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High pressure induces dramatic changes and novel phenomena in condensed volatiles

Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d -electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T , magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB 6 to address the question of whether FeSi is a d -electron analogue of an f -electron Kondo insulator and, in addition, a “topological Kondo insulator” (TKI). The overall behavior of the magnetoresistance of FeSi at temperatures above and below the onset temperature T S = 19 K of the CSS is similar to that of SmB 6 . The two energy gaps, inferred from the ρ( T ) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T S . Several studies of ρ( T ) under pressure on SmB 6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which T S vanishes, although the energy gaps in SmB 6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at T S ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at T S ≈ 4.5 K for SmB 6 .

Tetrahedral amorphous carbon(ta-C) thin films with high sp3 fraction have extraordinary mechanical properties and wide applications. Despite intensive effort to increase the thickness of ta-C thin ...

Physical review / B 105(22), 224201 (2022). doi:10.1103/PhysRevB.105.224201, Achieving effective tuning of glass structures between distinct states is intriguing for fundamental studies and applications, but has previously turned out to be challenging in practice. High-entropy metallic glasses (HEMGs), as an emerging type of metallic glasses (MGs) based on the high-entropy effect, are expected to have more disordered and frustrated chemical short-range structure compared with conventional MGs. Therefore, HEMGs may offer possibilities for structure and properties tuning in glasses. In this work, we employ pressure as a tuning parameter and monitor the atomic structural evolution of a senary HEMG, Ti16.7Zr16.7Hf16.7Cu16.7Ni16.7Be16.7, up to ∼40 GPa using in situ synchrotron x-ray diffraction. Analysis of its structure factor in reciprocal space and reduced pair distribution function in real space both reveal a pressure-induced structural crossover at ∼20 GPa with a dramatic change in short-range order (SRO), while no similar phenomenon is observed in a conventional MG, Cu36Zr64, as a control sample, suggesting the pressure-induced highly tunable SRO in HEMGs originates from the local chemical complexity, namely, the high-entropy effect. These results confirm that enhanced flexibility and tunability of atomic structures could be achieved by introducing the high-entropy effect into MGs. Therefore, configurational entropy could be another dimension for exploring MGs with highly tunable structures and properties for various potential applications., Published by Inst., Woodbury, NY

The electronic excitation of 3d transition metal oxide depends on the hybridization between orbitals of different ions, and is related to the materials properties. In this work, we systematically study the effect of hydrostatic pressure up to ∼20 GPa on the crystal structure and electronic structure of Pr0.67Sr0.33MnO3 powder, using Mn K-edge X-ray absorption near edge structure and X-ray diffraction at room temperature. With the increase of pressure, the energy position of non-local screened feature moves towards feature without screening due to the enhanced transition energy; however, the intensity (probability) of non-local screened feature increases with the pressure. Further study suggests that the pressure-induced antiferromagnetic phase, affecting hybridization between neighboring Mn ions with spin sensitivity, increase the probability of non-local charge transfer and corresponding intensity of non-local screened feature. The spin-sensitivity of non-local screening may be a useful parameter for characterization of material properties.

As a prototype refractory and hard transition-metal carbide, the stability of zirconium carbide (ZrC) under extreme conditions is critical for its applications. Despite the extensive theoretical st...

Spatial heterogeneity, as a crucial structural feature, has been intensively studied in metallic glasses (MGs) using various techniques, including two-dimensional nanoindentation mapping. However, the limiting spatial resolution of nanoindentation mapping on MGs remains unexplored. In this study, a comprehensive study on four representative MGs using nanoindentation mapping with a Berkovich indenter was carried out by considering the influence of a normalized indentation spacing d/h (indentation spacing/maximum indentation depth). It appeared to have no significant correlation with the measured hardness and elastic modulus when d/h > 10. The hardness and elastic modulus started to increase slightly (up to ~5%) when d/h < 10 and further started to decrease obviously when d/h < 5. The mechanism behind these phenomena was discussed based on a morphology analysis of residual indents using scanning electron microscopy and atomic force microscopy. It was found that the highest spatial resolution of ~200 nm could be achieved with d/h = 10 using a typical Berkovich indenter for nanoindentation mapping on MGs, which was roughly ten times the curvature radius of the Berkovich indenter tip (not an ideal triangular pyramid) used in this study. These results help to promote the heterogeneity studies of MGs using nanoindentation that are capable of covering a wide range of length scales with reliable and consistent results.

