Showing papers by "David Vanderbilt published in 1995"

First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3

Abstract: We carry out a completely first-principles study of the ferroelectric phase transitions in ${\mathrm{BaTiO}}_{3}$. Our approach takes advantage of two features of these transitions: the structural changes are small, and only low-energy distortions are important. Based on these observations, we make systematically improvable approximations which enable the parametrization of the complicated energy surface. The parameters are determined from first-principles total-energy calculations using ultrasoft pseudopotentials and a preconditioned conjugate-gradient scheme. The resulting effective Hamiltonian is then solved by Monte Carlo simulation. The calculated phase sequence, transition temperatures, latent heats, and spontaneous polarizations are all in good agreement with experiment. We find the transitions to be intermediate between order-disorder and displacive character. We find all three phase transitions to be of first order. The roles of different interactions are discussed.

Competing structural instabilities in cubic perovskites.

Abstract: 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 SrTi${\mathrm{O}}_{3}$ 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

Stability of periodic domain structures in a two-dimensional dipolar model

Abstract: We investigate the energetic ground states of a model two-phase system with 1/r^3 dipolar interactions in two dimensions. The model exhibits spontaneous formation of two kinds of periodic domain structure. A striped domain structure is stable near half filling, but as the area fraction is changed, a transition to a hexagonal lattice of almost-circular droplets occurs. The stability of the equilibrium striped domain structure against distortions of the boundary is demonstrated, and the importance of hexagonal distortions of the droplets is quantified. The relevance of the theory for physical surface systems with elastic, electrostatic, or magnetostatic 1/r^3 interactions is discussed.

Chemical hardness, linear response, and pseudopotential transferability

Abstract: 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 not only by including self-consistency, but also by generalizing to nondiagonal hardness matrices, thereby allowing us to test for transferability to nonspherically 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 alkaline 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.

Hydrogen, acceptors, and H-acceptor complexes in GaN

Abstract: We present ab-initio calculations on energetics and geometries of atomic hydrogen, of several candidate acceptors, and of H-acceptor complexes in wurtzite GaN For the H-Mg complex in Mg-doped GaN, we calculate the vibrational frequencies of H. Hydrogen is found to be a negative-U center. H-acceptor complex formation is always exothermic. Substitutional Be has a low formation energy and a shallow impurity level, which makes it a good candidate for p-doping in MBE growth. CN appears not to be shallow. Atomic hydrogen incorporation in undoped GaN is disfavored in an H2 atmosphere; it becomes favorable in p and n-type conditions in atomic H environments.

Coulomb interaction and ferroelectric phase transitions in perovskite compounds

Abstract: We approach the problem of ferroelectricity in cubic perovskite compounds by a three-stage procedure. First, a systematic set of first-principles calculations of structural energies and Coulomb interactions is carried out. We review this work, with an emphasis on the calculation of the Born effective charges, which we find to have anomalously large values. Second, these results are used to construct a model Hamiltonian for BaTiO3 with parameters fixed from the ab initio calculations. Third, we employ this Hamiltonian in a Monte-Carlo simulation of the ferroelectric phase transitions of BaTiO3. We obtain the correct experimental transition sequence cubic → tetragonal → orthorhombic → rhombohedral with decreasing temperature, but we underestimate the transition temperatures. This discrepancy is attributed to a small error in the ab initio lattice constant.

Structural and Electronic Properties of AlN, GaN And InN, and Band Offsets at AlN/GaN (1010) and (0001) Interfaces

Abstract: Ab initio local-density-functional calculations are presented for bulk A1N, GaN, and InN in the wurtzite, zincblende, and rocksalt structures. Structural transition pressures and deformation potentials of electronic gaps are investigated. In addition, we study the band offset at the polar (0001) and non-polar (1010) AIN/GaN interfaces. Within AIN-on-GaN epitaxial conditions, we obtain valence-band offset values close to 0.7 eV for both interfaces. From the macroscopic field appearing along the growth direction of the polar interface (tentatively attributed to AIN macroscopic polarization), an estimate of the macroscopic dielectric constant of GaN is extracted. All calculations employed conjugate-gradient total-energy minimizations, ultrasoft pseudopotentials, and plane waves at 25 Ryd cutoff.