In the absence of periodicity, the structure of glass is ill-defined, and a large number of struc... more In the absence of periodicity, the structure of glass is ill-defined, and a large number of structural states are found at similar energy levels. However, little is known about how these states are connected to each other in the potential energy landscape. We simulate mechanical relaxation by molecular dynamics for a prototypical Cu 64:5 Zr 35:5 metallic glass and follow the mechanical energy loss of each atom to track the change in the state. We find that the energy barriers separating these states are remarkably low, only of the order of 1 meV, implying that even quantum fluctuations can overcome these potential energy barriers. Our observation of numerous small ripples in the bottom of the potential energy landscape puts many assumptions regarding the thermodynamic states of metallic glasses into question and suggests that metallic glasses are not totally frozen at the local atomic level.
The atomistic mechanism of deformation in metallic liquids and glasses is discussed from the view... more The atomistic mechanism of deformation in metallic liquids and glasses is discussed from the view point of local topology of atomic connectivity, based mainly on the results of computer simulation. In crystals the topology of atomic connectivity network is fixed, and deviation from it defines lattice defects. In liquids and glasses, however, the topology is open and flexible, and fluctuates in time and space. We focus on the action of atomic bond being cut or formed, which changes the local topology of atomic connectivity, and discuss how the bond cutting and forming determine viscosity of liquids. We then discuss how collectivity of such actions, including shear-transformation-zone (STZ) and their avalanche, governs the macroscopic deformation behavior in supercooled liquids and glasses. The collectivity of topological change is directly connected to the local potential energy landscape (PEL), particularly the density of the local minima in PEL. The density of PEL minima defines the effective temperature, and is influenced by processing conditions, such as cooling rate through the glass transition. The description of the structure and dynamics of liquids and glasses through local topology of atomic connectivity and the atomic-level stress could advance the field to the level the current mean-field approach would not be able to attain.
Looking for new dual-functional molecular crystal is a emergency task to molecular electronics. T... more Looking for new dual-functional molecular crystal is a emergency task to molecular electronics. Two-dimensional honeycomb Cr III Mn II (C 2 O 4) 3anion as a building block succeeded on built up ferromagnetic conductors, so as zero-dimensional FeCl 4 anion to field-induced-organic superconductor with-d interaction between donor and anion. It will be interesting to explore uniform onedimensional anion with metal atom coordinated with (C 2 O 4) 2and Cl-. Several novel salts with one-dimensional [Fe(C 2 O 4)Cl 2 ]anion were synthesis, one iron atoms bonds to two Cl atoms and four oxygen atoms of two oxalato groups in cis-mode. Depending on the countercation from A + , R 4 N + and TTF series molecules, a uniform bindingarch or zigzag anion chain is found in the crystal. So dual-functional molecular crystal with magnetism property from paramagnetic, antiferromagnetic to ferromagnetic, conductivity from insulator, semiconductor, metal in charge-transfer complex and fast-ion conductor are constructed.
A general quantitative measure of the tendency towards phase separation is introduced for systems... more A general quantitative measure of the tendency towards phase separation is introduced for systems exhibiting phase transitions or crossovers controlled by charge carrier concentration. This measure is devised for the situations when the quantitative knowledge of various contributions to free energy is incomplete, and is applied to evaluate the chances of electronic phase separation associated with the onset of antiferromagnetic correlations in high-temperature cuprate superconductors. The experimental phenomenology of lanthanum-and yittrium-based cuprates was used as input to this analysis. It is also pointed out that Coulomb repulsion between charge carriers separated by the distances of 1-3 lattice periods strengthens the tendency towards phase separation by accelerating the decay of antiferromagnetic correlations with doping. Overall, the present analysis indicates that cuprates are realistically close to the threshold of phase separation-nanoscale limited or even macroscopic with charge density varying between adjacent crystal planes.
Metallurgical and Materials Transactions, Feb 24, 2010
It is difficult to formulate the statistical mechanical theory of liquids and glasses, because ph... more It is difficult to formulate the statistical mechanical theory of liquids and glasses, because phonons, which are the basis for the statistical mechanics of lattice dynamics in crystals, are strongly scattered and have a very short lifetime in liquids and glasses. Instead computer simulation and the “free-volume” theory are most frequently used in explaining experimental results on metallic glasses. However,
Metallurgical and Materials Transactions, May 15, 2008
ABSTRACT The effect of geometrical frustration in the atomic structure on the formability of bulk... more ABSTRACT The effect of geometrical frustration in the atomic structure on the formability of bulk metallic glasses is discussed from a general point of view. It is pointed out that there are two distinct and complementing pathways to easy glass formation: stabilizing the glass itself and destabilizing the corresponding crystalline state. While the discussions in the field tend to focus on the first one, the second in fact is a more effective approach. Examples of both will be discussed using soft-sphere, rather than hard-sphere, packing concepts.
