure cycling. We revealed three types of effects of this high-pressure treatment. (i) An increasing the level of defects and strains in the crystal greatly delays the first reconstructive phase transition, conserving the ambient-pressure tetragonal phase up to a record pressure of ∼1.4 GPa, much higher compared to its typical stability range of up to 0.3–0.5 GPa only. (ii) A time exposure of the crystal at a fixed pressure corresponding to the beginning of the second phase transition to a “destabilized” structure eliminated some of defects and strains; it down-shifted the phase boundaries back. (iii) A recrystallization of the sample after pressure-induced amorphization enhanced its structural stability against amorphization upon subsequent pressurization cycle. We proposed that the most abundant defects and strains, which could appear in the pressure-cycled MAPbI3 crystals, were related to migration of iodine atoms from their regular sites to the MA interstices. Temperature-dependent photoluminescence measurements indicated that the sample recovered after the high-pressure cycling crystallized back into the initial tetragonal crystal structure. Whereas, its stability range expanded toward lower temperatures by 20 K and a band gap slightly increased, from 1.59 to 1.625 eV (at 290 K). Using this organic-inorganic perovskite as an example, we show how the phase stability of such materials can be manipulated by creation and elimination of structural defects and local strains.

Pressure-induced phase transformations in Nb2O5 from ab initio molecular dynamics simulations

The equations of state for the H−, L– and T–Nb2O5 polymorphs were investigated with density functional theory. The shock Hugoniot curves were predicted from ab initio molecular dynamics simulations along 300–2500 K isotherms. Enthalpies derived from simulations at 300 K show that the H → L transition occurs at 6.2 GPa, in agreement with the measured value of ∼5.9 GPa, and is more favorable than the H → T transition. Pressure-induced amorphization (PIA) of L–Nb2O5 is in the range ∼18–25 GPa, consistent with experiments reporting PIA above 21.4 GPa. The simulated PIA of T–Nb2O5 between ∼16 and 26 GPa reproduces PIA observed above 19.2 GPa.

Strongly Modified Mechanical Properties and Phase Transition in AlPO4-17 Due to Insertion of Guest Species at High Pressure.

The porous aluminophosphate AlPO4-17 with a hexagonal erionite structure, exhibiting very strong negative thermal expansion, anomalous compressibility, and pressure-induced amorphization, was studied at high pressure by single-crystal and powder X-ray diffraction in the penetrating pressure transmitting media N2, O2, and Ar. Under pressure, these guest species were confirmed to enter the pores of AlPO4-17, thus completely modifying its behavior. Pressure-induced collapse in the xy plane of AlPO4-17 no longer occurred, and this plane exhibited close to zero area compressibility. Pressure-induced amorphization was also suppressed as the elastic instability in the xy plane was removed. Crystal structure refinements at a pressure of 5.5 GPa indicate that up to 28 guest molecules are inserted per unit cell and that this insertion is responsible for the reduced compressibility observed at high pressure. A phase transition to a new hexagonal structure with cell doubling along the a direction was observed above 4.4 GPa in fluid O2.

Structural, vibrational, and electronic behavior of two GaGeTe polytypes under compression

GaGeTe is a layered topological semimetal that has been recently found to exist in at least two different polytypes, α-GaGeTe (R3¯m) and β-GaGeTe (P63mc). Here we report a joint experimental and theoretical study of the structural, vibrational, and electronic properties of these two polytypes in high-pressure conditions. Both polytypes show anisotropic compressibility and two phase transitions, above 7 and 15 GPa, respectively, as confirmed by XRD and Raman spectroscopy measurements. Although the nature of the high-pressure phases could not be confirmed, comparison with other chalcogenides and total-energy calculations allow us to propose possible high-pressure phases for both polytypes with an increase in coordination for Ga and Ge atoms from 4 to 6. In particular, the simplification of the X-ray pattern for both polytypes above 15 GPa suggests a transition to a structure of relatively higher symmetry than the original one. This result is consistent with the rocksalt-like high-pressure phases observed in parent III-VI semiconductors, such as GaTe, GaSe, and InSe. Pressure-induced amorphization is observed upon pressure release. The electronic band structures of α-GaGeTe and β-GaGeTe and their pressure dependence also show similarities to III-VI semiconductors, thus suggesting that the germanene-like sublayer induces a semimetallic character in both GaGeTe polytypes. Above 3 GPa, both polytypes lose their topological features, due to the opening of the direct band gap, while the reduction of the interlayer space increases the thermal conductivity at high pressure.