The mechanism of plasticity in nanostructured Si has been intensively studied over the past decade but still remains elusive. Here, we used in situ high-pressure radial x-ray diffraction to simultaneously monitor the deformation and structural evolution of a large number of randomly oriented Si nanoparticles (SiNPs). In contrast to the high-pressure $\ensuremath{\beta}$-Sn phase dominated plasticity observed in large SiNPs ($\ensuremath{\sim}100\text{ }\text{ }\mathrm{nm}$), small SiNPs ($\ensuremath{\sim}9\text{ }\text{ }\mathrm{nm}$) display a high-pressure simple hexagonal phase dominated plasticity. Meanwhile, dislocation activity exists in all of the phases, but significantly weakens as the particle size decreases and only leads to subtle plasticity in the initial diamond cubic phase. Furthermore, texture simulations identify major active slip systems in all of the phases. These findings elucidate the origin of plasticity in nanostructured Si under stress and provide key guidance for the application of nanostructured Si.

As hard and refractory materials with high chemical resistance and mechanical strength, lanthanide hexaborides (LnB6) have attracted much attention. Among the family of LnB6, gadolinium hexaboride (GdB6) occupies a special position due to the half-filled 4f shell of Gd. Here, using in situ synchrotron radiation angle-dispersive X-ray diffraction in a diamond anvil cell at room temperature, the structural stability and compression behavior of GdB6 are investigated extensively, GdB6 is observed to be structurally stable up to 73 GPa, and the bulk modulus of 177 GPa is obtained under hydrostatic compression. In this paper, an interesting observation of pressure-induced spotty diffraction rings of GdB6 is reported, its formation mechanism can be well described by the reversible regional texture effect, which highlights the rearrangement of crystal grains under high pressure. The rearrangement mechanism has been well explained by investigating the pressure dependence of full width at half maximum, macro-differential stress and grain size. Collective grain rotation behavior motivated by stress difference is critical for the rearrangement process, the strong isotropy and strong stability of GdB6 structure also provide necessary conditions for high-pressure grain rotation behavior. These results will help to promote the understanding of high-pressure structural properties of GdB6, and provide novel insights on the high-pressure grain behavior in hard materials with strong isotropy and three-dimensional skeleton constituted of strong covalent bonds.

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.

Diamond owes its unique mechanical, thermal, optical, electrical, chemical, and biocompatible materials properties to its complete sp 3-carbon network bonding. Crystallinity is another major controlling factor for materials properties. Although other Group-14 elements silicon and germanium have complementary crystalline and amorphous forms consisting of purely sp 3 bonds, purely sp 3-bonded tetrahedral amorphous carbon has not yet been obtained. In this letter, we combine high pressure and in situ laser heating techniques to convert glassy carbon into “quenchable amorphous diamond”, and recover it to ambient conditions. Our X-ray diffraction, high-resolution transmission electron microscopy and electron energy-loss spectroscopy experiments on the recovered sample and computer simulations confirm its tetrahedral amorphous structure and complete sp 3 bonding. This transparent quenchable amorphous diamond has, to our knowledge, the highest density among amorphous carbon materials, and shows incompressibility comparable to crystalline diamond., Diamond’s properties are dictated by its crystalline, fully tetrahedrally bonded structure. Here authors synthesize a bulk sp 3-bonded amorphous form of carbon under high pressure and temperature, show that it has bulk modulus comparable to crystalline diamond and that it can be recovered under ambient conditions.