The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude ... more The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude before the liquid freezes into a glass. The rate of the change with temperature is characterized by liquid fragility. The mechanism of such a spectacular behavior and the origin of fragility have long been discussed, but it remains unresolved because of the difficulty of carrying out experiments and constructing theories that bridge over a wide timescale from atomic (ps) to bulk (minutes). Through the x-ray diffraction measurement and molecular dynamics simulation for metallic liquids we suggest that large changes in viscosity can be caused by relatively small changes in the structural coherence which characterizes the medium-range order. Here the structural coherence does not imply that of atomic-scale structure, but it relates to the coarse-grained density fluctuations represented by the peaks in the pair-distribution function (PDF) beyond the nearest neighbors. The coherence length is related to fragility and increases with decreasing temperature, and it diverges only at a negative temperature. This analysis is compared with several current theories which predict a phase transition near the glass transition temperature.
Metallic glasses are known for their outstanding mechanical strength. However, the microscopic me... more Metallic glasses are known for their outstanding mechanical strength. However, the microscopic mechanism of failure in metallic glasses is not well-understood. In this article we discuss elastic, anelastic and plastic behaviors of metallic glasses from the atomistic point of view, based upon recent results by simulations and experiments. Strong structural disorder affects all properties of metallic glasses, but the effects are more profound and intricate for the mechanical properties. In particular we suggest that mechanical failure is an intrinsic behavior of metallic glasses, a consequence of stress-induced glass transition, unlike crystalline solids which fail through the motion of extrinsic lattice defects such as dislocations.
The structure of liquid and glass is usually described by the atomic pair-distribution function (... more The structure of liquid and glass is usually described by the atomic pair-distribution function (PDF), g(r), which expresses the statistical distribution of distances between atoms. The PDF can be determined by diffraction experiments using x-rays or neutrons. However, liquid is dynamic in nature, and we have to be sensible about what the PDF means for liquid. For crystalline solids the atomic structure is determined by the elastic scattering of x-rays, neutrons or electrons, because the momentum is transferred to the whole rigid body of the sample in scattering. But the elastic scattering intensity from liquid is zero because of the lack of rigidity. The scattering from liquid is purely inelastic, described by the dynamic structure factor, S(Q, ω), where Q is the momentum transfer and E = hω/2π is the energy transfer in scattering. To measure S(Q, ω) we need an elaborate inelastic scattering instruments. In particular for inelastic x-ray scattering (IXS) we need a very high energy resolution with ~ meV and ΔE/E < 10-7. This can be achieved only with a large backscattering crystal analyzer with a long flight path. However, in regular x-ray diffraction measurement the energy resolution is poor, ~ 1 eV, far exceeding the typical energies of vibrational excitations. As a result, the measured structure function, S(Q), is the S(Q, ω) integrated over energy, thus representing the same-time correlation among atoms. Thus, the PDF, obtained by the Fourier-transformation of S(Q), is the same-time density correlation function which shows the time averaged snapshot of correlations. Therefore, the PDF does not describe the structure in a regular sense. For a long time, the PDF has been used in representing the structure, because it was the only readily available structural descriptor, and various theories have been proposed to predict dynamic properties from the PDF. On the other hand, the dynamic two-body correlation can be directly expressed by the Van Hove function, G(r, t), obtained by the double-Fourier-transformation of S(Q, ω). But, to carry out the double-Fourier-transformation accurately S(Q, ω) has to be measured over a wide Q-E space. Until recently this was unpractical, because the inelastic scattering measurement, typically done with a triple-axis-spectrometer, was extremely time-consuming. But the advent of pulsed neutron sources with large two-dimensional detector arrays and advances in the IXS instrumentation made it possible to determine the Van Hove function in a reasonable time, 4-12 hrs. We applied this technique to various liquids, including water, aqueous solutions of salt, metallic alloy liquids, liquid Ga, and organic electrolytes. New physical insights obtained by these measurements will be discussed. Now that this technique is available we should expand the definition of the "structure" of liquid to include the dynamic structure represented by the Van Hove function.