Pressure-Induced Phase Transition versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite.

The crystal structure of the CH3NH3PbBr3 perovskite has been investigated under high-pressure conditions by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm3̅m→Im3̅→Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies, which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to at least 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21; however, the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments, by visual observations, and by the fact that pressure-induced changes up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy, which is discussed using the structural information obtained from X-ray diffraction.

Dynamic high pressure phase transformation of ZrW2O8

Phase transformations under high strain rates (dynamic compression) are examined in situ on ZrW2O8, a negative thermal expansion ternary ceramic displaying polymorphism. Amorphization, consistent with prior quasi-static measurements, was observed at a peak pressure of 3.0 GPa under dynamic conditions, which approximate those expected during fabrication. Evidence of partial amorphization was observed at lower pressure (1.8 GPa) that may be kinetically restrained by the short (<∼150 ns) time scale of the applied high pressure. The impact of kinetics of pressure-induced amorphization from material fabrication methods is briefly discussed.

Nucleation and growth of crystalline ices from amorphous ices.

We here review mostly experimental and some computational work devoted to nucleation in amorphous ices. In fact, there are only a handful of studies in which nucleation and growth in amorphous ices are investigated as two separate processes. In most studies, crystallization temperatures Tx or crystallization rates RJG are accessed for the combined process. Our Review deals with different amorphous ices, namely, vapor-deposited amorphous solid water (ASW) encountered in many astrophysical environments; hyperquenched glassy water (HGW) produced from μm-droplets of liquid water; and low density amorphous (LDA), high density amorphous (HDA), and very high density amorphous (VHDA) ices produced via pressure-induced amorphization of ice I or from high-pressure polymorphs. We cover the pressure range of up to about 6GPa and the temperature range of up to 270K, where only the presence of salts allows for the observation of amorphous ices at such high temperatures. In the case of ASW, its microporosity and very high internal surface to volume ratio are the key factors determining its crystallization kinetics. For HGW, the role of interfaces between individual glassy droplets is crucial but mostly neglected in nucleation or crystallization studies. In the case of LDA, HDA, and VHDA, parallel crystallization kinetics to different ice phases is observed, where the fraction of crystallized ices is controlled by the heating rate. A key aspect here is that in different experiments, amorphous ices of different "purities" are obtained, where "purity" here means the "absence of crystalline nuclei." For this reason, "preseeded amorphous ice" and "nuclei-free amorphous ice" should be distinguished carefully, which has not been done properly in most studies. This makes a direct comparison of results obtained in different laboratories very hard, and even results obtained in the same laboratory are affected by very small changes in the preparation protocol. In terms of mechanism, the results are consistent with amorphous ices turning into an ultraviscous, deeply supercooled liquid prior to nucleation. However, especially in preseeded amorphous ices, crystallization from the preexisting nuclei takes place simultaneously. To separate the time scales of crystallization from the time scale of structure relaxation cleanly, the goal needs to be to produce amorphous ices free from crystalline ice nuclei. Such ices have only been produced in very few studies.

Kinetics and Mechanisms of Pressure-Induced Ice Amorphization and Polyamorphic Transitions in a Machine-Learned Coarse-Grained Water Model.

