Negative expansions of interatomic distances in metallic melts

Resolving decades of debate: the surprising role of high-temperature covalency in the structure of liquid gallium

Liquid metals (LMs) have the potential to revolutionise many important technologies, ranging from battery components to catalytic reactions. Low melting temperature gallium (Ga) is particularly promising as a solvent in many LM alloys, due to the low energy cost of maintaining its liquid state. However, despite 30+ years of study on the nature of Ga's liquid structure, it remains enigmatic with significant disagreement among the many published reports. In this work, we reconcile many of the conflicts through analysis of extensive ab initio molecular dynamics simulations of bulk Ga liquid at different temperatures. Contrary to previous assumptions, covalency becomes more important in the liquid at higher temperatures, meaning that covalency is not a significant feature of the liquid near the phase transition temperature. This explains the experimental observation of a decrease of resistivity of the metal upon melting, and its subsequent anomalously nonlinear increase with temperature. This revised understanding of structuring in the liquid has implications for the way these alloys are tailored for specific applications in the rapidly developing field of LMs.

Freezing Shrinkage Dynamics and Surface Dendritic Growth of Floating Refractory Alloy Droplets in Outer Space

The freezing shrinkage and dendritic growth are of great importance for various alloys solidified from high-temperature liquids to solids since they dominate microstructure patterns and follow-up processing. However, the microgravity freezing shrinkage dynamics is scarcely explored on the ground as it is hard to suppress the strong natural convection inside liquid alloys. Here, a series of in-orbit solidification experiments is conducted aboard the China Space Station with a long-term stable 10-5 g0 microgravity condition. The highest temperature up to 2265 K together with substantial liquid undercoolings far from a thermodynamically stable state are attained for both Nb82.7Si17.3 and Zr64V36 refractory alloys. Furthermore, the solidification under microgravity of a droplet is simulated to reveal the liquid-solid interface migration, temperature gradient, and flow field. The microgravity solidification process leads to freezing shrinkage cavities and distinctive surface dendritic microstructure patterns. The combined effects of shrinkage dynamics and liquid surface flow in outer space result in the dendrites growing not only along the tangential direction but also along the normal direction to the droplet surface. These space experimental results contribute to a further understanding of the solidification behavior of liquid alloys under a weaker convection condition, which is often masked by gravity on the ground.

Structural Changes in Metallic Glass-Forming Liquids on Cooling and Subsequent Vitrification in Relationship with Their Properties

The present review is related to the studies of structural changes observed in metallic glass-forming liquids on cooling and subsequent vitrification in terms of radial distribution function and its analogues. These structural changes are discussed in relationship with liquid's properties, especially the relaxation time and viscosity. These changes are found to be directly responsible for liquid fragility: deviation of the temperature dependence of viscosity of a supercooled liquid from the Arrhenius equation through modification of the activation energy for viscous flow. Further studies of this phenomenon are necessary to provide direct mathematical correlation between the atomic structure and properties.

Integration of negative, zero and positive linear thermal expansion makes borate optical crystals light transmission temperature-independent

Negative and zero thermal expansion (NTE and ZTE) materials are widely adopted to eliminate the harmful effect from the "heat expansion and cool contraction" effect and frequently embrace novel fundamental physicochemical mechanisms. To date, the manipulation of NTE and ZTE materials has mainly been realized by chemical component regulation. Here, we propose another method by making use of the anisotropy of thermal expansion in noncubic single crystals, with maximal tunability from the integration of linear NTE, ZTE and positive thermal expansion (PTE). We demonstrate this concept in borate optical crystals of AEB₂O₄ (AE = Ca or Sr) to make the light transmission temperature-independent by counterbalancing the thermal expansion and thermo-optics coefficient. We further reveal that such a unique thermal expansion behavior in AEB₂O₄ arises from the synergetic thermal excitation of bond stretching in ionic [AEO₈] and rotation between covalent [BO₃] groups. This work has significant implications for understanding the thermal excitation of lattice vibrations in crystals and promoting the functionalization of anomalous thermal expansion materials.