In the absence of periodicity the structure of glass is ill-defined, and a large number of struct... more In the absence of periodicity the structure of glass is ill-defined, and a large number of structural states are found at similar energy levels. However, little is known about how these states are connected to each other in the potential energy landscape. We simulate mechanical relaxation by molecular dynamics for a prototypical Cu 64.5 Zr 35.5 metallic glass and follow the mechanical energy loss of each atom to track the change in the state. We find that the energy barriers separating these states are remarkably low, only of the order of 1 meV, implying that even quantum fluctuations can overcome these potential energy barriers. Our observation of numerous small ripples in the bottom of the potential energy landscape puts many assumptions regarding the thermodynamic states of metallic glasses into question and suggests that metallic glasses are not 28 totally frozen at the local atomic level.
One would expect that the onset of superconductivity might leave a signature on the atomic struct... more One would expect that the onset of superconductivity might leave a signature on the atomic structure of the high temperature superconducting oxides, if the electron-lattice interaction is strong and totally or partially accounts for the occurance of superconductivity. For this reason the structural change at Tc has been the subject of considerable interest.
It is known that deformation in disordered materials such as metallic glasses and supercooled liq... more It is known that deformation in disordered materials such as metallic glasses and supercooled liquids occurs via the cooperative rearrangement of atoms or constituent particles at dynamical heterogeneities, commonly regarded as point-like defects. We show via molecular-dynamics simulations that there is no apparent relationship between atomic rearrangements and the local atomic environment as measured by the atomic-level stresses, kinetic and potential energies, and the per-atom Voronoi volume. In addition, there is only a weak correlation between atomic rearrangements and the largest and smallest eigenvalues of the dynamical matrix. Our results confirm the transient nature of dynamical heterogeneities and suggest that the notion of defects may be less relevant than that of a propensity for rearrangement.
The conventional approach to elucidate the atomic structure of liquid and glass is to start with ... more The conventional approach to elucidate the atomic structure of liquid and glass is to start with local structural units made of several atoms, and to use them as building blocks to form a global structure, the bottom-up approach. We propose to add an alternative top-down approach in which we start with a global high-temperature gas state and then apply interatomic potentials to all atoms at once. This causes collective density wave instability in all directions with the same wavelength. These two driving forces, local and global, are in competition and are mutually frustrated. The final structure is determined through the compromise of frustration between these two, which creates the medium-range-order. This even-handed approach on global and local potential energy landscapes explains the distinct natures of short-range order and medium-range order, and strong temperature dependence of various properties of liquid.
In the absence of periodicity, the structure of glass is ill-defined, and a large number of struc... more In the absence of periodicity, the structure of glass is ill-defined, and a large number of structural states are found at similar energy levels. However, little is known about how these states are connected to each other in the potential energy landscape. We simulate mechanical relaxation by molecular dynamics for a prototypical Cu 64:5 Zr 35:5 metallic glass and follow the mechanical energy loss of each atom to track the change in the state. We find that the energy barriers separating these states are remarkably low, only of the order of 1 meV, implying that even quantum fluctuations can overcome these potential energy barriers. Our observation of numerous small ripples in the bottom of the potential energy landscape puts many assumptions regarding the thermodynamic states of metallic glasses into question and suggests that metallic glasses are not totally frozen at the local atomic level.
The atomistic mechanism of deformation in metallic liquids and glasses is discussed from the view... more The atomistic mechanism of deformation in metallic liquids and glasses is discussed from the view point of local topology of atomic connectivity, based mainly on the results of computer simulation. In crystals the topology of atomic connectivity network is fixed, and deviation from it defines lattice defects. In liquids and glasses, however, the topology is open and flexible, and fluctuates in time and space. We focus on the action of atomic bond being cut or formed, which changes the local topology of atomic connectivity, and discuss how the bond cutting and forming determine viscosity of liquids. We then discuss how collectivity of such actions, including shear-transformation-zone (STZ) and their avalanche, governs the macroscopic deformation behavior in supercooled liquids and glasses. The collectivity of topological change is directly connected to the local potential energy landscape (PEL), particularly the density of the local minima in PEL. The density of PEL minima defines the effective temperature, and is influenced by processing conditions, such as cooling rate through the glass transition. The description of the structure and dynamics of liquids and glasses through local topology of atomic connectivity and the atomic-level stress could advance the field to the level the current mean-field approach would not be able to attain.
