G.A. de Wijs

Spin-polarization in half-metals (invited)

Half-metals are defined by an electronic structure, which shows conduction by charge carriers of one spin direction exclusively. Consequently, the spin polarization of the conduction electrons should be 100%. In reality this complete spin polarization is not always observed. Since the experimental search for half-metals is tedious and the verification of the expected spin polarization is involved, electronic structure calculations have played an important role in this area. So, an important question is, how the approximations in such calculations influence the resulting spin polarization of the conduction. Another aspect is the well-known fact that bulk properties can be very different from surface and interface properties. Indeed, measurements of the spin polarization in the bulk for, e.g., NiMnSb, show results different from surface sensitive measurements. In this respect it is important to realize that the origin of half-metallic behavior is not unique. Consequently, the deviations from the bulk behavior at the surface/interface can be important. Three different categories of half-metals can be distinguished and their expected surface properties will be discussed. Finally, ways will be described to control the properties at interfaces.

Electronic structure and optical properties of lightweight metal hydrides

We study the dielectric functions of the series of simple hydrides LiH, NaH, MgH2, and AlH3, and of the complex hydrides Li3AlH6, Na3AlH6, LiAlH4, NaAlH4, and Mg(AlH4)2, using first-principles density-functional theory and GW calculations. All compounds are large gap insulators with GW single-particle band gaps varying from 3.5 eV in AlH3 to 6.6 eV in LiAlH4. Despite considerable differences between the band structures and the band gaps of the various compounds, their optical responses are qualitatively similar. In most of the spectra the optical absorption rises sharply above 6 eV and has a strong peak around 8 eV. The quantitative differences in the optical spectra are interpreted in terms of the structure and the electronic structure of the compounds. In the simple hydrides the valence bands are dominated by the hydrogen atoms, whereas the conduction bands have mixed contributions from the hydrogens and the metal cations. The electronic structure of the aluminium compounds is determined mainly by aluminium hydride complexes and their mutual interactions.

Phonon Spectrum of ZnAl{sub 2}O{sub 4} spinel from inelastic neutron scattering and first-principles calculations.

The phonon spectrum of ZnAl 2 O 4 spinel was investigated jointly by inelastic neutron-scattering and first-principles calculations. The results permit an assessment of important mechanical and thermodynamical properties such as the bulk modulus, elastic constants, lattice specific heat, vibration energy, and Debye temperature. The observed generalized phonon density of states shows a gapless spectrum extending to a cutoff energy of ∼840 cm - 1 . The theoretical results reproduce all of the features of the phonon density of states. The calculated Raman-and infrared-active phonon frequencies agree well with the data in the literature. A comparison of the lattice dynamics of ZnAl 2 O 4 and MgAl 2 O 4 spinels was carried out using a simple rigid-ion model, which shows that the major difference in the phonon frequencies of the two materials can be accounted for by the mass effects between the Zn and Mg ions.

Amorphous WO3: a first-principles approach

Results of first-principles calculations on the structure and electronic structure of amorphous WO3 are presented. The effect of non-stoichiometry is investigated. In particular, we discuss the pairing of W5+ species in oxygen-deficient films resulting in deep in-gap states and its possible consequences for the electrochromic coloration efficiency. To this end, we make a connection with Raman experiments by calculating the vibrational density from molecular-dynamics simulation. For the W5+-W5+ stretch we find a low frequency mode at ~200 cm-1 which agrees well with the Raman data. We also estimate the stability of isolated W5+ species in stoichiometric and oxygen-deficient tungstentrioxide.

The continuing drama of the half-metal/semiconductor interface

In this article, based on electronic structure calculations, the conditions are discussed under which a genuine half-metallic interface between a heusler C1b half-metal and a semiconductor can exist. An explanation is given why for the III–V semiconductors the double anion terminated (111) interface is the only possible interface. For semiconductors, based on transition metals, a much wider variety of interfaces are found to be possible.

