Amorphous-amorphous Transition Research Articles

Study on the structural origins of glass-forming ability and crystallization behaviors in Ni[sbnd]Nb binary alloys

The limited glass forming ability (GFA) poses a pervasive challenge in different metallic glass (MG) systems. This study systematically investigated the GFA of NiNb binary alloys, particularly those near the eutectic composition, aiming to explore the intrinsic factors contributing to the disparities in GFA among NiNb binary MGs. The relationship between liquid-like atomic structures and resistance to devitrification is thoroughly examined through the application of k-nearest neighbor machine learning methods. The results indicate that the critical diameter of Ni61Nb39, Ni61.5Nb38.5, and Ni62Nb38 MGs reaches upto 2 mm. An amorphous-amorphous transition is discerned within NiNb MGs exhibiting superior GFAs. These alloys exhibit low thermodynamic driving forces for crystallization, resulting in correspondingly elevated fragility coefficients. Furthermore, the presence of icosahedral-like clusters and the probability density of crystal-like clusters are observed to be intricately linked to GFA and the resistance to crystallization in NiNb MGs. The heightened abundance of crystal-like clusters within the liquid-like domain renders the alloy more susceptible to crystallization. Concurrently, icosahedral-like ordered clusters exert a “pinning effect” on the crystal-like regions, effectively inhibiting crystallization. These results collectively shed light on the fundamental nature of glass formation in NiNb binary MGs.

Microscopic mechanisms of pressure-induced amorphous-amorphous transitions and crystallisation in silicon

Some low-coordination materials, including water, silica, and silicon, exhibit polyamorphism, having multiple amorphous forms. However, the microscopic mechanism and kinetic pathway of amorphous-amorphous transition (AAT) remain largely unknown. Here, we use a state-of-the-art machine-learning potential and local structural analysis to investigate the microscopic kinetics of AAT in silicon after a rapid pressure change. We find that the transition from low-density-amorphous (LDA) to high-density-amorphous (HDA) occurs through nucleation and growth, resulting in non-spherical interfaces that underscore the mechanical nature of AAT. In contrast, the reverse transition occurs through spinodal decomposition. Further pressurisation transforms LDA into very-high-density amorphous (VHDA), with HDA serving as an intermediate state. Notably, the final amorphous states are inherently unstable, transitioning into crystals. Our findings demonstrate that AAT and crystallisation are driven by joint thermodynamic and mechanical instabilities, assisted by preordering, occurring without diffusion. This unique mechanical and diffusion-less nature distinguishes AAT from liquid-liquid transitions.

Molecular dynamics analysis of elastic properties and new phase formation during amorphous ices transformations

Unlike conventional first-order phase transitions, the kinetics of amorphous-amorphous transitions has been much less studied. The ultrasonic experiments on the transformations between low-density and high-density amorphous ice induced by pressure or heating provided the pressure and temperature dependencies of elastic moduli. In this article, we make an attempt to build a microscopic picture of these experimentally studied transformations using the molecular dynamics method with the TIP4P/Ice water model. We study carefully the dependence of the results of elastic constants calculations on the deformation rates. The system size effects are considered as well. The comparison with the experimental data enriches our understanding of the transitions observed. Our modeling gives new information about the formation mechanisms of new phase clusters during the transition between low-density and high-density amorphous ices. We analyse the applicability of the term “nucleation” for these processes.

Liquid-liquid transition and polyamorphism.

Two or more liquid states may exist even for single-component substances, which is known as liquid polymorphism, and the transition between them is called liquid-liquid transition (LLT). On the other hand, the existence of two or more amorphous states is called polyamorphism, and the transition between them is called amorphous-amorphous transition (AAT). Recently, we have accumulated a lot of experimental and numerical evidence for LLT and AAT. These intriguing phenomena provide crucial information on the fundamental nature of liquid and amorphous states. Here, we review the recent progress in this field and discuss how we can physically rationalize the existence of two or more liquids (glasses) for a single-component substance. We also discuss the relationship between liquid-, amorphous-, and crystal-polymorphisms, putting a particular focus on the roles of thermodynamics, mechanics, and kinetics.

Evidence of polyamorphic transitions during densified SiO2 glass annealing.

In situ X-ray scattering monitoring is carried out during temperature annealing on different densified SiO2 glasses. Density fluctuations and intermediate range coherence from X-ray scattering (small-angle X-ray scattering) and diffraction (wide angle X-ray scattering) evidence a maximum in their evolution at the same relaxation time. These extrema confirm the existence of an intermediate transitory disordered state between the two more ordered high and low density amorphous states. We propose that the existence of this transitory state confirms the existence of two mega basins in the energy landscape and therefore an amorphous-amorphous transition. Including older Raman results, we show that this intermediate disorder state implies similar mechanisms at all length scales from a few angstroms to 5 nm.