Theoretical and experimental study of the microstructure of a metallic melt in an In50Bi50 alloy based on the Wulff cluster model

In this paper, the Wulff cluster model which has been proved to successfully describe the melt structure of pure metals, homogenous alloys and eutectic alloys has been extended to an alloy with intermetallic compounds (In₅₀Bi₅₀). According to the cohesive energy and the solid-state XRD patterns, the most possible types of clusters in the melt are Bi and InBi. At relatively high temperatures, the superimposed XRD (simulated) patterns of Bi and InBi clusters are in good agreement with the experimental HTXRD patterns in terms of the position and intensity of the peaks. With the decrease of temperature, there is an obvious deviation in the simulated XRD value at the second peak caused by the nucleation process of Bi clusters, which would be modified by adding simulated XRD patterns of the Bi bulk. The proportion of the superimposed Bi bulk XRD pattern increases with the decrease of temperature suggesting that the nucleation process of the Bi cluster begins at 160 °C.

Ultrafast atomic view of laser-induced melting and breathing motion of metallic liquid clusters with MeV ultrafast electron diffraction

Intense lasers can be used to drive materials into transient states far from equilibrium. Investigations of such states and processes at the atomic scale are of fundamental significance in understanding a material’s behavior under extreme conditions. Herein, an ultrafast electron diffraction technique is used to track the atomic pathway of the entire melting process of aluminum and reveal a coherent breathing motion of polyhedral clusters in transient liquid aluminum at high temperature and high pressure. The negative expansion behavior of interatomic distances in a superheated liquid state upon heating is observed. These findings provide insight into ultrafast structural transformations and transient atomic dynamics under extreme conditions.

Under the irradiation of an ultrafast intense laser, solid materials can be driven into nonequilibrium states undergoing an ultrafast solid–liquid phase transition. Understanding such nonequilibrium states is essential for scientific research and industrial applications because they exist in various processes including laser fusion and laser machining yet challenging in the sense that high resolution and single-shot capability are required for the measurements. Herein, an ultrafast diffraction technique with megaelectron-volt (MeV) electrons is used to resolve the atomic pathway over the entire laser-induced ultrafast melting process, from the initial loss of long-range order and the formation of high-density liquid to the progressive evolution of short-range order and relaxation into the metastable low-density liquid state. High-resolution measurements using electron pulse compression and a time-stamping technique reveal a coherent breathing motion of polyhedral clusters in transient liquid aluminum during the ultrafast melting process, as indicated by the oscillation of the interatomic distance between the center atom and atoms in the nearest-neighbor shell. Furthermore, contraction of interatomic distance was observed in a superheated liquid state with temperatures up to 6,000 K. The results provide an atomic view of melting accompanied with internal pressure relaxation and are critical for understanding the structures and properties of matter under extreme conditions.

Folded network and structural transition in molten tin

The fundamental relationships between the structure and properties of liquids are far from being well understood. For instance, the structural origins of many liquid anomalies still remain unclear, but liquid-liquid transitions (LLT) are believed to hold a key. However, experimental demonstrations of LLTs have been rather challenging. Here, we report experimental and theoretical evidence of a second-order-like LLT in molten tin, one which favors a percolating covalent bond network at high temperatures. The observed structural transition originates from the fluctuating metallic/covalent behavior of atomic bonding, and consequently a new paradigm of liquid structure emerges. The liquid structure, described in the form of a folded network, bridges two well-established structural models for disordered systems, i.e., the random packing of hard-spheres and a continuous random network, offering a large structural midground for liquids and glasses. Our findings provide an unparalleled physical picture of the atomic arrangement for a plethora of liquids, shedding light on the thermodynamic and dynamic anomalies of liquids but also entailing far-reaching implications for studying liquid polyamorphism and dynamical transitions in liquids.

Unraveling the structural origin of liquid anomalies remains a challenging topic. Xu et al. propose a folded-network structural model for molten tin and provide insights into the observed second-order-like structural transition.

Investigation of structural evolution in the Cu-Zr metallic glass at cryogenic temperatures by using molecular dynamics simulations