Electronic band structure of tetracene-TCNQ and perylene-TCNQ compounds

The relationship between the crystal structures, band structures, and electronic properties of acene-TCNQ complexes has been investigated. We focus on the newly synthesized crystals of the charge-transfer salt tetracene-TCNQ and similar to it perylene-TCNQ, potentially interesting for realization of ambipolar transport. The band structures were calculated from first principles using density-functional theory (DFT). Despite the similarity in the crystal structures of the acene-TCNQ complexes studied here, the band structures are very different. Hole and electron transport properties are predicted to be equally good in perylene-TCNQ, in contrast to the tetracene-TCNQ, which has good transport properties for electrons only. The estimated degree of charge transfer for tetracene-TCNQ is 0.13e and for perylene-TCNQ 0.46e.

First‐principles molecular‐dynamics simulation of liquid CsPb

Many alkali–post‐transition group IV alloy systems exhibit clearly defined equiatomic compounds together with a pronounced intermediate range ordering, indicated by a first sharp diffraction peak at ≊0.9 A−1. These phenomena have been explained assuming that tetrahedral group IV anions, ‘‘Zintl’’ ions, survive in the liquid state. As a prototype system we considered liquid CsPb, for which several experimental results are available, and studied it by means of first‐principles molecular‐dynamics. Agreement with experiment is satisfactory, provided the 5s and 5p electrons of cesium are explicitly taken into account in the computation of the electronic valence charge density. In particular, our calculations reproduce the structure factor prepeak reasonably well. The local liquid structure however is quite complex. This can be described as a disordered network, which still has many features in common with the ‘‘Zintl’’ ion model. For instance, the average Pb‐Pb coordination is close to 3, the value for perfect...

Recent developments in ab initio thermodynamics

It has recently become possible to calculate the free energy and other thermodynamic functions of solids and liquids using density functional theory to treat the quantum mechanics of the electrons. We present the main ideas that have made this possible, emphasizing the key role of thermodynamic integration and the importance of well-adapted reference systems in the computation of the free energy. We show how ab initio methods have been used to calculate the melting and other phase-equilibrium properties of simple materials, and the thermal-equilibrium concentrations of point defects in crystals. We point out the possibility of adapting techniques that are already available in order to calculate chemical potentials, solubilities, equilibrium constants, and other thermodynamic functions that are important in physical chemistry. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 871-879, 2000

Ab initiostudy ofMg(AlH4)2

Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential material for hydrogen storage. We apply ab initio calculations to characterize structural, electronic and energetic properties of Mg(AlH4)2. Density functional theory calculations within the generalized gradient approximation (GGA) are used to optimize the geometry and obtain the electronic structure. The latter is also studied by quasi-particle calculations at the GW level. Mg(AlH4)2 is a large band gap insulator with a fundamental band gap of 6.5 eV. The hydrogen atoms are bonded in AlH4 complexes, whose states dominate both the valence and the conduction bands. On the basis of total energies, the formation enthalpy of Mg(AlH4)2 with respect to bulk magnesium, bulk aluminum and hydrogen gas is 0.17 eV/H2 (at T = 0). Including corrections due to the zero point vibrations of the hydrogen atoms this number decreases to 0.10 eV/H2. The enthalpy of the dehydrogenation reaction Mg(AlH4)2 -> MgH2 +2Al+3H2(g) is close to zero, which impairs the potential usefulness of magnesium alanate as a hydrogen storage material.

Li intercalation in graphite: A van der Waals density-functional study

Modeling layered intercalation compounds from first principles poses a problem, as many of their properties are determined by a subtle balance between van der Waals interactions and chemical or Madelung terms, and a good description of van der Waals interactions is often lacking. Using van der Waals density functionals we study the structures, phonons and energetics of the archetype layered intercalation compound Li-graphite. Intercalation of Li in graphite leads to stable systems with calculated intercalation energies of −0.2 to −0.3 eV/Li atom, (referred to bulk graphite and Li metal). The fully loaded stage 1 and stage 2 compounds LiC 6 and Li 1/2 C 6 are stable, corresponding to two-dimensional 3 √ ×3 √ lattices of Li atoms intercalated between two graphene planes. Stage N>2 structures are unstable compared to dilute stage 2 compounds with the same concentration. At elevated temperatures dilute stage 2 compounds easily become disordered, but the structure of Li 3/16 C 6 is relatively stable, corresponding to a 7 √ ×7 √ in-plane packing of Li atoms. First-principles calculations, along with a Bethe-Peierls model of finite temperature effects, allow for a microscopic description of the observed voltage profiles