Effect of Cu on dehydrogenation and thermal stability of amorphous Mg-Ce-Ni-Cu alloys

The alloying element component is very crucial in improving the hydrogen storage performance of amorphous alloys. In this work, quaternary amorphous Mg70−xCe10Ni20Cux (x = 3, 7.5, 10) alloys were prepared by melt-spinning and the effect of Cu on hydrogenation and dehydrogenation were investigated in comparison with the Mg70−xCe10Ni20 amorphous alloys. The initial hydrogenation kinetics of amorphous Mg70−xCe10Ni20Cux (x = 0, 3, 7.5, 10) was improved with the increase of Cu content according to the kinetics measured at a temperature below crystallization temperature. As hydrogen is absorbed, an amorphous-amorphous transition occurred, and relatively high Cu content would lead to phase separation in the hydrogenation process. Amorphous phase have much higher crystallization temperature after it absorbs hydrogen and the addition of Cu could increase the crystallization activation energy of amorphous hydrides. In addition, the increase of Cu content could reduce the dehydrogenation temperature of amorphous hydrides, which gives a significant indication for future improving research of the hydrogen desorption performance of Mg based amorphous hydrides.

Kinetically Controlled Two-Step Amorphization and Amorphous-Amorphous Transition in Ice.

We report the results of insitu structural characterization of the amorphization of crystalline ice Ih under compression and the relaxation of high-density amorphous (HDA) ice under decompression at temperatures between 96 and 160K by synchrotron x-ray diffraction. The results show that ice Ih transforms to an intermediate crystalline phase at 100K prior to complete amorphization, which is supported by molecular dynamics calculations. The phase transition pathways show clear temperature dependence: direct amorphization without an intermediate phase is observed at 133K, while at 145K a direct Ih-to-IX transformation is observed; decompression of HDA shows a transition to low-density amorphous ice at 96K and ∼1 Pa, to ice Ic at 135K and to ice IX at 145K. These observations show that the amorphization of compressed ice Ih and the recrystallization of decompressed HDA are strongly dependent on temperature and controlled by kinetic barriers. Pressure-induced amorphous ice is an intermediate state in the phase transition from the connected H-bond water network in low pressure ices to the independent and interpenetrating H-bond network of high-pressure ices.

High-pressure behavior of amorphous selenium from ultrasonic measurements and Raman spectroscopy

The high-pressure behavior of melt-quenched amorphous selenium (a-Se) has been investigated via ultrasonic measurements and Raman scattering at room temperature. The ultrasonic measurements were conducted on a-Se in a multi-anvil apparatus with two different sample assemblies at pressures of up to 4.5 and 4.8 GPa. We discovered that similar kinks occur in the slopes of the pressure dependence characteristics of the travel time and the sound velocity in both shear and longitudinal waves in the 2.0–2.5 GPa range. These kinks are independent of the sample assemblies, indicating an intrinsic transformation of the a-Se. Additionally, we deduced the pressure-volume relationship of a-Se from the sound velocity characteristics using the Birch–Murnaghan equation of state, and the results agreed well with those of previous reports. In situ high-pressure Raman scattering measurements of a-Se were conducted in a diamond anvil cell with an 830 nm excitation line up to a pressure of 4.3 GPa. We found that the characteristic band of a-Se at ∼250 cm−1 experienced a smooth shift to a lower frequency with pressure, but a sharp slope change in the band intensity versus pressure occurred near 2.5 GPa. The results of X-ray diffraction and differential scanning calorimetry measurements indicate that the samples remain in their amorphous states after decompression. Thus, we proposed that the abnormal compression behavior of a-Se in the 2.0–2.5 GPa range can be attributed to pressure-induced local atomic reconfiguration, implying an amorphous-amorphous transition of the elementary selenium.

A reconstructive polyamorphous transition in borosilicate glass induced by irreversible compaction

Understanding the response of glasses to high pressure is of key importance for clarifying energy-dissipation and the origin of material damage during mechanical load. In the absence of shear bands or motile dislocations, pressure-induced deformation is governed by elastic and inelastic structural changes which lead to compaction of the glass network. Here, we report on a pressure-induced reconstructive amorphous-amorphous transition which was detected in sodium borosilicate glass by Raman and Brillouin scattering. The transition occurs through the formation of four-membered danburite-type rings of BO4 and SiO4-tetrahedra. We suggest that the inelastic pressure-resistance is governed by the Si-O-Si-backbone of the mixed borosilicate network. We further show that compaction is accompanied by increasing structural homogeneity and interpret this as a universal phenomenon in non-crystalline materials.

