High-density Amorphous State Research Articles

Amorphous GaN: Polyamorphism and crystallization at high pressure

Employing constant pressure ab initio simulations, we have shed light on the previously unknown high-pressure behavior of amorphous gallium nitride. Our findings reveal a two-step transformation sequence under pressure. The initial transition involves a polyamorphic transformation from a low-density amorphous (LDA) phase to a high-density amorphous (HDA) phase with an average coordination number of 5.4. Upon pressure release, the HDA state partially reverts to a denser amorphous network with a higher coordination number (4.34) compared to the original LDA phase. Further pressurization triggers the crystallization of the HDA state into a rocksalt structure. Remarkably, the electronic structure of the amorphous forms of GaN exhibits insignificant sensitivity to changes in coordination number, maintaining a band gap of approximately 1.7–2.0 eV across all phases.

Structural and electronic transformations of GeSe2 glass under high pressures studied by X-ray absorption spectroscopy

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above [Formula: see text] 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure.

Quantifying a partial polyamorphic transition in a cerium-based metallic glass during cooling

Cerium-based metallic glasses are prototype polyamorphous systems with pressure-induced polyamorphic transitions extensively reported. Cooling typically has a similar effect on materials as compression with regard to reducing volume. However, previous studies show dramatically different behavior of Ce-based metallic glasses between cooling and compression, whose origin remains unclear. Here, using in situ low-temperature synchrotron high-energy x-ray diffraction, the structural evolution of a Ce68Al10Cu20Co2 metallic glass is accurately determined and analyzed by a structure factor and a reduced pair distribution function (PDF) during cooling from 298 to 83 K. An unusually large linear thermal expansion coefficient is revealed, which is associated with both continuous but inconsistent structural changes between the two subpeaks of the first atomic shell in terms of average bond lengths and coordination numbers. These phenomena are suggested to be attributed to a gradual 4f electron delocalization of only a minimal amount (∼2.6% at 83 K) of Ce atoms by quantitative analysis of the PDF data. However, a previously expected global polymorphic transition from a low-density amorphous state to a high-density amorphous state with an abrupt volume collapse is not observed. Moreover, electrical resistivity also shows a continuous increase during cooling without any sharp change. It is clarified that cryogenic temperatures could facilitate but are not powerful enough alone to trigger a global polymorphic transition in the Ce68Al10Cu20Co2 metallic glass, suggesting a wide distribution of its local atomic environment.

Dual crystallization modes of sputter-deposited amorphous SiGe films

The crystallization behavior of sputter-deposited films of amorphous Si (a-Si) and SiGe alloys (a-SiGe) induced by electron irradiation at room temperature and by thermal annealing was investigated by in situ transmission electron microscopy. On electron irradiation at room temperature, extremely rapid crystallization, so-called explosive crystallization, occurred at a higher electron flux but not at a lower electron flux. On in situ thermal annealing, explosive crystallization occurred preferentially and partially at low temperatures in Ge-rich a-SixGe100−x for x < 50 but not for x > 50. These results indicate that the increase of Si content in a-SiGe prevents the occurrence of explosive crystallization. We previously proposed that explosive crystallization can occur in pristine a-Ge films via the interface of a liquid-like, high-density amorphous state at the growth front. An increase in the instability of this high-density amorphous state caused by the increase of Si in a-SiGe apparently gives rise to the suppression of explosive crystallization.

Polyamorphic transition in a transition metal based metallic glass under high pressure

Pressure-induced glass-glass transition (GGT) has been reported in a few rare-earth based and main-group metallic glasses (MGs), a process during which the compressibility decreases with the transformation from a low-density to a high-density amorphous state. Herein, we report an unexpected GGT behavior in Pd-based MGs under pressure, which is characterized by an anomalous increase in the compressibility with the pressure. State-of-the-art high-energy synchrotron technique, coupled with theoretical simulations, reveals that this unique GGT behavior is primarily caused by the change in bonding characteristics, i.e., some covalentlike atomic bonds in the as-prepared state change to metallic ones under high pressure, which also leads to the increase in the compressibility. Our current findings shed light on the understanding of the nature of polyamorphic transitions in MGs.