In the present work, investigation of structural evolution of Cu₃₃Zr₆₇ specimen during the cooling process from 2500 down to the 300 K, 200 K, 150 K, 100 K, 50 K, and 10 K has been performed at cooling rate of 5 K/ps using molecular dynamics simulation. The pair distribution function (PDF) reveals that Zr‒Zr pair causes the splitting of the first peak of the Cu₃₃Zr₆₇ glass at a lower temperature with an increase in height. Splitting of the first and second peaks supports the presence of the inhomogeneous structure with a statistical average of crystal-like and disordered structural regions in the Cu₃₃Zr₆₇ glass. Voronoi cluster analysis indicated that quasi icosahedral clusters such as < 284 > , < 0285 > , and < 0282 > ; mixed-type cluster such as < 0364 > ; and crystal-like clusters such as < 0446 > are responsible for stabilization of glassy phase at 300 K, 200 K, 150 K, 100 K, 50 K, and 10 K. Similarly, the maximum population of the Cu-centered and Zr-centered < 0286 > quasi icosahedral clusters support the stability of the glassy phase over the studied temperature range. Besides, the maximum population of Cu-centered < 0367 > and Zr-centered < 0364 > , < 0367 > , < 0363 > , and < 0365 > mixed-type clusters and Cu-centered < 0448 > and Zr-centered < 0448 > , < 0445 > , < 0446 > , and < 0444 > crystal-like clusters support the possibility of the presence of intermediate phase of CuZr₂ at lower temperatures as observed from PDFs. Mean square displacement (MSD) for the Cu₃₃Zr₆₇ glass shows that the diffusion coefficient of Cu and Zr atoms reduces with decreasing temperature from 300 to 10 K. Diversity parameter (d) was found to decrease with decreasing temperature.

The structure of metallic melts in binary homogenous alloys: a thermodynamical understanding from the Wulff cluster model

In the present work, a new liquid metal model (Wulff cluster model) which has been proved to describe the structures of pure metal melts has been extended to binary homogeneous alloy melts (Cu-Ni and Ag-Au). The shapes of the nano-particles are determined by surface energies of different families of crystal planes, calculated by density functional theory (DFT), whereas the size was given by the pair distribution function (PDF) g(r) which was converted from experimental high-temperature X-ray diffraction (HTXRD). We demonstrated that the simulated X-ray diffraction (XRD) curves from present models match the experimental results quite well at high temperatures above the liquid-solid two-phase region, including not only the position and width of the peaks but also the relative intensity of the first and second peaks. Moreover, when the temperature is near the liquid-solid two-phase region, our model also fits the experimental strength curve well after modification using the solid XRD pattern of a relatively high melting point metal instead of its nano-particle. The agreement indicates the nucleation processes in homogeneous alloy melts.

Probing the Microstructure in Pure Al & Cu Melts: Theory Meets Experiment

In the present work, a new model of the atomic cluster structure, which is determined by metal Wulff construction with the crystal structure inside, is proposed to describe the structures of metallic melts. The shapes of the structures are determined by surface energies of different crystal plane groups, calculated from density functional theory (DFT), while the size is given by the pair distribution function (PDF) of the experimental high-temperature X-ray diffraction (HTXRD). Taking Aluminum (Al) and Copper (Cu) as the representative examples, we demonstrate that the simulated XRD curves from present models match the experimental results quite well, not only regarding the position and width of the peaks but also the relative intensity of the first and second peaks. These results indicate a successful model to describe the properties of metallic melts. The model also explains a main peak deviation phenomenon between the XRD of metallic melt and the solid ones in pure metal Al. Finally, a physical picture of metallic melt is given, which is mainly composed of atomic cluster structures and free atoms around them.

Vitrification and nanocrystallization of pure liquid Ni studied using molecular-dynamics simulation

Structural variation, vitrification, and crystallization processes in liquid nickel are simulated on continuous cooling and isothermal holding using a classical molecular-dynamics computer simulation procedure with an embedded-atom method potential at constant pressure. Structural changes are monitored with direct structure observation in the simulation cells, as well as by pair distribution and radial distribution functions created using the atomic coordinates. A cluster analysis is also performed. The crystallization kinetics is analyzed under isothermal conditions by monitoring density and energy variation as a function of time. As a result, a time-temperature-transformation diagram can be constructed over a wide temperature range.

Lead-Bismuth Eutectic: Atomic and Micro-Scale Melt Evolution

Element clustering and structural features of liquid lead-bismuth eutectic (LBE) alloy have been investigated up to 720 °C by means of high temperature X-ray diffraction (HT-XRD), X-ray Photoemission Spectroscopy (XPS) and Scanning Photoemission Microscopy (SPEM) at the Elettra synchrotron in Trieste. The short-range order in liquid metal after melting corresponds to the cuboctahedral atomic arrangement and progressively evolves towards the icosahedral one as temperature increases. Such process, that involve a negative expansion of the alloy, is mainly connected to the reduction of atom distance in Pb–Pb pairs which takes place from 350 °C to 520 °C. On an atomic scale, it is observed a change of the relative number of Bi–Bi, Pb–Pb, and Pb–Bi pairs. The Pb–Bi pairs are detected only at a temperature above ~350 °C, and its fraction progressively increases, giving rise to a more homogeneous distribution of the elements. SPEM results showed evidence that the process of chemical homogenization on an atomic scale is preceded and accompanied by homogenization on micro-scale. Clusters rich of Bi and Pb, which are observed after melting, progressively dissolve as temperature increases: Only a few residuals remain at 350 °C, and no more clusters are detected a 520 °C.