Polyamorphism in cerium based bulk metallic glasses: Electronic and structural properties under pressure and temperature by x-ray absorption techniques

High pressure and high temperature x-ray absorption near edge spectroscopy experiments have been carried out on Ce60Al20Cu20 bulk metallic glass showing an electronic delocalization of the 4f-electron of cerium under pressure. In parallel, high pressure extended x-ray absorption fine structure spectroscopy reveals large structural modifications of the cerium local environment. This study provides experimental evidence that an electronic driven structural transformation occurs in cerium based bulk metallic glasses (Ce-BMGs). The effect of temperature on the hysteresis of this amorphous-amorphous phase transition is also discussed, suggesting the existence of a critical point in the phase diagram of Ce-BMGs. This work will encourage further investigations on Ce-based metallic glasses phase diagrams in order to support, or refute, the actual theoretical understanding of polyamorphism.

Pressure-induced structural change of intermediate-range order in poly(4-methyl-1-pentene) melt

High-pressure in situ x-ray diffraction and specific-volume measurements on isotactic poly(4-methyl-1-pentene) melt have uncovered abrupt changes in the pressure dependence of microscopic structure as well as that of macroscopic density. The first sharp diffraction peak of the polymer melt, which is related to the intermediate-range order and is explained as resulting from the correlations between main chains, is suppressed at pressures less than 1 kbar. These changes in intermediate-range order show similarities to those seen in liquid-liquid or amorphous-amorphous transitions in simpler small molecule based systems, suggesting that this kind of phenomenon may occur in a wide range of materials.

High-pressure EXAFS study of vitreousGeO2up to 44 GPa

High-pressure extended x-ray absorption fine-structure measurements were performed on amorphous ${\text{GeO}}_{2}$ over increasing and decreasing pressure cycles at pressures up to 44 GPa. Several structural models based on crystalline phases with fourfold, fivefold, and sixfold coordination were used to fit the Ge-O first shell. The Ge-O bond lengths gradually increased up to 30 GPa. Three different pressure regimes were identified in the pressure evolution of the Ge-O bond distances. Below 13 GPa, the local structure was well described by a fourfold ``quartzlike'' model whereas a disordered region formed by a mixture of four- and five-coordinated germanium-centered polyhedra was observed in the intermediate pressure range between 13 and 30 GPa. Above 30 GPa the structural transition to the maximum coordination could be considered complete. The present results shed light on the ${\text{GeO}}_{2}$ densification process and on the nature of the amorphous-amorphous transition, suggesting that the transition is more gradual and continuous than what has been previously reported.

Effects of deformation and physical aging on nano-scale free volumes in polycarbonate studied by positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) has been employed to measure the variation of free volume hole parameters in poly(carbonate) (PC) after plastic deformation below the glass transition temperature (Tg). Changes in nano-scale free volume holes (Vh) and in their fraction (%fv) due to plane-strain and uniaxial compression and aging time were investigated at temperatures below Tg. For both deformation modes, %Δfv showed weak variations up to 30% thickness reduction followed by significant decrease at higher thickness reduction. As would be expected, trends in the variation of Vh were discernibly different for plane-strain and uniaxial deformation owing to shape transformation in free volumes. Vh showed a decrease followed by an increase during physical aging for both modes of deformation indicating the possibility of an amorphous-amorphous transition. Δfv showed a decrease only under uniaxial compression. The corresponding Vh size distributions showed high dependency on the mode of plastic deformation.

Interplay between morphology and metallization in amorphous-amorphous transitions

We present spectacular evidence of metallization of amorphous Ge films under high pressure using optical micrographs and Raman-scattering and x-ray-absorption spectroscopies. This transformation is identified as a low-density amorphous (LDA) to high-density amorphous (HDA) transition, relevant to a large class of systems including amorphous ice, semiconductors, and oxides. We have discovered that this transition initiates at the surface of the LDA sample, characterized intrinsically by a thickness dependent density of voids. Contrary to the case of transitions involving stable crystalline solid phases, our observations show that pressure-induced phase transitions in inhomogeneous amorphous samples are morphology driven and are favored for lower defect (void) densities. The metal disordered phase is observed first at 8 GPa for a lower density of voids, transforming to a metastable ordered phase upon depressurization. The local HDA structure is characterized by an increase in the first-neighbor coordination number, average distance, and variance.

High-pressure behavior ofAs2O3: Amorphous-amorphous and crystalline-amorphous transitions

The room temperature compression of ${\text{As}}_{2}{\text{O}}_{3}$ has been studied by in situ diamond anvil cell Raman and x-ray diffraction up to a pressure of 35 GPa. Upon compression, we discovered a crystal-to-amorphous transition in arsenolite, while claudetite, which is another crystalline polymorph of ${\text{As}}_{2}{\text{O}}_{3}$, remains crystalline up to at least 40 GPa. We have also observed an amorphous-amorphous transition in ${\text{As}}_{2}{\text{O}}_{3}$ glass at 25 GPa. This transition is characterized by a dramatic change in the Raman spectrum, which is reversible upon decompression. In situ diamond anvil cell x-ray diffraction of ${\text{As}}_{2}{\text{O}}_{3}$ glass reveals that the amorphous-amorphous transition is associated with an increase in the arsenic coordination. The amorphous-amorphous transition is reversible with little hysteresis.

