V. E. Van Doren

High pressure structural phase transformation in gallium nitride

Abstract Under normal conditions GaN crystallizes in the wurtzite structure. At high pressure (30–50 GPa) GaN undergoes a structural phase transformation to the rocksalt structure. The total energy of both structures as well as of the zincblende structure is calculated, for different unit cell volumes, using first-principles non-local pseudopotentials. For the wurtzite structure we obtain a = 3.126 A , c = 5.119 A and an internal parameter u = 0.3767. In the rocksalt structure we get a = 4.098 A and the zinchlende lattice constant is found to be a = 4.419 A . At low pressure the wurtzite structure has the lowest energy. At 55.1 GPa there is a phase transformation to the rocksalt structure. No transition to the zincblende structure is observed.

Ground state properties and structural phase transformation of berylliumsulphide

Abstract The electronic structure, the charge density and the total energy of BeS in the rocksalt (B1), cesium chloride (B2), zincblende (B3), wurtzite (B4) nickel arsenide (B81) and iron silicide (B28) structures are studied using first-principles self-consistent localdensity calculations in a large plane wave basis employing soft non-local pseudopotentials. The zincblende structure is the calculated ground state with a = 4.773 A B0 = 101.9 GPa and B′0 = 3.70. The experimental value is a = 4.85 A . The wurtzite structure is energetically very close to the zincblende structure. The difference at the minimal energy in these two structures is only 6.2 meV. At a pressure of 58 GPa we observe a transition to the six-fold coordinated nickelarsenide structure. From that structure on no other transition is found to any of the calculated structures.

Ground-state properties and structural phase transformation of beryllium oxide

The electronic structure, the charge density and the total energy of BeO in the rocksalt (B1), caesium chloride (B2), zincblende (B3), wurtzite (B4), nickel arsenide (f) and iron silicide (B28) structures are studied using first-principles self-consistent local-density calculations in a large plane wave basis employing soft non-local pseudopotentials. Experimentally no transition was observed up to a pressure of 55 GPa. The wurtzite structure is the calculated ground state with a = 2.639 A, c = 4.299 A, c/a = 1.629 and an internal parameter u = 0.377. The experimental values are respectively 2.699 A, 4.373 A, 1.62 and 0.378. The zincblende structure is energetically very close to the wurtzite structure. The difference at the minimal energy between these two structures is only 5.6 meV. At a pressure of 137 GPa we observe a transition to the sixfold-coordinated rocksalt structure. From that structure on, no other transition is found to any of the calculated structures.

Density functional calculations on the structure of crystalline polyethylene under high pressures

The geometrical structures of the crystalline polyethylene under several different external pressures up to 10 GPa are optimized by a pseudopotential plane wave density functional method. Both local density (LDA) and generalized gradient (GGA) approximations for exchange-correlation energy and potential are used. It is found that LDA heavily underestimate the geometry parameters under ambient pressure but GGA successfully correct them and get results in good agreements with the experimental geometry. The calculated GGA volume is about 94 A3 in comparison with the x-ray scattering value of about 92 A3 and the neutron scattering value of 88 A3. The bulk and Young’s modulus are calculated by means of several different methods. The Young’s modulus along the chain ranges from about 350 to about 400 GPa which is in good agreement with the experimental results. But the bulk modulus is several times larger than those of experiments, indicating a different description of the interchain interactions by both LDA and...

First-principles calculation of the conformation and electronic structure of polyparaphenylene

In this article, an all-electron first-principles total energy calculation with Gaussian-type functions for the wave functions, for the exchange correlation potential, and for the charge density has been applied for single chains of polyparaphenylene (PPP). A local-density approximation within a helical band structure approach has been used. The calculated torsional potential shows a minimum at the torsion angle of 34.8°. The internal coordinates were optimized in the equilibrium conformation and are in good agreement with experimental and other theoretical results. The calculated direct band gap is 2.54 eV compared with the experimental result from UPS spectra of 3.4 eV for the gas phase. The band structure strongly depends on the conformation which suggests that the electronic properties can be modified in a wide range through doping or addition of side groups.

Density functional calculations of the structure of crystalline urea under high pressure

Abstract The geometry of the urea crystal is optimized for several different pressures using a pseudopotential density functional (DFT) approach with a plane-wave basis set and the Ceperley–Alder local density correlation potential. All the optimized intra- and intermolecular geometrical parameters, as well as the lattice vectors, are found to be in good agreement with experiment. Under high pressures, the crystal changes anisotropically; the lattice constants a and b reduce by almost 10 times more with the pressure than the constant c. Changes of the geometrical parameters within the molecule as a function of the external pressure are found to be in good agreement with the donor–acceptor theory.

Pressure dependent properties of cubic boron nitride

Abstract Results are presented of ground state and electronic properties of BN made with large numbers of plane waves. Several equations of state are compared with each other and with recent experiments. An estimate, based on LDA calculations, is given of the difference of the energy shift between the Γ 15 c and X 1 c points in the lowest conduction band due to the self-energy corrections. It is found that the shift at these points is not the same. The first and second order pressure coefficients have been calculated and are used together with the experimental Γ 15 v to Γ 15 c and to X 1 c band gaps in order to predict a cross-over pressure from indirect to direct band gap of 11.60 Mbar.

The effect of pressure and temperature on the vibrational spectra of different hydrogen bonded systems

The effect of pressure and temperature on the vibrational spectra of hydrogen bonded systems has been studied on amides, thioamides, carboxylic acids and urea. The compounds under investigation are indicative for the kind of hydrogen bonding changing from pure intermolecular to intramolecular and dimeric forms. The discussion of the temperature dependence on the fundamentals involved in the hydrogen bonding is straightforward but the pressure data are much more complicated and only if the changes in the crystalline state at different pressures are known, we will have a better understanding of the dependence of some fundamentals in the hydrogen bonded systems. A clear example of this approach is given for urea.

Monte Carlo simulations of dielectric relaxation in Na-mordenites

Abstract The chemical composition of zeolites is defined by the average ratio between the number of 4-coordinated Si and Al atoms of the lattice (ratio Si/Al). Each Al atom adds a net negative charge to the otherwise SiO 2 lattice which is counterbalanced by an extra-framework mobile cation. These counter-ions are responsible for the major polarization of the material at low frequencies (10 6 Hz). The nature, localization and diffusion of these cations depend on the Si/Al ratio and influence the catalytic properties of the material. Here, we present a joint theory-experiment study of these properties at the atomic level. The thermally stimulated depolarization current (TSDC) of Na-mordenites is measured for increasing Si/Al ratios. The analysis of these dielectric relaxation data leads to activation energy barriers for the Na + “jumps” responsible for the polarization change. Using semi-empirical inter-atomic potentials and Monte Carlo algorithms we propose a possible mechanism for the cation motions occurring in TSDC experiments.

Non-local kinetic energy functional for an arbitrary number of Fermions moving independently in one-dimensional harmonic oscillator potential

Abstract For one-dimensional Fermions bound by a general one-body potential V(x), the Pauli potential is first related to the kinetic energy and the particle density ρ(x). For the model of the harmonic oscillator, V(x)= 1 2 x 2 , this equation leads to a non-local kinetic energy functional in which only first-order derivatives of ρ(x) enter. This example shows the usefulness of a new concept, the Pauli function, which encompasses the Pauli principle in terms of the electronic density. For the harmonic oscillator model, the kinetic energy can then be expressed exactly in terms of the Thomas–Fermi kinetic energy functional, together with the von Weizsacker inhomogeneity term, but now in a fully non-local way.