Search

Molecular dynamics simulations were carried out for different structural models of the Si/3C-SiC interface using the Tersoff SiC potential that can model both Si and SiC. We find that the bonding at the Si/3C-SiC interface has a strong effect on the crystallization of the Si phase and that a degree of intermixing is present between the two materials with some C atoms migrating from the 3C-SiC (hereinafter referred to as SiC) into the Si region. The degree of intermixing is likely to exhibit a strong dependence on the temperature and most likely also increases with time, which would lead to changes in the Si/SiC interface during the life of the Si/SiC composite. The inter-mixing also creates disorder and defects of threefold and fivefold bonded atoms in the vicinity of the interfaces. In particular, {111} 1 2 ⟨ 110 ⟩ misfit dislocations were formed at all three types of interfaces [(100), (110), and (111)] in order to relieve the local stress due to lattice mismatch. Additionally, the Si(110)/SiC(110) and Si(111)/SiC(111) interfaces prepared at higher temperatures show the formation of the {111} 1 6 ⟨ 112 ⟩ partial dislocation which arises due to intrinsic stacking faults. We find that the bonding at the crystalline(c) c-Si/SiC interface is weaker than that in bulk crystalline Si, whereas bonding at the amorphous(a)-Si/SiC interface is stronger than that in amorphous Si. Therefore, the rupture in the yield stress occurs at the vicinity of the Si/SiC interface and in the Si region for the a-Si/SiC systems, respectively. Finally, for both bulk and Si/SiC interface systems, a strong variation of the yield strength with temperature was observed. [ABSTRACT FROM AUTHOR]

CO adsorption on the Pt(111) surface is studied using first-principles methods. As found in a recent study [Feibelman et al., J. Phys. Chem. B 105, 4018 (2001)], we find the preferred adsorption site within density functional theory to be the hollow site, whereas experimentally it is found that the top site is preferred. The influence of pseudopotential and exchange-correlation functional error on the CO binding energy and site preference is carefully investigated. We also compare the site preference energy of CO on Pt(111) with the reaction energy of formaldehyde formation from H[sub 2] and CO. We show that the discrepancies between the experimental and theoretical results are due to the generalized gradient approximation (GGA) treating different bond orders with varying accuracy. We can therefore expect that GGA results will contain significant error whenever bonds of different bond order are broken and formed. [ABSTRACT FROM AUTHOR]

Relaxor ferroelectrics have been a focus of intense attention due to their fascinating physical properties. Their diffuse phase transitions have been explained by the polar nanoregion model. Nevertheless, fundamental characterization of structure and dynamics in relaxors is still a long-standing challenge. Better scientific understanding of the microscopic origins of relaxor behavior is also required to improve efficiencies of relaxor based devices. Our molecular dynamics studies in 0.75PbMg1/3Nb2/3O3-0.25PbTiO3showed good agreement with experimental data and revealed conflicts with the current polar nanoregion model. Here, we review our work and propose an alternate model for structure and dynamics in the relaxor phase. [ABSTRACT FROM PUBLISHER]

The motion of ferroelectric domain walls (DWs) is critical for various applications of ferroelectric materials. One important question that is of interest both scientifically and technologically, is whether the ferroelectric DW has significant inertial response. To address this problem, we performed canonical ensemble molecular dynamics simulations of 180° and 90° DWs under applied electric fields. Examination of the evolution of the polarization and local structure of DWs reveals that they stop moving immediately after the removal of electric field. Thus, our computational study shows that ferroelectric DWs do not have significant intrinsic inertial response. [ABSTRACT FROM AUTHOR]

Relaxor ferroelectrics have been a focus of intense attention due to their anomalous properties, and understanding the structure and dynamics of relaxors has been one of the long-standing challenges in solid-state physics. We investigate the local structure and dynamics in 75%PbMg1/3Nb2/3O3-25%PbTiO3 using molecular dynamics simulations and the dynamic pair distribution function technique. We show that relaxor transitions can be described by local order parameters. The relaxor phase is characterized by the presence of highly anisotropic correlations between the local cation displacements that resemble the hydrogen bond network in water. This contradicts the current model of polar nanoregion inside a nonpolar matrix. We therefore suggest a new model of a homogeneous random network of anisotropically coupled dipoles. [ABSTRACT FROM AUTHOR]