Amorphous Fe 80 Si 9 B 10 Cu 1 ribbons were annealed at various temperatures below the crystallization temperature. The structural change was investigated by the synchrotron radiation X-ray diffraction and reverse Monte Carlo method. Intensities of the first peaks on partial pair distribution functions g Fe-Fe ( r ) and g Fe-Cu ( r ) rise up with the increase of annealing temperature, which implies the Cu clustering and concomitant Fe aggregation. Employing the Voronoi tessellation method, the enhanced phase separation during annealing was revealed by the variation in population of Fe-centered clusters. These findings are then corroborated by Fe 57 Mossbauer spectroscopy. Moreover, annealing enhances the average hyperfine field of the alloy ribbons and induces the rotation of easy axis to the ribbon plane.

Metallic glasses are expected to have quite tunable structures in their configuration space, without the strict constraints of a well-defined crystalline symmetry and large energy barriers separating different states in crystals. However, effectively modulating the structure of metallic glasses is rather difficult. Here, using complementary in situ synchrotron x-ray techniques, we reveal thermal-driven structural ordering in a Ce65Al10Co25 metallic glass, and a reverse disordering process via a pressure-induced rejuvenation between two states with distinct structural order characteristics. Studies on other metallic glass samples with different compositions also show similar phenomena. Our findings demonstrate the feasibility of two-way structural tuning states in terms of their dramatic ordering and disordering far beyond the nearest-neighbor shells with the combination of temperature and pressure, extending accessible states of metallic glasses to unexplored configuration spaces., While metallic glasses are expected to have tunable structures, these have rarely been demonstrated. Here, the authors combine temperature and pressure to show a two-way structural tuning in rare earth-based metallic glasses beyond the nearest-neighbor atomic shells.

As a new class of multi-principal component oxides with high chemical disorder, high-entropy oxides (HEOs) have attracted much attention. The stability and tunability of their structure and properties are of great interest and importance, but remain unclear. By using in situ synchrotron radiation X-ray diffraction, Raman spectroscopy, ultraviolet–visible absorption spectroscopy, and ex situ high-resolution transmission electron microscopy, here we show the existence of lattice distortion in the crystalline (Ce0.2La0.2Pr0.2Sm0.2Y0.2)O2−δ HEO according to the deviation of bond angles from the ideal values, and discover a pressure-induced continuous tuning of lattice distortion (bond angles) and band gap. As continuous bending of bond angles, pressure eventually induces breakdown of the long-range connectivity of lattice and causes amorphization. The amorphous state can be partially recovered upon decompression, forming glass–nanoceramic composite HEO. These results reveal the unexpected flexibility of the structure and properties of HEOs, which could promote the fundamental understanding and applications of HEOs. High entropy oxides can exhibit remarkable properties which may be amenable to pressure tuning. Here a fluorite-type high entropy oxide is shown to undergo pressure-induced lattice distortion with associated changes to optical behaviour.

Glassy carbon (GC) distinguishes itself from other carbon materials by its unique atomic structure and properties. Cold-compressed GC gives rise to new physical properties; however, the atomistic mechanism for the transitions remains elusive. In this study, by combining in situ high-pressure x-ray diffraction with first-principles calculations, we observe pressure-induced disappearance of the initial intermediate range order of GC, followed by formation of local tetrahedral structural domains and ${sp}^{3}$ bonds. Correspondingly, the resistance of GC increases by four orders of magnitude during compression from $\ensuremath{\sim}20$ to $\ensuremath{\sim}61\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Both the structural and resistance transitions are partially reversible upon decompression, with noticeable hysteresis. Our results highlight the central role of layer distortions in inducing the ${sp}^{2}$-to-${sp}^{3}$ bonding transition and provide the structural underpining for the various transitions observed in cold-compressed glassy carbon.