Metastable States in Pressurized Bulk and Mesoporous Germanium

Combination of porosity and hydrostaticity during compression is used with a view to explore the energy landscape of germanium. In this work, pressure-induced phase transformations in mesoporous crystalline Ge has been investigated by in situ Raman spectroscopy. A pressure-induced amorphization to a low-density amorphous (LDA) state was observed prior to a reversible polyamorphic transformation between LDA and high-density amorphous states. These pressure-induced transformations show some similarities with the behavior previously reported in nanoparticles. Thermodynamics models developed in the case of nanoparticles are successfully used indicating that, in both cases, the large surface-to-volume ratio leads to an increase of the system energy and that mesoporous materials may be considered as the negative image of a collection of nanoparticles. However, an inhomogeneous stress distribution is expected in porous materials because of it being a network with hyperbolic geometry. A control experiment is presented using a reference bulk germanium sample. The diamond-to-β-tin transformation is observed starting at around 8.0 GPa. On decompression, the metastable ST12 (Ge-III) phase is observed. Ab initio simulations are used to assign and interpret Raman spectra of this phase.

Permanent densification of amorphous zinc oxide under pressure: A first principles study

Ab initio simulations within a generalized gradient approximation are carried out to investigate the densification mechanism of amorphous zinc oxide (a-ZnO) under hydrostatic pressure. In contrast to the crystalline ZnO, the densification of a-ZnO is found to proceed gradually and is associated with a structural modification from a low density amorphous state to a high density amorphous state. Accompanied by the phase transformation, the mean coordination number increases from ~4.0 to ~5.5. The high-density amorphous form of ZnO has a local structure, partially comparable with that of the rocksalt type ZnO crystal and presents a semiconducting behavior. The phase change is irreversible because upon pressure release, an amorphous model largely consisting of fivefold coordination is recovered. The decompressed model can be, therefore, classified as an intermediate phase between the wurtzite-like and the rocksalt-like amorphous configurations.

Densification of amorphous boron under pressure

The densification mechanism of amorphous boron under pressure is investigated using a constant pressure ab initio technique and found to be associated with two consecutive amorphous-to-amorphous phase transformations. Amorphous boron gradually transforms into a high density amorphous phase, followed by a first order phase transformation into another high density amorphous state. The high density amorphous phases of boron are not quenchable to ambient pressure. Most quasimolecular B12 icosahedra in the model are found to be stable at the highest the theoretical pressure of 280GPa reached in the present work and thus the phase transformations are principally due to the re-structural arrangements in the parts of the model connecting B12 icosahedra.

Elastic Anomaly and Polyamorphic Transition in (La, Ce)-based Bulk Metallic Glass under Pressure

Pressure-induced polyamorphism in Ce-based metallic glass has attracted significant interest in condensed matter physics. In this paper, we discover that in association with the polyamorphism of La32Ce32Al16Ni5Cu15 bulk metallic glass, the acoustic velocities, measured up to 12.3 GPa using ultrasonic interferometry, exhibit velocity minima at 1.8 GPa for P wave and 3.2 GPa for S wave. The low and high density amorphous states are distinguished by their distinct pressure derivatives of the bulk and shear moduli. The elasticity, permanent densification, and polyamorphic transition are interpreted by the topological rearrangement of solute-centered clusters in medium-range order (MRO) mediated by the 4f electron delocalization of Ce under pressure. The precisely measured acoustic wave travel times which were used to derive the velocities and densities provided unprecedented data to document the evolution of the bulk and shear elastic moduli associated with a polyamorphic transition in La32Ce32Al16Ni5Cu15 bulk metallic glass and can shed new light on the mechanisms of polyamorphism and structural evolution in metallic glasses under pressure.

High-density amorphous phase of CdO

An amorphous CdO model is densified up to a theoretical pressure of 200GPa. A continues phase transformation from a low-density amorphous (LDA) phase to a high-density amorphous (HDA) phase is observed through the simulation. Associated with the phase transformation, the average coordination raises progressively from ~5.5 to ~7.0. The sevenfold coordination is the most abundant motifs in the HDA state. Therefore its short-range order differs principally from that of the known CdO crystals. Upon pressure release, an amorphous state being intermediate between the LDA structure and the HDA phase is recovered with a mean coordination number of 5.9.