Water glasses have attracted considerable attention due to their potential connection to a liquid-liquid transition in supercooled water. Here we use molecular simulations to investigate the formation and phase behavior of water glasses using the machine-learned bond-order parameter (ML-BOP) water model. We produce glasses through hyperquenching of water, pressure-induced amorphization (PIA) of ice, and pressure-induced polyamorphic transformations. We find that PIA of polycrystalline ice occurs at a lower pressure than that of monocrystalline ice and through a different mechanism. The temperature dependence of the amorphization pressure of polycrystalline ice for ML-BOP agrees with that in experiments. We also find that ML-BOP accurately reproduces the density, coordination number, and structural features of low-density (LDA), high-density (HDA), and very high-density (VHDA) amorphous water glasses. ML-BOP accurately reproduces the experimental radial distribution function of LDA but overpredicts the minimum between the first two shells in high-density glasses. We examine the kinetics and mechanism of the transformation between low-density and high-density glasses and find that the sharp nature of these transitions in ML-BOP is similar to that in experiments and all-atom water models with a liquid-liquid transition. Transitions between ML-BOP glasses occur through a spinodal-like mechanism, similar to ice crystallization from LDA. Both glass-to-glass and glass-to-ice transformations have Avrami-Kolmogorov kinetics with exponent n = 1.5 ± 0.2 in experiments and simulations. Importantly, ML-BOP reproduces the competition between crystallization and HDA→LDA transition above the glass transition temperature Tg, and separation of their time scales below Tg, observed also in experiments. These findings demonstrate the ability of ML-BOP to accurately reproduce water properties across various regimes, making it a promising model for addressing the competition between polyamorphic transitions and crystallization in water and solutions.

Pressure-Optimized Band Gap and Enhanced Photoelectric Response of Graphitic Carbon Nitride with Nitrogen Vacancies

Graphitic carbon nitride ($g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$) shows favorable performance as a photocatalyst and has attracted widespread attention in recent years. As its wide band gap of 2.70 eV limits light absorption in the visible range, many efforts have been made to optimize the band gap. In this report, pressure is used to engineer the band gap and photoelectric response of nitrogen-deficient $g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$ nanoflakes. The band gap of the sample is first narrowed to 2.40 eV due to the introduction of nitrogen vacancies and then further narrowed to 1.70 eV by pressure, which is the lowest value reported in the literature for undoped $g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$. Accordingly, the photoelectric response increases by nearly 50% because of the enhanced light absorption at high pressure. More interestingly, after depressurization to ambient pressure, the optimized band gap survives with a minimum value of 1.87 eV accompanied by enhanced photoelectric responsivity. In situ synchrotron x-ray diffraction and Raman spectra suggest that the tunable band gap originates from irreversible pressure-induced amorphization with the assistance of vacancies for $g$-${\mathrm{C}}_{3}{\mathrm{N}}_{4}$. The collaborative approach of introducing deficiency and pressure treatment adopted here shows the ability to engineer the band gap continuously over a prominently wider region than that for the single band-gap-narrowing technique, and thus, enhances the photoelectric performance for broadened semiconductors.

Synthesis of Edge-Shared Octahedral MAPbBr3 via Pressure- and Temperature-Induced Multiple-Stage Processes

Organic–inorganic halide perovskites possess flexible structures with tunable optical properties at high pressures, but the pressure-induced amorphization limits the understanding and exploration of the structure–property relationships. Herein, we report an edge-shared octahedral MAPbBr3 via multiple-stage transformation of the traditional corner-shared three-dimensional perovskites at high-pressure and high-temperature (P–T) conditions. In situ synchrotron X-ray diffraction reveals that upon heating at 8.7 GPa, the amorphized MAPbBr3 transforms into a high P–T structure above 654 K via multiple-stage crystallization. The high P–T phase is determined to be a monoclinic P2/m structure with layered edge-shared Pb–Br octahedra, which can be quenched to RT at high pressures but converts back to the initial cubic perovskite structure below 0.7 GPa under decompression. The Raman and optical measurements demonstrate that the high P–T phase has similar vibrational behavior and tunable optical properties to those of the pressure-induced amorphous phase, indicating the similarity in the local structure with the edge-shared PbBr6 octahedral frame and the MA organic cations between the high P–T phase and the amorphous state. The present results demonstrate the structural diversity and interconversion in MAPbBr3 by overcoming the large kinetic barrier derived from the anisotropic organic cations and disordering arrangement of the Pb–Br octahedra, providing new insights into the understanding of structural transitions and amorphization of halide perovskites.