A 4:1 (volume ratio) methanol–ethanol (ME) mixture and silicone oil are two of the most widely used liquid pressure-transmitting media (PTM) in high-pressure studies. Their hydrostatic limits have been extensively studied using various methods; however, the evolution of the atomic structures associated with their emerging nonhydrostaticity remains unclear. Here, we monitor their structures as functions of pressure up to ∼30 GPa at room temperature using in situ high-pressure synchrotron x-ray diffraction (XRD), optical micro-Raman spectroscopy, and ruby fluorescence spectroscopy in a diamond anvil cell. No crystallization is observed for either PTM. The pressure dependence of the principal diffraction peak position and width indicates the existence of a glass transition in the 4:1 ME mixture at ∼12 GPa and in the silicone oil at ∼3 GPa, beyond which a pressure gradient emerges and grows quickly with pressure. There may be another liquid-to-liquid transition in the 4:1 ME mixture at ∼5 GPa and two more glass-to-glass transitions in the silicone oil at ∼10 GPa and ∼16 GPa. By contrast, Raman signals only show peak weakening and broadening for typical structural disordering, and Raman spectroscopy seems to be less sensitive than XRD in catching these structural transitions related to hydrostaticity variations in both PTM. These results uncover rich pressure-induced transitions in the two PTM and clarify their effects on hydrostaticity with direct structural evidence. The high-pressure XRD and Raman data on the two PTM obtained in this work could also be helpful in distinguishing between signals from samples and those from PTM in future high-pressure experiments.

The pressure effect on structures and properties of a Fe78Si9B13 metallic glass was investigated by in situ high-pressure synchrotron Fe Kβ x-ray emission spectroscopy and x-ray diffraction, and electrical resistivity measurements up to ∼51 GPa. The study reveals a reversible and continuous pressure-induced high- to low-spin crossover of Fe atoms in an amorphous structure. The changes of the local spin moment can be scaled to match its average atomic distance shrinkage very well during compression. The crossover of electronic spin states in the Fe78Si9B13 metallic glass resembles that of typical crystalline Fe-bearing materials but without a sharp atomic volume collapse and an abrupt electrical resistivity jump. These findings could help guide applications of Fe-based metallic glasses as a soft ferromagnetic material at extreme conditions and also improve our understanding of magnetism and coupling of its changes with disordered atomic structures and other properties in metallic glasses.

Amorphous Fe 81 Si 9 B 10 melt-spun ribbons were annealed at various temperatures to relieve the internal stress. Magnetic studies manifest the occurrence of magnetic softening. The structure evolution of samples was then investigated by synchrotron radiation X-ray diffraction. It was found that, from short-range order to medium-range order, a positive and negative sign for volumetric thermal strain alternates upon heating and the amplitude of strain decays. Moreover, extending to outer shells, thermal strain seems less temperature-dependent in terms of the rate of increment. Magnetic softening in Fe 81 Si 9 B 10 metallic glasses upon annealing may result from the inhomogeneous thermal strains.

Significance This work establishes a general rule correlating the bulk properties [volume ( V )] with atomic structure information (principal diffraction peak position q 1 ) for metallic glasses, i.e., V ∝(1/ q 1 ) 2.5 . It is shown that the 2.5 power law is strictly followed by any metallic glass with its volume tuned by pressure and/or composition. This general 2.5 power law is attributed to the well-constrained structure change/modification that inevitably happens during pressure and/or composition tuning of metallic glasses, which brings insight into the structure of metallic glasses.