Looking for new dual-functional molecular crystal is a emergency task to molecular electronics. T... more Looking for new dual-functional molecular crystal is a emergency task to molecular electronics. Two-dimensional honeycomb Cr III Mn II (C 2 O 4) 3anion as a building block succeeded on built up ferromagnetic conductors, so as zero-dimensional FeCl 4 anion to field-induced-organic superconductor with-d interaction between donor and anion. It will be interesting to explore uniform onedimensional anion with metal atom coordinated with (C 2 O 4) 2and Cl-. Several novel salts with one-dimensional [Fe(C 2 O 4)Cl 2 ]anion were synthesis, one iron atoms bonds to two Cl atoms and four oxygen atoms of two oxalato groups in cis-mode. Depending on the countercation from A + , R 4 N + and TTF series molecules, a uniform bindingarch or zigzag anion chain is found in the crystal. So dual-functional molecular crystal with magnetism property from paramagnetic, antiferromagnetic to ferromagnetic, conductivity from insulator, semiconductor, metal in charge-transfer complex and fast-ion conductor are constructed.
A general quantitative measure of the tendency towards phase separation is introduced for systems... more A general quantitative measure of the tendency towards phase separation is introduced for systems exhibiting phase transitions or crossovers controlled by charge carrier concentration. This measure is devised for the situations when the quantitative knowledge of various contributions to free energy is incomplete, and is applied to evaluate the chances of electronic phase separation associated with the onset of antiferromagnetic correlations in high-temperature cuprate superconductors. The experimental phenomenology of lanthanum-and yittrium-based cuprates was used as input to this analysis. It is also pointed out that Coulomb repulsion between charge carriers separated by the distances of 1-3 lattice periods strengthens the tendency towards phase separation by accelerating the decay of antiferromagnetic correlations with doping. Overall, the present analysis indicates that cuprates are realistically close to the threshold of phase separation-nanoscale limited or even macroscopic with charge density varying between adjacent crystal planes.
Metallurgical and Materials Transactions, Feb 24, 2010
It is difficult to formulate the statistical mechanical theory of liquids and glasses, because ph... more It is difficult to formulate the statistical mechanical theory of liquids and glasses, because phonons, which are the basis for the statistical mechanics of lattice dynamics in crystals, are strongly scattered and have a very short lifetime in liquids and glasses. Instead computer simulation and the “free-volume” theory are most frequently used in explaining experimental results on metallic glasses. However,
Metallurgical and Materials Transactions, May 15, 2008
ABSTRACT The effect of geometrical frustration in the atomic structure on the formability of bulk... more ABSTRACT The effect of geometrical frustration in the atomic structure on the formability of bulk metallic glasses is discussed from a general point of view. It is pointed out that there are two distinct and complementing pathways to easy glass formation: stabilizing the glass itself and destabilizing the corresponding crystalline state. While the discussions in the field tend to focus on the first one, the second in fact is a more effective approach. Examples of both will be discussed using soft-sphere, rather than hard-sphere, packing concepts.
The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude ... more The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude before the liquid freezes into a glass. The rate of the change with temperature is characterized by liquid fragility. The mechanism of such a spectacular behavior and the origin of fragility have long been discussed, but it remains unresolved because of the difficulty of carrying out experiments and constructing theories that bridge over a wide timescale from atomic (ps) to bulk (minutes). Through the x-ray diffraction measurement and molecular dynamics simulation for metallic liquids we suggest that large changes in viscosity can be caused by relatively small changes in the structural coherence which characterizes the medium-range order. Here the structural coherence does not imply that of atomic-scale structure, but it relates to the coarse-grained density fluctuations represented by the peaks in the pair-distribution function (PDF) beyond the nearest neighbors. The coherence length is related to fragility and increases with decreasing temperature, and it diverges only at a negative temperature. This analysis is compared with several current theories which predict a phase transition near the glass transition temperature.
Metallic glasses are known for their outstanding mechanical strength. However, the microscopic me... more Metallic glasses are known for their outstanding mechanical strength. However, the microscopic mechanism of failure in metallic glasses is not well-understood. In this article we discuss elastic, anelastic and plastic behaviors of metallic glasses from the atomistic point of view, based upon recent results by simulations and experiments. Strong structural disorder affects all properties of metallic glasses, but the effects are more profound and intricate for the mechanical properties. In particular we suggest that mechanical failure is an intrinsic behavior of metallic glasses, a consequence of stress-induced glass transition, unlike crystalline solids which fail through the motion of extrinsic lattice defects such as dislocations.
