Elastic constants of Cu and the instability of its bcc structure

Molecular Dynamics Simulation of Bulk Cu Material under Various Factors

In this paper, the molecular dynamics (MD) method was used to study the influence of factors of bulk Cu material, such as the effect of the number of atoms (N) at temperature (T), T = 300 K, temperature T, and annealing time (t) with Cu5324 on the structure properties, phase transition, and glass temperature Tg of the bulk Cu material. The obtained results showed that the glass transition temperature (Tg) of the bulk Cu material was Tg = 652 K; the length of the link for Cu-Cu had a negligible change; r = 2.475 Å; and four types of structures, FCC, HCP, BCC, Amor, always existed. With increasing the temperature the FCC, HCP, and BCC decrease, and Amorphous (Amor) increases. With an increasing number of atoms and annealing time, the FCC, HCP, and BCC increased, and Amor decreased. The simulated results showed that there was a great influence of factors on the structure found the gradient change, phase transition, and successful determination of the glass temperature point above Tg of the bulk Cu material. On the basis of these results, essential support will be provided for future studies on mechanical, optical, and electronic properties.

Strain driven phase transition and mechanism for Fe/Ir(111) films

By way of introducing heterogeneous interfaces, the stabilization of crystallographic phases is critical to a viable strategy for developing materials with novel characteristics, such as occurrence of new structure phase, anomalous enhancement in magnetic moment, enhancement of efficiency as nanoportals. Because of the different lattice structures at the interface, heterogeneous interfaces serve as a platform for controlling pseudomorphic growth, nanostructure evolution and formation of strained clusters. However, our knowledge related to the strain accumulation phenomenon in ultrathin Fe layers on face-centered cubic (fcc) substrates remains limited. For Fe deposited on Ir(111), here we found the existence of strain accumulation at the interface and demonstrate a strain driven phase transition in which fcc-Fe is transformed to a bcc phase. By substituting the bulk modulus and the shear modulus and the experimental results of lattice parameters in cubic geometry, we obtain the strain energy density for different Fe thicknesses. A limited distortion mechanism is proposed for correlating the increasing interfacial strain energy, the surface energy, and a critical thickness. The calculation shows that the strained layers undergo a phase transition to the bulk structure above the critical thickness. The results are well consistent with experimental measurements. The strain driven phase transition and mechanism presented herein provide a fundamental understanding of strain accumulation at the bcc/fcc interface.

Instability of the body-centered cubic lattice within the sticky hard sphere and Lennard-Jones model obtained from exact lattice summations

A smooth path of rearrangement from the body-centered cubic (bcc) to the face-centered cubic (fcc) lattice is obtained by introducing a single parameter to lattice vectors of a cuboidal unit cell. As a result, we obtain analytical expressions in terms of lattice sums for the cohesive energy where the interaction is described by a Lennard-Jones (LJ) interaction potential or a sticky hard-sphere (SHS) model with a r^{-n} long-range attractive term. These lattice sums are evaluated to computer precision by expansions in terms of a fast converging Bessel function series. Applying the whole range of lattice parameters for the SHS and LJ potentials we prove that the bcc phase is unstable (or, at best, metastable) toward distortion into the fcc phase in the low temperature and pressure limit. Even if more accurate potentials are used, such as the extended LJ potential for argon or chromium, the bcc phase remains unstable. This strongly indicates that the appearance of a low temperature bcc phase for several elements in the periodic table is due to higher than two-body forces in atomic interactions.

A practical method for fabricating superparamagnetic films and the mechanism involved

Due to the widespread applications of biosensors, such as in magnetic resonance imaging, cancer detection and drug delivery, the use of superparamagnetic materials for preparing biosensors has increased greatly. We report herein on a strategy toward fabrication of a nanoscale biosensor composed of superparamagnetic films. On increasing the film thickness of magnetic layers, a phase transition typically occurs from either a low-Curie-temperature state or a superparamagnetic state to a ferromagnetic state. A new finding is demonstrated wherein a phase transition of such a superparamagnetic phase can be induced by controlling the thickness of ultrathin ferromagnetic layers with perpendicular magnetic anisotropy. Both the M-H curve with zero coercive force at 300 K and deviations of the normalized hysteresis loop at 2 K confirm the superparamagnetic state of Co/Ir(111) at room temperature. An overstrained film transforming into clusters (OFTC) model based on the new finding and our experimental evidence is proposed for modeling this phenomenon. From the energetic point of view of the OFTC model, we propose a limited distortion mechanism that can be useful in determining the critical thickness for the phase transition. This mechanism considers the balance between interfacial strain energy and surface free energy. A method for producing superparamagnetic films by taking advantage of the accumulation of strain and relaxation is reported.