Polyamorphism discovered in lanthanide-rich metallic glasses (MGs) has been attributed to the electronic transition of the lanthanide element as a solvent element. In this work, we report that pressure-induced polyamorphism still exists in a Ce-poor Al93Ce7 binary MG where the 4f electron element serves as a solute and solute–solute avoidance is expected. The polyamorphic transition, observed by in situ high-pressure synchrotron x-ray diffraction, is accompanied by a volume collapse of ∼0.78% and occurs over a narrow pressure range from ∼0.8 to ∼1.8 GPa. Further synchrotron Ce L3-edge x-ray absorption spectroscopy measurements reveal that pressure-induced 4f electron delocalization underlies the polyamorphic transition. Molecular dynamics simulations confirm that the Ce atoms in the MG are completely isolated by the solvent Al atoms. This result demonstrates that 4f element-bearing alloys with extremely dilute concentrations can also exhibit polyamorphic states originating from electronic transitions, extending the compositional space of polyamorphism of MGs into very dilute regions. Our work suggests that tunable properties under compressive stress could be achieved in MGs by even minor doping of elements prone to electronic transitions.

Materials today advances 8, 100102 (2020). doi:10.1016/j.mtadv.2020.100102, High-entropy oxides (HEOs) stabilize multiple cations in a single solid solution phase, providing a new opportunity for property engineering in almost infinite compositional space. The structural stability and tunability of HEO are of great interest and importance but has not been well understood, especially under pressure. Here, we studied the structure evolution of a rock salt phase (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O HEO using in situ synchrotron X-ray diffraction, pair distribution function, Raman spectroscopy up to ~43 GPa, and ex situ transmission electron microscopy, a pressure-induced reversible rock salt to highly distorted cubic phase transition was observed. These results suggest highly tunable lattice distortion in HEOs under pressure, which could promote the fundamental understanding and also guide applications of HEOs., Published by Elsevier, Amsterdam

Recently, solid-state amorphization through temperature-induced martensitic transition has been reported. The stability of the amorphous martensite phase, which co-exists with a crystalline counterpart, is intriguing but remains unclear. In this work, we studied the structural stability of a Ti59.1Zr37Cu2.3Fe1.6 amorphous martensite alloy by combining in situ high-pressure synchrotron x-ray diffraction with ex situ transmission electron microscopy. During compression at room temperature, an irreversible pressure-induced crystallization of the amorphous martensite phase into a β-Ti phase is revealed. Qualitative analysis reveals the important role of the local atomic strain in stabilizing/destabilizing amorphous martensite alloys and its high tunability of the local atomic strain under high pressure.

High-entropy alloys (HEAs) as a new class of alloy have been at the cutting edge of advanced metallic materials research in the last decade. With unique chemical and topological structures at the atomic level, HEAs own a combination of extraordinary properties and show potential in widespread applications. However, their phase stability/transition, which is of great scientific and technical importance for materials, has been mainly explored by varying temperature. Recently, pressure as another fundamental and powerful parameter has been introduced to the experimental study of HEAs. Many interesting reversible/irreversible phase transitions that were not expected or otherwise invisible before have been observed by applying high pressure. These recent findings bring new insight into the stability of HEAs, deepens our understanding of HEAs, and open up new avenues towards developing new HEAs. In this paper, we review recent results in various HEAs obtained using in situ static high-pressure synchrotron radiation x-ray techniques and provide some perspectives for future research.

Structural heterogeneity has been proposed as a key intrinsic feature and mechanism underline unique properties and dynamic behavior of metallic glasses; however, it is still challenging in precisely describing and effectively characterizing the atomic-scale heterogeneity in metallic glasses. In this work, we employed the Meissner effect of superconductivity in magnetic susceptibility measurements as a sensitive bulk probe and successfully revealed the three-dimensional structural heterogeneity and its two-way evolution tuned by structural aging or rejuvenation in a La-based metallic glass. Compared with the resistivity measurements which only signal the most superconductive loop in inhomogeneous materials, the diamagnetic susceptibility signal of the Meissner effect maps the volumetric distribution of all superconductive regions and the corresponding structural heterogeneity in metallic glasses with high sensitivity. The experimental results reported in this article can be well interpreted based on a structural model of tunable “soft liquid-like” regions with soft vibration modes mixed with “hard solid-like” regions, validating the heterogeneity models of metallic glasses with new experimental data and approach.