Direct observation of pressure-induced amorphization of methane/ethane hydrates using Raman and infrared spectroscopy.

The pressure-induced amorphization (PIA) of ice and clathrate hydrates occurs at temperatures significantly below their melting and decomposition points. The PIA of type I clathrate hydrates containing methane and ethane as guest molecules was investigated using Raman and infrared (IR) spectroscopy. With isothermal compression at 100 K, methane hydrate (MH) underwent PIA at 2-3.5 GPa, whereas ethane hydrate (EH) underwent PIA at 4.0-5.5 GPa. The type I clathrate structure consists of small (512) and large (51262) cages. The Raman results revealed that the collapsed small and large cages in the amorphous forms of MH and EH were not distinguishable. The collapsed cages, including the methane and ethane molecules, were similar to the small and large cages, respectively. Their water networks were folded or expanded during the PIA process so that the cavity sizes of the collapsed cages were compatible with those of the guest molecules. Peaks in the IR spectra of crystalline MH assignable to the ro-vibrational transition of methane in large cages were observed in the C-H stretching wavenumber region below 40 K. The ro-vibrational IR band disappeared after amorphization, suggesting that the rotational motion of the methane molecule in the large cage was frozen by the collapse, as reported in previous dielectric spectroscopic and simulation studies. This study contributes to a better understanding of the changes in the local structure around guest molecules during PIA and the dynamics of the guest molecules.

Pressure-Tuning Photothermal Synergy to Optimize the Photoelectronic Properties in Amorphous Halide Perovskite Cs3 Bi2 I9.

Effective modification of the structure and properties of halide perovskites via the pressure engineering strategy has attracted enormous interest in the past decade. However, sufficient effort and insights regarding the potential properties and applications of the high-pressure amorphous phase are still lacking. Here, the superior and tunable photoelectric properties that occur in the pressure-induced amorphization process of the halide perovskite Cs3 Bi2 I9 are demonstrated. With increasing pressure, the photocurrent with xenon lamp illumination exhibits a rapid increase and achieves an almost five orders of magnitude increment compared to its initial value. Impressively, a broadband photoresponse from 520 to 1650nm with an optimal responsivity of 6.81mA W-1 and fast response times of 95/96ms at 1650nm is achieved upon successive compression. The high-gain, fast, broadband, and dramatically enhanced photoresponse properties of Cs3 Bi2 I9 are the result of comprehensive photoconductive and photothermoelectric mechanisms, which are associated with enhanced orbital coupling caused by an increase in BiI interactions in the [BiI6 ]3- cluster, even in the amorphous state. These findings provide new insights for further exploring the potential properties and applications of amorphous perovskites.

Experimental and molecular dynamic studies of phase transformation and amorphization in silicon during high-energy ball milling