New limits of secondary \u03b2-relaxation

Glass is an ultraviscous liquid that ceases to flow on a laboratory timescale but continues to relax on a geological timescale. Quintessentially, it has become hopeless for humans to explore the equilibrium behavior of glass, although the technology of glass making witness a remarkable advance. In this work, we propose a novel thermodynamic path to prepare a high density amorphous state of matter (carvedilol dihydrogen phosphate) using high pressure. In addition, we provide the impeccable experimental evidence of heterogeneous nature of secondary β-relaxation and probe its properties to understand the various aspects of pressure densified glass, such as dynamics, packing and disorder. These features are expected to provide new horizons to glass preparation and functional response to pharmaceutical applications.

Polyamorphism in silicon nanocrystals under pressure

Many works have been devoted to describing mechanisms of pressure-induced polyamorphism. This phenomenon is apparent in the phase transition between low- and high-density amorphous states (LDA and HDA) upon the application of pressure, resulting in substantial changes in the structure and physical properties of the amorphous state. The HDA–LDA transition in Si nanocrystals is observed when recording Raman spectra in situ during decompression at 6.68 GPa.

Amorphous boron nitride at high pressure

The pressure-induced phase transformation in hexagonal boron nitrite and amorphous boron nitrite is studied using ab initio molecular dynamics simulations. The hexagonal-to-wurtzite phase transformation is successfully reproduced in the simulation with a transformation mechanism similar to one suggested in experiment. Amorphous boron nitrite, on the other hand, gradually transforms to a high-density amorphous phase with the application of pressure. This phase transformation is irreversible because a densified amorphous state having both sp3 and sp2 bonds is recovered upon pressure release. The high-density amorphous state mainly consists of sp3 bonds and its local structure is quite similar to recently proposed intermediate boron nitrite phases, in particular tetragonal structure (P42/mnm), rather than the known the wurtzite or cubic boron nitrite due to the existence of four membered rings and edge sharing connectivity. On the basis of this finding we propose that amorphous boron nitrite might be best candidate as a starting structure to synthesize the intermediate phase(s) at high pressure and temperature (probably below 800 °C) conditions.

The dynamic crossover in water does not require bulk water

Many of the anomalous properties of water may be explained by invoking a second critical point that terminates the coexistence line between the low- and high-density amorphous states in the liquid. Direct experimental evidence of this point, and the associated polyamorphic liquid-liquid transition, is elusive as it is necessary for liquid water to be cooled below its homogeneous-nucleation temperature. To avoid crystallization, water in the eutectic LiCl solution has been studied but then it is generally considered that "bulk" water cannot be present. However, recent computational and experimental studies observe cooperative hydration in which case it is possible that sufficient hydrogen-bonded water is present for the essential characteristics of water to be preserved. For femtosecond optical Kerr-effect and nuclear magnetic resonance measurements, we observe in each case a fractional Stokes-Einstein relation with evidence of the dynamic crossover appearing near 220 K and 250 K respectively. Spectra obtained in the glass state also confirm the complex nature of the hydrogen-bonding modes reported for neat room-temperature water and support predictions of anomalous diffusion due to "worm-hole" structure.

Pressure-induced polyamorphism inTiO2nanoparticles

Two different nanometric (6 nm) ${\text{TiO}}_{2}$ compounds, anatase polycrystals and amorphous particles, were investigated under high pressure using Raman spectroscopy. Nanoanatase undergoes a pressure-induced amorphization. The pressure-induced transformations of this mechanically prepared amorphous state are compared with those of a chemically prepared amorphous particles. In the mechanically prepared amorphous state, a reversible transformation from a low-density amorphous state to high-density amorphous state (HDA1) is observed in the range 13--16 GPa. In the chemically prepared sample, a transformation to a new high-density amorphous state (HDA2) is observed at around 21 GPa. Further compression leads to the transformation $\text{HDA}2\ensuremath{\rightarrow}\text{HDA}1$ at $\ensuremath{\sim}30\text{ }\text{GPa}$. We demonstrate that depending on the starting amorphous material, the high-pressure polyamorphic transformations may differ. This observation indicates that pressure is a suited tool to discriminate between nanomaterials apparently similar at ambient conditions.