Temperature Dependences of Peak Positions in Pair Distribution Function of Metallic Liquids

The temperature dependences of the peak positions in pair distribution functions G(r) of pure metallic zinc (Zn) and indium (In) liquids have been studied using high-energy X-ray diffraction together with ab initio molecular dynamic simulations. It has been demonstrated that the first peak positions in G(r) of both Zn and In move to small r, whereas the second peak positions exhibit opposite movements with increasing temperature, originating from different thermal responses of polyhedron connections. However, the third, above peaks in G(r) in both liquids shift to large r with the expansion coefficients smaller than the values of bulk liquids.

Structure and dynamical properties of liquid Ni64Zr36 and Ni65Hf35 alloys: an ab initio molecular dynamics study

Ab initio molecular dynamics simulations are performed to investigate the atomic structures and dynamics of Ni₆₄Zr₃₆ and Ni₆₅Hf₃₅ metallic liquids in a temperature range of 1400-2500 K. Calculated results are in good agreement with recently reported high temperature experimental data. Local atomic structures are analyzed and compared for Ni₆₄Zr₃₆ and Ni₆₅Hf₃₅ metallic liquids in terms of average bond length, coordination number, Honey-Andersen index, Bond-orientation order, spatial correlation and Voronoi tessellation methods. It is found that Zr-Zr bonds have larger average length of 3.32 Å than 3.22 Å for Hf-Hf bonds, causing sluggish diffusion in Ni₆₅Hf₃₅ liquids. Zr and Hf atom-centered clusters with higher coordination numbers are inclined to aggregate with high-coordinated clusters, while Ni atom-centered clusters with lower coordination numbers prefer to avoiding to be the nearest neighbor with each other. Temperature dependent diffusion coefficients reveal the decoupled diffusion in both liquids, which are related with different spatial correlations for Ni- and Zr- (or Hf-) centered clusters.

A re-evaluation of thermal expansion measurements of metallic liquids and glasses from x-ray scattering experiments

Previous studies reported a number of anomalies when estimates of linear thermal expansion coefficients of metallic liquids and glasses from x-ray scattering experiments were compared with direct measurements of volume/length changes with temperature. In most cases, the first peak of the pair correlation function showed a contraction, while the structure factor showed an expansion, but both at rates much different from those expected from the direct volume measurements. In addition, the relationship between atomic volume and the characteristic lengths obtained from the structure factor from scattering experiments was found to have a fractional exponent instead of one equal to three, as expected from the Ehrenfest relation. This has led to the speculation that the atomic packing in liquids and glasses follow a fractal behavior. These issues are revisited in this study using more in-depth analysis of recent higher resolution data and some new ideas suggested in the literature. The main conclusion is that for metallic alloys, at least to a large extent, most of these anomalies arise from complicated interplays of the temperature dependences of the various partial structure factors, which contribute to the total intensities of the scattering peaks.

Temperature dependent structural evolution in liquid Ag50Ga50 alloy

The temperature dependence of atomic structural evolution in liquid Ag₅₀Ga₅₀ alloy has been studied using an in situ high energy x-ray diffraction (XRD) experiment combined with first-principles molecular dynamics (FPMD) simulations. The experimental data show a reversible structural crossover at the temperature of about 1050 K. Changes in both electrical resistivity and absolute thermoelectric power at about 1100 K strongly support the XRD results. Additionally, FPMD simulations reveal the abnormal temperature dependent behavior of partial coordination number and atomic diffusivity at about 1200 K, elucidating that the partition experimentally observed changes in structure and properties could be linked with the repartition between Ag and Ga atoms in the liquid at around 1050-1200 K. This finding will trigger more studies on the structural evolution of noble-polyvalent metals in particular and metallic liquids in general.

Perspective on Structural Evolution and Relations with Thermophysical Properties of Metallic Liquids

The relationship between the structural evolution and properties of metallic liquids is a long-standing hot issue in condensed-matter physics and materials science. Here, recent progress is reviewed in several fundamental aspects of metallic liquids, including the methods to study their atomic structures, liquid-liquid transition, physical properties, fragility, and their correlations with local structures, together with potential applications of liquid metals at room temperature. Involved with more experimentally and theoretically advanced techniques, these studies provide more in-depth understanding of the structure-property relationship of metallic liquids and promote the design of new metallic materials with superior properties.