Atomic packing of three Fe-based metallic glasses (MGs) has been studied by X-ray diffraction and X-ray absorption fine structure, combining with reverse Monte Carlo (RMC) and ab initio molecular dynamics (AIMD) simulations. It is found that the addition of Y and Nb atoms to the binary Fe–B MG mainly stabilizes large high-coordinated polyhedra by substituting the center Fe atoms, promoting the formation of icosahedron-like clusters and their connectivity with high-coordinated polyhedra. Moreover, the heterogeneities in local structures are found to increase due to element segregations of Fe, Y and Nb atoms. The obtained results indicate that the large-sized Y and Nb atoms can frustrate the geometry of competing crystalline phases and improve the atomic packing efficiency, therefore leading to the enhanced glass forming ability.

The atomic dynamics and its structure dependence in a glass are fundamental issues but still little understood for decades. Through state-of-the-art experiment and simulation, we reveal that the annihilation of excess free volume in a β-relaxation pronounced La–Al–Ni metallic glass slows down the atomic motion by one order of magnitude upon annealing, which is mainly caused by the change of subatomic cavities and enhanced network structure resulting from the presence of Ni atoms, showing significant difference from a β-relaxation unpronounced La–Al–Cu glass. Our findings provide insights into the dynamics and atomic structure of La-based metallic glasses and will be helpful in understanding the microscopic dynamic behavior of metallic glasses in general.

The structures and dynamics of liquid Ag74Ge26 alloy at different temperatures were investigated by high-energy X-ray diffraction and ab initio molecular dynamics simulation. The calculated structure factors agree well with the experimental data. Local atomic structure orders have been characterized by angular distribution functions, Honeycutt-Andersen index, Voronoi tessellation and the atomic cluster alignment. We reveal that (i) , , , and polyhedral dominate in the undercooled Ag74Ge26 liquid. (ii) The icosahedral-like short-range order is enhanced as the temperature decreases while , , , and polyhedral become the major short-range orders at low temperature. (iii) The medium-range order which was formed for amorphous phase at ambient temperature persists in the undercooled liquid. In addition, it is found that the diffusion constant of Ag is lower than that of Ge over the studied temperature range. Our findings elucidate that highly dominated short-range order associated with nearest-neighbors shell could empower the development of medium-range order and the formation of metallic glasses. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In situ high-pressure X-ray diffraction experiments and reverse Monte Carlo simulations on Pd81Si19 glassy alloy were performed up to similar to 35 GPa. High-quality structure factor data with a q range up to 13.5 angstrom(-1) were obtained. The pressure dependence of the relative volume of Pd81Si19 glassy alloy can be well described by a third-order Birch Murnaghan equation of state with bulk modulus of B-0 = 229 GPa and a pressure derivative of B-0(') = 2.7. The continuous and monotonic change of relative volume as a function of pressure does not indicate any first-order phase transitions in Pd81Si19 glassy alloy in the studied pressure range. The pressure dependence of the atomic structures was systematically investigated by means of bond angle distribution, the Honeycutt and Andersen index, Voronoi tessellation, total and partial coordination numbers, and local chemical ordering analyses. It is found that the topological and chemical atomic structures of the Pd81Si19 glassy alloy do not change much with pressure, indicating the stability of this material under pressure. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The local atomic structure evolution in Al2Au alloy during solidification from 2000 K to 400 K was studied by ab initio molecular dynamics simulations and analyzed using the structure factor, pair correlation functions, bond angle distributions, the Honeycutt–Anderson (HA) index and Voronoi tessellation methods. It was found that the icosahedral-like clusters are negligible in the Al2Au stable liquid and supercooled liquid states, and the most abundant clusters are those having HA indices of 131 and 120 or Voronoi indices of 〈0, 4, 4, 0〉, 〈0, 3, 6, 0〉 and 〈0, 4, 4, 2〉 with coordination numbers of 8, 9 and 10, respectively. These clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, revealing the existence of structure heredity between liquid and crystalline phase in Al2Au alloy.