Nano-sized and amorphous silicon (α-Si) have a wide range of properties and applications, but their atomistic structure and the process of crystal evolution are still unclear. In this study, a combined action is used to present a mechanism for the crystal-to-amorphous solid–solid phase transition in silicon during mechanical alloying (MA) at room temperature. For this purpose, the ball milling of silicon was performed and changes in crystallographic structure and amorphous phase were studied by scanning electron microscopy (SEM), X-ray diffraction patterns (XRD), and energy dispersive X-ray spectroscopy (EDX). The process of changing phase was determined by quantitative phase analysis (MAUD/Rietveld refinement). In the simulation part, a crystal structure of silicon consisting of 64 atoms was created and uniaxial and biaxial external pressures were applied randomly by BIOVIA Materials Studio. The time step and simulation time were 1 and 300 Ps, respectively. Important atomic properties including density, mean square displacement (MSD), angle length and bonds distribution, and the fraction of broken bands were measured after applying per compression. Atomic structure and changes in crystalline phase to nano and amorphous phases were studied and these results had a considerable agreement with the experimental part and well-known previous studies. The diamond cubic structure shows a remarkable toughness during the modeling and so amorphization was performed at an exponential evolution. The Results of the simulations showed that the bond angle and length distribution and MSD, were increased significantly, and in contrast, density was decreased to 1.254 g/cm3. The percentage of the broken Si-Si bonds was 58.12 % and a new definition of Si amorphization based on the fraction of covalent bonds was proposed. The amorphization started from the surface of Si and spread by the partial distribution toward the crystal structure. These findings show that the proposed mechanism based on the fraction of breaking bonds and pressure-induced amorphization could be used for the accurate simulation of the solid–solid phase transformation.

Pressure-Induced Superconductivity in PdTeI with Quasi-One-Dimensional PdTe Chains

The quasi-one-dimensional material PdTeI exhibits unusual electronic transport properties at ambient pressure. Here, we systematically investigate both the structural and electronic responses of PdTeI to external pressure, through a combination of electrical transport, synchrotron X-ray diffraction (XRD), and Raman spectroscopy measurements. The charge density wave (CDW) order in PdTeI is fragile and the transition temperature TCDW decreases rapidly with the application of external pressure. The resistivity hump is indiscernible when the pressure is increased to ~1 GPa. Upon further compression, the resistivity dropping is observed approximately ~15 GPa and zero resistance is established above ~20 GPa, suggesting the occurrence of superconductivity. Combined XRD and Raman data evidence that the emergence of superconductivity is accompanied by a pressure-induced amorphization of PdTeI.

Origin of negative thermal expansion and pressure-induced amorphization in zirconium tungstate from a machine-learning potential

Understanding various macroscopic pressure-volume-temperature properties of materials on the atomistic level has always been an ambition for physicists and material scientists. Particularly, some materials such as zirconium tungstate (ZrW2O8), exhibit multiple exotic properties including negative thermal expansion (NTE) and pressure-induced amorphization (PIA). Here, using machine-learning based deep potential, we trace both of the phenomena in ZrW2O8 back to a common atomistic origin, where the nonbridging O atoms play a critical role. We demonstrate that the nonbridging O atoms confer great flexibility to vibration of polyhedrons, and kinetically drive volume shrinking on heating, or NTE. In addition, beyond a certain critical pressure, we find that the migration of nonbridging O atoms leads to additional bond formation that lowers the potential energy, suggesting that the PIA is a potential-driven first-order phase transition. Most importantly, we identify a second critical pressure beyond which the amorphous phase of ZrW2O8 undergoes a hidden phase transition from a reversible phase to an irreversible one.

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Degenerate Lattice-Instability-Driven Amorphization under Compression in Metal Halide Perovskite CsPbI3.

Halide perovskites have been intensively investigated for photovoltaic applications because of their good optoelectronic properties and low cost. Various high-pressure experiments have shown that these materials generally undergo reversible phase transitions between different crystalline phases as well as between crystalline and amorphous phases under external pressure. Herein, using first-principles density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations, we investigate the origin of the pressure-induced amorphization in CsPbI3. We find that the amorphous-like structures obtained from AIMD calculations become more stable than the orthorhombic Pbnm phase above 6.66 GPa, in good agreement with the experimental value (4.44 GPa). We further find that an imaginary flat band appears in the phonon dispersion of the orthorhombic CsPbI3 phase across the Brillouin zone at 10 GPa, leading to degenerate lattice instabilities. These energetically degenerate phonon modes are related to PbI6 octahedral tilting modes and provide random local distortions, leading to amorphization.