Materials Theory

We propose a systematic method of analyzing pseudopotential transferability based on linear-response properties of the free atom, including self-consistent chemical hardness and polarizability. Our calculation of hardness extends the approach of Teter\cite{teter} not only by including self-consistency, but also by generalizing to non-diagonal hardness matrices, thereby allowing us to test for transferability to non-spherically symmetric environments. We apply the method to study the transferability of norm-conserving pseudopotentials for a variety of elements in the Periodic Table. We find that the self-consistent corrections are frequently significant, and should not be neglected. We prove that the partial-core correction improves the pseudopotential hardness of alkali metals considerably. We propose a quantity to represent the average hardness error and calculate this quantity for many representative elements as a function of pseudopotential cutoff radii. We find that the atomic polarizabilities are usually well reproduced by the norm-conserving pseudopotentials. Our results provide useful guidelines for making optimal choices in the pseudopotential generation procedure.

Competing magnetically ordered structures in polymerized orthorhombic A1C60 are studied. A mean-field theory for the equilibrium phases is developed using an Ising model and a classical Heisenberg model to describe the competition between inter- and intra-chain magnetic order in the solid. In the Ising model, the limiting commensurate one-dimensional and three-dimensional phases are separated by a commensurate three-sublattice state and by two sectors containing higher-order commensurate phases. For the Heisenberg model the quasi-1D phase is never the equilibrium state; instead the 3D commensurate phases exhibits a transition to a continuum of coplanar spiral magnetic phases.

This paper explains the origin of the ferroelectric instability in LiNbO3 and LiTaO3 and compares the electronic structures and energetics of the two materials.

We study the antiferrodistortive instability and its interaction with ferroelectricity in cubic perovskite compounds. Our first-principles calculations show that coexistence of both instabilities is very common. We develop a first-principles scheme to study the thermodynamics of these compounds when both instabilities are present, and apply it to SrTiO 3 . We find that increased pressure enhances the antiferrodistortive instability while suppressing the ferroelectric one. Moreover, the presence of one instability tends to suppress the other. A very rich P -- T phase diagram results.

Ab initio molecular dynamics simulations are used to investigate the structure and electronic properties of the liquid Ag-Se system at three compositions. The realism of the simulations is demonstrated by comparison with diffraction data for the stoichiometric case Ag 2 Se. As the Se content is increased beyond the stoichiometric value, short-lived Se n complexes are formed. The concentration of complexes and the associated changes of electronic structure can be explained using a simple ionic model.

Ab initio molecular dynamics simulation is used to study the structure and electronic properties of the liquid Ga-Se system at the three compositions Ga 2 Se, GaSe and Ga 2 Se 3 , and of the GaSe and Ga 2 Se 3 crystals. The calculated equilibrium structure of GaSe crystal agrees well with available experimental data. The neutron-weighted liquid structure factors calculated from the simulations are in reasonable agreement with recent neutron diffraction measurements. Simulation results for the partial radial distribution functions show that the liquid structure is closely related to that of the crystals. A close similarity between solid and liquid is also found for the electronic density of states and charge density. The calculated electronic conductivity decreases strongly with increasing Se content, in accord with experimental measurements.

A recently proposed computational scheme based on local increments has been applied to the calculation of correlation contributions to the cohesive energy of the CaO crystal. Using ab-initio quantum chemical methods for evaluating individual increments, we obtain 80% of the difference between the experimental and Hartree-Fock cohesive energies. Lattice constants corrected for correlation effects deviate by less than 1% from experimental values, in the case of MgO and CaO.

Using first-principles calculations, the phonon frequencies at the Γ point and the dielectric tensor are determined and analysed for the cubic and rhombohedral phases of BaTiO 3 . The dipole-dipole interaction is then separated à la Cochran from the remaining short-range forces, in order to investigate their respective influence on lattice dynamics. This analysis highlights the delicate balance of forces leading to an unstable phonon in the cubic phase and demonstrates the extreme sensitivity of this close compensation to minute effective charge changes. Within our decomposition, the stabilization of the unstable mode in the rhombohedral phase or under isotropic pressure has a different origin.

We investigate the elastic properties of small droplets under compression. The compression of a bubble by two parallel plates is solved exactly and it is shown that a lowest-order expansion of the solution reduces to a form similar to that obtained by Morse and Witten. Other systems are studied numerically and results for configurations involving between 2 and 20 compressing planes are presented. It is found that the response to compression depends on the number of planes. The shear modulus is also calculated for common lattices and the stability crossover between f.c.c.\ and b.c.c.\ is discussed.

The success of density functional theory for the description of the adsorption of atoms on surfaces is well established, and based on recent calculations using gradient corrections, it has been shown that it also describes well the dissociative adsorption of molecules at surfaces - admittedly however, the data base for reactions at surfaces is still somewhat small. In the present paper the power of density functional theory calculations is demonstrated by investigations for two different adsorption systems, namely, one with a strongly electropositive adsorbate [Na on Al(111)] and one with a strongly electronegative adsorbate [O on Ru(0001)]. In each case, new hitherto not expected adsorbate phases have been predicted by the theory: For Na on Al(111) the stability of a "four-layer" surface alloy was identified while for O on Ru(0001) it was predicted that the formation of a (1 x 1)-O adlayer should be possible which implies that the apparent saturation coverage of 1/2 is due to kinetic hindering.