Comparative study of crystallization process in metallic melts using ab initio molecular dynamics simulations

The crystallization process of liquid metals is studied using ab initio molecular dynamics simulations. The evolution of short-range order during quenching in Pb and Zn liquids is compared with body-centered cubic (bcc) Nb and V, and hexagonal closed-packed (hcp) Mg. We found that the fraction and type of the short-range order depends on the system under consideration, in which the icosahedral symmetry seems to dominate in the body-centered cubic metals. Although the local atomic structures in stable liquids are similar, liquid hcp-like Zn, bcc-like Nb and V can be deeply supercooled far below its melting point before crystallization while the supercooled temperature range in liquid Pb is limited. Further investigations into the nucleation process reveal the process of polymorph selection. In the body-centered cubic systems, the polymorph selection occurs in the supercooled state before the nucleation is initiated, while in the closed-packed systems it starts at the time of onset of crystallization. Atoms with bcc-like lattices in all studied supercooled liquids are always detected before the polymorph selection. It is also found that the bond orientational ordering is strongly correlated with the crystallization process in supercooled Zn and Pb liquids.

Validity of the Stokes-Einstein relation in liquids: simple rules from the excess entropy

It is becoming common practice to consider that the Stokes-Einstein relation D/T~ η ^-1 usually works for liquids above their melting temperatures although there is also experimental evidence for its failure. Here we investigate numerically this commonly-invoked assumption for simple liquid metals as well as for their liquid alloys. Using ab initio molecular dynamics simulations we show how entropy scaling relationships developed by Rosenfeld can be used to predict the conditions for the validity of the Stokes-Einstein relation in the liquid phase. Specifically, we demonstrate the Stokes-Einstein relation may break down in the liquid phase of some liquid alloys mainly due to the presence of local structural ordering as evidenced in their partial two-body excess entropies. Our findings shed new light on the understanding of transport properties of liquid materials and will trigger more experimental and theoretical studies since excess entropy and its two-body approximation are readily obtainable from standard experiments and simulations.

Excess Entropy Scaling Law for Diffusivity in Liquid Metals

Understanding how dynamic properties depend on the structure and thermodynamics in liquids is a long-standing open problem in condensed matter physics. A very simple approach is based on the Dzugutov contribution developed on model fluids in which a universal (i.e. species-independent) connection relates the pair excess entropy of a liquid to its reduced diffusion coefficient. However its application to "real" liquids still remains uncertain due to the ability of a hard sphere (HS) reference fluid used in reducing parameters to describe complex interactions that occur in these liquids. Here we use ab initio molecular dynamics simulations to calculate both structural and dynamic properties at different temperatures for a wide series of liquid metals including Al, Au, Cu, Li, Ni, Ta, Ti, Zn as well as liquid Si and B. From this analysis, we demonstrate that the Dzugutov scheme can be applied successfully if a self-consistent method to determine the packing fraction of the hard sphere reference fluid is used as well as the Carnahan-Starling approach to express the excess entropy.

Evolution of atomic structure in Al75Cu25 liquid from experimental and ab initio molecular dynamics simulation studies

X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al(75)Cu(25) alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al(75)Cu(25) alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt-Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al(75)Cu(25) alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid-liquid transition appears in the bulk liquid Al(75)Cu(25) alloy in the studied temperature range.

Size-and phase-dependent structure of copper(ii) oxide nanoparticles

Core (3 nm diameter) and outer surface layer (0.5 nm width) of a CuO nanoparticle of 4 nm in diameter.

Molten uranium dioxide structure and dynamics

Uranium dioxide (UO2) is the major nuclear fuel component of fission power reactors. A key concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its zirconium cladding and steel containment. Yet, the very high temperatures (>3140 kelvin) and chemical reactivity of molten UO2 have prevented structural studies. In this work, we combine laser heating, sample levitation, and synchrotron x-rays to obtain pair distribution function measurements of hot solid and molten UO2. The hot solid shows a substantial increase in oxygen disorder around the lambda transition (2670 K) but negligible U-O coordination change. On melting, the average U-O coordination drops from 8 to 6.7 ± 0.5. Molecular dynamics models refined to this structure predict higher U-U mobility than 8-coordinated melts.