The mechanism of pressure-induced amorphous-to-amorphous transitions (AATs) in metallic glasses (MGs) has been a subject of intense research. Most AATs in MGs were found in lanthanide-based alloys and shown to originate from $4f$ orbital delocalization. Recently, evidence of an unexpected AAT in the main-group Ca-Al MGs was reported without a satisfactory explanation. Here, based on the results of first-principles molecular dynamics calculations, the suggested AAT at 12--15 GPa in the $\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$ MG is confirmed. Contrary to the common belief that the coordinate of metallic glasses with close packing cannot be increased further, the coordination around Al atoms is found to increase suddenly at the transition as a consequence of atomic migration and the aggregation of Al atoms. This transition originates from pressure-enhanced bonding between Ca $3d$ and Al $3p$ orbitals and is confirmed by the good agreement on the predicted and measured electrical conductivities. The theoretical analysis not only uncovers a mechanism of pressure-induced AAT in main-group MGs, but it can be generalized to establish a different perspective to guide the understanding of transformation phenomena in compressed MGs.

The glass forming ability of Zr 46 Cu 37.64− x Ag 8.36 Al 8 Be x ( x = 0, 6 and 10 at.%) bulk metallic glasses (BMGs) were significantly improved by Be addition. The critical size of amorphous rods can be over 35 mm diameter. The high GFA achieved is mainly due to the decrease of melting point and liquidus temperature, and suppression of the formation of crystalline phases during solidification from liquid state. The high stabilization with supercooled liquid regime of 115 K was found for the BMG with x = 10 at.%. Two independent exothermic events happen in x = 0 and 6 at.% BMGs, corresponding to the formation of primary crystalline phases Cu 10 Zr 7 and AgZr, then transforming to final stable crystalline phases Zr 2 Cu and AlCu 2 Zr. However, in the x = 10 at.% BMG, the precipitation of primary phases and transformation to final stable phases are within the first exothermic event and the AlCu 2 Zr phase is totally suppressed.

Recently, an irreversible polymorphic transition from face-centered cubic to hexagonal close-packing was surprisingly observed under high pressure in the prototype CoCrFeMnNi high-entropy alloys (HEAs) by various research groups. This unexpected phase transition brings new insights into the stability of HEAs, and its irreversibility stimulates exploration for new HEAs via high-pressure compression synthesis. However, the onset pressure for the phase transition was reported to fluctuate over a vast range from ∼7 to above 49 GPa in the reported experiments. The reason for this inconsistency remains unclear and puzzles the HEA community. To address this problem, this work systematically investigates the effects of non-hydrostaticity and grain size. Our results demonstrate that larger deviatoric stress induced by the non-hydrostaticity of the pressure medium and larger grain size of the initial sample can both promote a phase transition and, therefore, considerably depress the onset pressure.Recently, an irreversible polymorphic transition from face-centered cubic to hexagonal close-packing was surprisingly observed under high pressure in the prototype CoCrFeMnNi high-entropy alloys (HEAs) by various research groups. This unexpected phase transition brings new insights into the stability of HEAs, and its irreversibility stimulates exploration for new HEAs via high-pressure compression synthesis. However, the onset pressure for the phase transition was reported to fluctuate over a vast range from ∼7 to above 49 GPa in the reported experiments. The reason for this inconsistency remains unclear and puzzles the HEA community. To address this problem, this work systematically investigates the effects of non-hydrostaticity and grain size. Our results demonstrate that larger deviatoric stress induced by the non-hydrostaticity of the pressure medium and larger grain size of the initial sample can both promote a phase transition and, therefore, considerably depress the onset pressure.