The structure of liquid and glass is usually described by the atomic pair-distribution function (... more The structure of liquid and glass is usually described by the atomic pair-distribution function (PDF), g(r), which expresses the statistical distribution of distances between atoms. The PDF can be determined by diffraction experiments using x-rays or neutrons. However, liquid is dynamic in nature, and we have to be sensible about what the PDF means for liquid. For crystalline solids the atomic structure is determined by the elastic scattering of x-rays, neutrons or electrons, because the momentum is transferred to the whole rigid body of the sample in scattering. But the elastic scattering intensity from liquid is zero because of the lack of rigidity. The scattering from liquid is purely inelastic, described by the dynamic structure factor, S(Q, ω), where Q is the momentum transfer and E = hω/2π is the energy transfer in scattering. To measure S(Q, ω) we need an elaborate inelastic scattering instruments. In particular for inelastic x-ray scattering (IXS) we need a very high energy resolution with ~ meV and ΔE/E < 10-7. This can be achieved only with a large backscattering crystal analyzer with a long flight path. However, in regular x-ray diffraction measurement the energy resolution is poor, ~ 1 eV, far exceeding the typical energies of vibrational excitations. As a result, the measured structure function, S(Q), is the S(Q, ω) integrated over energy, thus representing the same-time correlation among atoms. Thus, the PDF, obtained by the Fourier-transformation of S(Q), is the same-time density correlation function which shows the time averaged snapshot of correlations. Therefore, the PDF does not describe the structure in a regular sense. For a long time, the PDF has been used in representing the structure, because it was the only readily available structural descriptor, and various theories have been proposed to predict dynamic properties from the PDF. On the other hand, the dynamic two-body correlation can be directly expressed by the Van Hove function, G(r, t), obtained by the double-Fourier-transformation of S(Q, ω). But, to carry out the double-Fourier-transformation accurately S(Q, ω) has to be measured over a wide Q-E space. Until recently this was unpractical, because the inelastic scattering measurement, typically done with a triple-axis-spectrometer, was extremely time-consuming. But the advent of pulsed neutron sources with large two-dimensional detector arrays and advances in the IXS instrumentation made it possible to determine the Van Hove function in a reasonable time, 4-12 hrs. We applied this technique to various liquids, including water, aqueous solutions of salt, metallic alloy liquids, liquid Ga, and organic electrolytes. New physical insights obtained by these measurements will be discussed. Now that this technique is available we should expand the definition of the "structure" of liquid to include the dynamic structure represented by the Van Hove function.
In the absence of periodicity the structure of glass is ill-defined, and a large number of struct... more In the absence of periodicity the structure of glass is ill-defined, and a large number of structural states are found at similar energy levels. However, little is known about how these states are connected to each other in the potential energy landscape. We simulate mechanical relaxation by molecular dynamics for a prototypical Cu 64.5 Zr 35.5 metallic glass and follow the mechanical energy loss of each atom to track the change in the state. We find that the energy barriers separating these states are remarkably low, only of the order of 1 meV, implying that even quantum fluctuations can overcome these potential energy barriers. Our observation of numerous small ripples in the bottom of the potential energy landscape puts many assumptions regarding the thermodynamic states of metallic glasses into question and suggests that metallic glasses are not 28 totally frozen at the local atomic level.
One would expect that the onset of superconductivity might leave a signature on the atomic struct... more One would expect that the onset of superconductivity might leave a signature on the atomic structure of the high temperature superconducting oxides, if the electron-lattice interaction is strong and totally or partially accounts for the occurance of superconductivity. For this reason the structural change at Tc has been the subject of considerable interest.
It is known that deformation in disordered materials such as metallic glasses and supercooled liq... more It is known that deformation in disordered materials such as metallic glasses and supercooled liquids occurs via the cooperative rearrangement of atoms or constituent particles at dynamical heterogeneities, commonly regarded as point-like defects. We show via molecular-dynamics simulations that there is no apparent relationship between atomic rearrangements and the local atomic environment as measured by the atomic-level stresses, kinetic and potential energies, and the per-atom Voronoi volume. In addition, there is only a weak correlation between atomic rearrangements and the largest and smallest eigenvalues of the dynamical matrix. Our results confirm the transient nature of dynamical heterogeneities and suggest that the notion of defects may be less relevant than that of a propensity for rearrangement.
The conventional approach to elucidate the atomic structure of liquid and glass is to start with ... more The conventional approach to elucidate the atomic structure of liquid and glass is to start with local structural units made of several atoms, and to use them as building blocks to form a global structure, the bottom-up approach. We propose to add an alternative top-down approach in which we start with a global high-temperature gas state and then apply interatomic potentials to all atoms at once. This causes collective density wave instability in all directions with the same wavelength. These two driving forces, local and global, are in competition and are mutually frustrated. The final structure is determined through the compromise of frustration between these two, which creates the medium-range-order. This even-handed approach on global and local potential energy landscapes explains the distinct natures of short-range order and medium-range order, and strong temperature dependence of various properties of liquid.
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Papers by Takeshi Egami