B. Johansson

Density functional theory for calculation of elastic properties of orthorhombic crystals: Application to TiSi2

A theoretical formalism to calculate the single crystal elastic constants for orthorhombic crystals from first principle calculations is described. This is applied for TiSi2 and we calculate the elastic constants using a full potential linear muffin-tin orbital method using the local density approximation (LDA) and generalized gradient approximation (GGA). The calculated values compare favorably with recent experimental results. An expression to calculate the bulk modulus along crystallographic axes of single crystals, using elastic constants, has been derived. From this the calculated linear bulk moduli are found to be in good agreement with the experiments. The shear modulus, Young’s modulus, and Poisson’s ratio for ideal polycrystalline TiSi2 are also calculated and compared with corresponding experimental values. The directional bulk modulus and the Young’s modulus for single crystal TiSi2 are estimated from the elastic constants obtained from LDA as well as GGA calculations and are compared with the ...

Experimental and theoretical identification of a new high-pressure phase of silica

Following the discovery of stishovite (the highest-pressure polymorph of silica known from natural samples), many attempts have been made to investigate the possible existence of denser phases of silica at higher pressures. Based on the crystal structures observed in chemical analogues of silica, high-pressure experiments on silica and theoretical studies, several possible post-stishovite phases have been suggested. But the likely stable phase of silica at pressures and temperatures representative of Earths lower mantle remains uncertain. Here we report the results of an X-ray diffraction study of silica that has been heated to temperatures above ∼2,000 K and maintained at pressures between 68 and 85 GPa. We observe the occurrence of a new high-pressure phase which we identify with the aid of first-principles total-energy calculations. The structure of this phase (space group Pnc2) is intermediate between the α-PbO2 and ZrO2 structures, and is denser than other known silica phases.

ELASTIC AND HIGH PRESSURE PROPERTIES OF ZNO

We have studied the elastic and structural properties of ZnO by means of accurate first-principles total energy calculations using the full potential linear muffin tin orbital method. The calculations are based on the density functional theory and we have used the local density Hedin–Lundqvist parametrization and the generalized gradient approximation of Perdew and Wang for the exchange and correlation potential. The calculated values for the equilibrium volume, bulk modulus, and elastic constants are generally in very good agreement with experiments. At elevated pressures ZnO undergoes a structural phase transition from the relatively open wurtzite structure into the more dense NaCl atomic arrangement. The calculated transition pressure is in good agreement with experiment.

Meta-magnetism in UCoAl

The authors have undertaken a detailed theoretical investigation of the meta-magnetic transition in UCoAl. Using the fixed spin moment method in their calculations they can account for the observed meta-magnetic transition and find that the magnetic properties of UCoAl are dominated by a moment located on the uranium atom. This uranium moment is found to consist of an orbital moment that is larger and anti-parallel to the spin moment. The spin and orbital moments were calculated using the local spin-density approximation and the recently reported formalism for orbital polarisation together with the spin-orbit interaction.

Surface energy and magnetism of the 3d metals

Abstract We present an ab initio study of surface energies, surface magnetism and work functions of the 3d transition metals. The calculations are performed by means of a spin-polarized Greens function technique based on the tight-binding linear muffin-tin orbitals method within the atomic sphere approximation. In addition to the conventional paramagnetic and spin-polarized calculations we use the fixed spin-moment method to clarify the effect of magnetism on the surface energies. The results are shown to be consistent with a Friedel model of d-electron bonding combined with spin-split state densities as well as with a Stoner-type description. It is established that the anomaly in the surface energy of the 3d metals deduced from surface tension measurements is purely a magnetic solid state effect. In addition, it is found that magnetism reduces the surface energy of open surfaces, e.g. the (001) crystal faces, to the extent that the usual anisotropy of the surface energy is reversed. Thus a complete realization of the surface energy anomaly only takes place in the less close-packed surface facets.

Rare-earth transition-metal intermetallics

Abstract The combination of itinerant transition metal (M = Fe) and localized rare-earth (R = Gd-Y) magnetism in RFe 2 compounds has been investigated in self-consistent energy band calculations. The computed and measured total moments are in good agreement for all cases where single crystal data are available. We find, however, that there is a significant contribution to the moment from the R-5d partial moments coupled antiparallel to the Fe-3d moment which results from 3d–5d hybridization. The R-4f moments interact with the conduction band system solely by local exchange interactions, which are calculated ab initio from density functional theory. A sum rule for the total 3d + 5d moment is shown to be obeyed and the effective ferrimagnetic exchange interaction between rare-earth and transition-metal moments is discussed. Finally, the spin wave spectra of these materials are evaluated in terms of a model arising from these calculations.

Theory of the ternary layered system Ti-Al-N

We present calculations on the physical properties of Ti4AlN3 obtained from first principles, as well as some general observations concerning this group of materials. We further report mechanical data obtained through numerical simulations for some representative phases, and elaborate on the issue of high pressure stability. By examining the density of states of Ti4AlN3, we conclude that it is a semimetal, and we predict an opening up of the band gap around the Fermi level with increasing pressure.

Electronic and optical properties of lead iodide

The electronic properties and the optical absorption of lead iodide (PbI2) have been investigated experimentally by means of optical absorption and spectroscopic ellipsometry, and theoretically by a full-potential linear muffin-tin-orbital method. PbI2 has been recognized as a very promising detector material with a large technological applicability. Its band-gap energy as a function of temperature has also been measured by optical absorption. The temperature dependence has been fitted by two different relations, and a discussion of these fittings is given.

First-principle calculations of optical properties of wurtzite AlN and GaN

The imaginary part of the dielectric function of wurtzite AIN and GaN has been calculated in the long wavelength limit, using two different first-principle electronic structure methods. The first m ...

The structure of the metallic high-pressure Fe3O4 polymorph: experimental and theoretical study

The structure of the metallic high-pressure Fe3O4 polymorph : experimental and theoretical study