Vibrational properties of amorphous germanium under pressure and its thermal expansion and Grüneisen parameters

We study the vibrational properties of amorphous germanium at high pressures using a constant pressure ab initio technique. With the application of pressure, the low-frequency modes gradually soften with a decrease in their acoustic-like nature while the high-frequency modes shift to larger frequencies. The acoustic-like vibrations are found to have negative Grüneisen parameters whereas the optic-like ones have positive values of about +1. A negative thermal expansion is produced below 200 K. Furthermore, we find that pressure reduces the number of localized states. Accompanied by the phase transition into a metallic high-density amorphous state, the nature of the low-frequency vibrations is changed from acoustic-like to optic-like and from bond bending to bond stretching.

High-density amorphous phase ofGeS2glass under pressure

The pressure-induced phase transition in amorphous germanium disulfide $(a{\text{-GeS}}_{2})$ is studied using an ab initio constant-pressure technique. With the application of hydrostatic pressure, $a{\text{-GeS}}_{2}$ undergoes a gradual phase transition from a semiconducting low-density amorphous state to a metallic high-density amorphous state. The transition is associated with a local coordination change in both the Ge atoms and S atoms. Upon pressure release, the high-density phase transforms back to a low-density amorphous state. The physical origin of the gradual phase transformation is discussed. The pressure-induced changes in the electronic and vibrational properties are studied with details. Additionally the pressure-induced phase transition of the monoclinic ${\text{GeS}}_{2}$ is compared with that of amorphous state.

Polyamorphism and the evolution of intermediate-range order in moltenZnCl2

The pressure- and temperature-dependent properties of molten zinc chloride areinvestigated by means of molecular dynamics computer simulation. The potential modelused is based on a pair potential augmented with a description of the (many-body) anionpolarization. The static experimental and simulated structural properties arebriefly reviewed. The structural properties obtained from the simulation modelare compared with those obtained from experimental investigations and trendswith the change in system pressure and temperature are discussed. Preliminarystudies of the pressure behaviour of the simulated glassy states show the existenceof low-and high-density amorphous states (LDA and HDA). The LDA/HDAcoexistence line is established and its relationship to the underlying phase diagram isdiscussed.

Evidence for a high-density amorphous form in indomethacin from Raman scattering investigations

Pressure-induced transformation of $\ensuremath{\gamma}$-IMC [1-($p$-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid] is analyzed from Raman scattering investigations in the low-frequency range of $10--250\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and the high frequency region between 1550 and $1750\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, where $\mathrm{C}\mathrm{O}$ stretching vibrations are usually observed. At room temperature, by pressurization from atmospheric pressure up to $4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, $\ensuremath{\gamma}$-IMC undergoes a collapse transformation into a high-pressure crystalline form, induced by large rearrangement in the hydrogen-bonded network associated with molecular conformational changes. The Raman spectrum of the high-pressure crystal is similar to that of the $\ensuremath{\alpha}$ form, which is denser than the $\ensuremath{\gamma}$ form and metastable with respect to $\ensuremath{\gamma}$-IMC at atmospheric pressure. Upon further compression a solid-state amorphization is observed via the breakdown of hydrogen bonds. The Raman line shape of the high-pressure amorphous form is different from that of the vitreous state (or thermal glass obtained by quenching the liquid), suggesting the existence of a high-density amorphous state. By release of pressure, this high-density amorphous state transforms into the thermal glass. This transformation can be interpreted as a transformation between a high-density amorphous to a low-density amorphous state, which could be associated with a polyamorphic transformation.

Ab initiosimulation of polyamorphic phase transition in hydrogenated silicon

We study the pressure-induced phase transition of a hydrogenated amorphous silicon model with a void, using a constant pressure ab initio technique within the generalized gradient approximation. With the application of pressure, the 223 atom model of hydrogenated amorphous silicon with a 5-atom void undergoes a first order phase transition from a semiconducting low density amorphous state to a metallic high density amorphous state with a discontinuous volume change of about 16%. The transition is accompanied by a coordination change from fourfold to about sixfold.