Electrical, thermal, and species transport properties of liquid eutectic Ga-In and Ga-In-Sn from first principles

Using ab initio molecular dynamics, the atomic structure and transport properties of eutectic Ga-In and Ga-In-Sn are investigated. The Kubo-Greenwood (K-G) and the Ziman-Faber (Z-F) formulations and the Wiedemann-Franz (W-F) law are used for the electrical and electronic thermal conductivity. The species diffusivity and the viscosity are also predicted using the mean square displacement and the Stokes-Einstein (S-E) relation. Alloying Ga causes more disordered structure, i.e., broadening the atomic distance near the In and Sn atoms, which reduces the transport properties and the melting temperature. The K-G treatment shows excellent agreement with the experimental results while Z-F treatment formula slightly overestimates the electrical conductivity. The predicted thermal conductivity also shows good agreement with the experiments. The species diffusivity and the viscosity are slightly reduced by the alloying of Ga with In and Sn atoms. Good agreements are found with available experimental results and new predicted transport-property results are provided.

The crystallization process of liquid vanadium studied by ab initio molecular dynamics

We present a study of the crystallization process in liquid vanadium over a temperature range from 3000 K down to 1500 K using ab initio molecular dynamics simulations. Short-range order evolution during solidification is studied using various structural analysis methods. We show that the icosahedral-like short-range order is detected in the stable liquid phase and grows upon supercooling. The system undergoes a first-order phase transition (from a liquid to a solid state) at a temperature of about 1600 K. The crystal nucleation process is further studied using the time-temperature transformation mechanism by annealing the system at 1650 K. The nucleation is examined using bond-orientational order and density fluctuation analysis. Our finding is that various precursors appear in the region of high bond-orientational order with the majority having body-centered cubic (bcc)-like symmetry. This bcc-like region grows on annealing via thermal fluctuations. Our results reveal that the bond-orientational order precedes the density fluctuation, and is the main driving factor for nucleation.

Atomic structure evolution during solidification of liquid niobium from ab initio molecular dynamics simulations

Atomic structure transitions of liquid niobium during solidification, at different temperatures from 3200 to 1500 K, were studied by using ab initio molecular dynamics simulations. The local atomic structure variations with temperature are investigated by using the pair-correlation function, the structure factor, the bond-angle distribution function, the Honeycutt-Anderson index, Voronoi tessellation and the cluster alignment methods. Our results clearly show that, upon quenching, the icosahedral short-range order dominates in the stable liquid and supercooled liquid states before the system transforms to crystalline body-center cubic phase at a temperature of about 1830 K.

Correlation between atomic structure evolution and strength in a bulk metallic glass at cryogenic temperature

A model Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 (at.%) bulk metallic glass (BMG) is selected to explore the structural evolution on the atomic scale with decreasing temperature down to cryogenic level using high energy X-ray synchrotron radiation. We discover a close correlation between the atomic structure evolution and the strength of the BMG and find out that the activation energy increment of the concordantly atomic shifting at lower temperature is the main factor influencing the strength. Our results might provide a fundamental understanding of the atomic-scale structure evolution and may bridge the gap between the atomic-scale physics and the macro-scale fracture strength for BMGs.

A facile heating cell for in situ transmittance and fluorescence X-ray absorption spectroscopy investigations

A facile heating cell has been designed for in situ transmittance and fluorescence X-ray absorption spectroscopy (XAS) measurements up to 1273 K under vacuum or an inert atmosphere. These high temperatures are achieved using a tantalum heating element by ohmic heating. Because of the small specific heat capacity, the temperature can be changed in a matter of minutes from room temperature to high temperature. Furthermore, a commercial power controller was adapted to provide stable temperature control. The construction of the heat shielding system provides a novel approach to reducing the beam's path length and the cell's size. The cell is inexpensive and easy to build. Its performance was evaluated by in situ XAS measurements of the temperature-dependent structure of ceria nanocrystals. Some preliminary results for the structural mechanism in ceria nanocrystal redox applications are given.

Short range order in liquid pnictides

Liquid pnictides have anomalous physical properties and complex radial distribution functions. The quasi-crystalline model of liquid structure is applied to interpret the three-dimensional structure of liquid pnictides. It is shown that all the column V elements can be characterized by a short range order lattice symmetry similar to that of the underlying solid, the A7 structure, which originates from a Peierls distorted simple cubic lattice. The evolution of the liquid structure down the column as well as its temperature and pressure dependence is interpreted by means of the effect of thermodynamic parameters on the Peierls distortion. Surprisingly, it is found that the Peierls effect increases with temperature and the nearest neighbour distances exhibit negative thermal expansion.