A metastable phase of shocked bulk single crystal copper: an atomistic simulation study

Structural phase transformation in bulk single crystal Cu in different orientation under shock loading of different intensities has been investigated in this article. Atomistic simulations, such as, classical molecular dynamics using embedded atom method (EAM) interatomic potential and ab-initio based molecular dynamics simulations, have been carried out to demonstrate FCC-to-BCT phase transformation under shock loading of 〈100〉 oriented bulk single crystal copper. Simulated x-ray diffraction patterns have been utilized to confirm the structural phase transformation before shock-induced melting in Cu(100).

Metastable cubic and tetragonal phases of transition metals predicted by density-functional theory

DFT calculations for 24 transition metals predict eleven metastable allotropes in fcc or bct phase and support a relation between nonequilibrium crystal structures observable in nanostructures and corresponding metastable isostructural bulk phases.

Continuous transformations of C60 crystals: polymorphs, polymers, and the ideal strength of fullerites

Application of pressure is a versatile method to tailor the properties of organic semiconductors. For example, it is known that high pressure can transform C60 face-centred-cubic (FCC) crystals to polymer structures with inter-molecular bonds. Here we use first-principles calculations to describe continuous crystalline transformation paths that include the FCC and polymer structures as distinct local energy minima. In addition to analysing the atomic-scale details of polymerization, we obtain the ideal strength of FCC-C60, identify metastable C60 crystalline polymorphs, and characterize their electronic properties-all key features for the performance of C60 crystals in organic electronic devices.

Catalyst-free direct vapor-phase growth of Zn1-xCuxO micro-cross structures and their optical properties

We report a simple catalyst-free vapor-phase method to fabricate Zn1-xCuxO micro-cross structures. Through a series of controlled experiments by changing the location of the substrate and reaction time, we have realized the continuous evolution of product morphology from nanorods into brush-like structures and micro-cross structures at different positions, together with the epitaxial growth of branched nanorods from the central stem with the time extended. The growth mechanism of the Zn1-xCuxO micro-cross structures has been proposed to involve the synthesis of Cu/Zn square-like core, surface oxidation, and the secondary growth of nanorod arrays. By the detailed structural analysis of the yielded Zn1-xCuxO samples at different locations, we have shown that the CuO phases were gradually formed in Zn1-xCuxO, which is significant to induce the usual ZnO hexagonal structures changing into four-folded symmetrical hierarchical micro-cross structures. Furthermore, the visible luminescence can be greatly enhanced by the introduction of Cu, and the observed inhomogeneous cathode luminescence in an individual micro-cross structure is caused by the different distributions of Cu.

Strain relief in Cu-Pd heteroepitaxy

We present experimental and theoretical studies of Pd/Cu(100) and Cu/Pd(100) heterostructures in order to explore their structure and misfit strain relaxation. Ultrathin Pd and Cu films are grown by pulsed laser deposition at room temperature. For Pd/Cu, compressive strain is released by networks of misfit dislocations running in the [100] and [010] directions, which appear after a few monolayers (ML) already. In striking contrast, for Cu/Pd the tensile overlayer remains coherent up to about 9 ML, after which multilayer growth occurs. The strong asymmetry between tensile and compressive cases is in contradiction with continuum elasticity theory and is also evident in the structural parameters of the strained films. Molecular dynamics calculations based on classical many-body potentials confirm the pronounced tensile-compressive asymmetry and are in good agreement with the experimental data.

Bcc and Fcc transition metals and alloys: a central role for the Jahn-Teller effect in explaining their ideal and distorted structures

Transition metal elements, alloys, and intermetallic compounds often adopt the body centered cubic (bcc) and face centered cubic (fcc) structures. By comparing quantitative density functional with qualitative tight-binding calculations, we analyze the electronic factors which make the bcc and fcc structures energetically favorable. To do so, we develop a tight-binding function, DeltaE(star), a function that measures the energetic effects of transferring electrons within wave vector stars. This function allows one to connect distortions in solids to the Jahn-Teller effect in molecules and to provide an orbital perspective on structure determining deformations in alloys. We illustrate its use by considering first a two-dimensional square net. We then turn to three-dimensional fcc and bcc structures, and distortions of these. Using DeltaE(star), we rationalize the differences in energy of these structures. We are able to deduce which orbitals are responsible for instabilities in seven to nine valence electron per atom (e(-)/a) bcc systems and five and six e(-)/a fcc structures. Finally we demonstrate that these results account for the bcc and fcc type structures found in both the elements and binary intermetallic compounds of group 4 through 9 transition metal atoms. The outline of a theory of metal structure deformations based on loss of point group operation rather than translational symmetry is presented.