Structural Transition in Compressed Amorphous Sulfur

Properties of amorphous sulfur (a-S) were investigated by synchrotron x-ray diffraction up to 100 GPa between 40 and 175 K. Measurements of the structure factor yielded the radial distribution function and the densities of two amorphous forms. a-S undergoes a structural transition above 65 GPa, accompanied by density discontinuity of 7%. These results indicate the amorphous-amorphous transition, from a low-density to a high-density form, and open up the possibility for the direct measurements of density of liquid-amorphous materials at extreme conditions.

Water polyamorphism: Reversibility and (dis)continuity

An understanding of water's anomalies is closely linked to an understanding of the phase diagram of water's metastable noncrystalline states. Despite the considerable effort, such an understanding has remained elusive and many puzzles regarding phase transitions in supercooled liquid water and their possible amorphous proxies at low temperatures remain. Here, decompression of very high density amorphous ice (VHDA) from 1.1 to 0.02 GPa at 140 K is studied by means of dilatometry and powder x-ray diffraction of quench-recovered states. It is shown that the three amorphous states of ice are reversibly connected to each other, i.e., LDA<-->e-HDA<-->VHDA. However, while the downstroke VHDA-->e-HDA transition takes place in the pressure range of 0.06 GPa<p<0.40 GPa, the e-HDA-->LDA transition takes place quasi-discontinuously at p approximately 0.06 GPa. That is, two amorphous-amorphous transitions of a distinct nature are observed for the first time in a one-component system-a first-order-like transition (e-HDA-->LDA) and a transition which is not first-order like but possibly of higher order (VHDA-->e-HDA). VHDA and e-HDA are established as the most stable and limiting states in the course of the transition. We interpret this as evidence disfavoring the hypothesis of multiple first-order liquid-liquid transitions (and the option of a third critical point), but favoring a single first-order liquid-liquid transition (and the option of a second critical point).

Amorphous-amorphous transition and the two-step replica symmetry breaking phase

The nature of polyamorphism and amorphous-to-amorphous transitions is investigated by means of an exactly solvable model with quenched disorder, the spherical $s+p$ multispin interaction model. The analysis is carried out in the framework of replica symmetry breaking theory and leads to the identification of low-temperature glass phases of different kinds. Besides the usual one-step solution, known to reproduce all basic properties of structural glasses, also a physically consistent two-step solution arises. More complicated phases are found as well, as temperature is further decreased, expressing a complex variety of metastable-state structures for amorphous systems.

Spherical2+pspin-glass model: An analytically solvable model with a glass-to-glass transition

We present the detailed analysis of the spherical s+p spin glass model with two competing interactions: among p spins and among s spins. The most interesting case is the 2+p model with p > 3 for which a very rich phase diagram occurs, including, next to the paramagnetic and the glassy phase represented by the one step replica symmetry breaking ansatz typical of the spherical p-spin model, other two amorphous phases. Transitions between two contiguous phases can also be of different kind. The model can thus serve as mean-field representation of amorphous-amorphous transitions (or transitions between undercooled liquids of different structure). The model is analytically solvable everywhere in the phase space, even in the limit where the infinite replica symmetry breaking ansatz is required to yield a thermodynamically stable phase.

Simulation of pressure-induced phase transition in liquid and amorphousAl2O3

We investigated the pressure-induced structural transformation in liquid and amorphous ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ by the molecular dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with Born-Mayer type pair potentials. The structure of the amorphous ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ model with real density at ambient pressure is in good agreement with Lamparter's experiment. In order to study the amorphous-amorphous phase transition, 23 models of amorphous alumina at the temperature of $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and at densities ranging from $2.83\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5.0\phantom{\rule{0.3em}{0ex}}\mathrm{g}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ had been built. The microstructure of the ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ systems had been analyzed through pair radial distribution functions, coordination number distributions, interatomic distances, and bond-angle distributions. Here we found clear evidence of a structural transition in amorphous alumina from a tetrahedral to an octahedral network upon compression. According to our results, this transformation occurred at densities ranging from $3.6\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}4.05\phantom{\rule{0.3em}{0ex}}\mathrm{g}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We also presented the amorphous-amorphous phase transition from an octahedral to a tetrahedral network structure upon decompression at densities ranging from $5.00\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.83\phantom{\rule{0.3em}{0ex}}\mathrm{g}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Also, the same study was carried out for the liquid state of the system at the temperature of $3500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the liquid-liquid phase transition had been discussed.