arXiv subjects

David Vanderbilt

Publications and source records attributed to David Vanderbilt.

At least 217 records · Page 12Linked to original sources

arXiv2000

Finite-temperature properties of PZT alloys from first principles

A first-principles-derived approach is developed to study finite-temperature properties of PbZr{1-x}Ti{x}O3 (PZT) solid solutions near the morphotropic phase boundary (MPB). Structural and piezoelectric predictions are in excellent agreement with experimental data and direct first-principles results. A low-temperature monoclinic phase is confirmed to exist, and is demonstrated to act as a bridge between the well-known tetragonal and rhombohedral phases delimiting the MPB. A successful explanation for the large piezoelectricity found in PZT ceramics is also provided.

cond-mat.mtrl-sci↗

arXiv2000

Compositional Inversion Symmetry Breaking in Ferroelectric Perovskites

Ternary cubic perovskite compounds of the form A_(1/3)A'_(1/3)A''_(1/3)BO_3 and AB_(1/3)B'_(1/3)B''_(1/3)O_3, in which the differentiated cations form an alternating series of monolayers, are studied using first-principles methods. Such compounds are representative of a possible new class of materials in which ferroelectricity is perturbed by compositional breaking of inversion symmetry. For isovalent substitution on either sublattice, the ferroelectric double-well potential is found to persist, but becomes sufficiently asymmetric that minority domains may no longer survive. The strength of the symmetry breaking is enormously stronger for heterovalent substitution, so that the double-well behavior is completely destroyed. Possible means of tuning between these behaviors may allow for the optimization of resulting materials properties.

cond-mat.mtrl-sci↗

arXiv1999

Theory of PbTiO3, BaTiO3, and SrTiO3 Surfaces

First-principles total-energy calculations are carried out for (001) surfaces of the cubic perovskite ATiO3 compounds PbTiO3, BaTiO3, and SrTiO3. Both AO-terminated and TiO2-terminated surfaces are considered, and fully-relaxed atomic configurations are determined. In general, BaTiO3 and SrTiO3 are found to have a rather similar behavior, while PbTiO3 is different in many respects because of the partially covalent character of the Pb-O bonds. PbTiO3 and BaTiO3 are ferroelectrics, and the influence of the surface upon the ferroelectric distortions is studied for the case of a tetragonal ferroelectric distortion parallel to the surface. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric order parameter is modest, and is qualitatively as well as quantitatively different for the two materials. Surface energies and electronic properties are also computed. It is found that for BaTiO3 and SrTiO3 surfaces, both AO-terminated and TiO2-terminated surfaces can be thermodynamically stable, whereas for PbTiO3 only the PbO surface termination is stable.

cond-mat.mtrl-sci↗

arXiv1999

The virtual crystal approximation revisited: Application to dielectric and piezoelectric properties of perovskites

We present an approach to the implementation of the virtual crystal approximation (VCA) for the study of properties of solid solutions in the context of density-functional methods. Our approach can easily be applied to any type of pseudopotential, and also has the advantage that it can be used to obtain estimates of the atomic forces that would arise if the real atoms were present, thus giving insight into the expected displacements in the real alloy. We have applied this VCA technique within the Vanderbilt ultrasoft-pseudopotential scheme to predict dielectric and piezoelectric properties of the Pb(Zr[0.5],Ti[0.5])O[3] solid solution in its paraelectric and ferroelectric phases, respectively. Comparison with calculations performed on ordered alloy supercells and with data on parents compounds demonstrates the adequacy of using the VCA for perovskite solid solutions. In particular, the VCA approach reproduces the anomalous Born effective charges and the large value of the piezoelectric coefficients.

cond-mat.mtrl-sci↗

arXiv1999

Thermal Contraction and Disordering of the Al(110) Surface

Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface support this anomalous contraction, and provide a novel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behavior originates in the free-electron-gas bonding at a loosely packed surface.

cond-mat.mtrl-sci↗

arXiv1999

Berry-phase theory of proper piezoelectric response

Recent theoretical advances have established that the electric polarization in an insulating crystal can be viewed as a multivalued quantity that is determined by certain Berry phases associated with the occupied Bloch bands. The application of this approach to the computation of piezoelectric coefficients is not entirely straightforward, since a naive determination of the (``improper'') piezoelectric coefficients from finite differences of the polarization at nearby strain states leads to a dependence upon the choice of ``branch'' of the polarization. The purpose of the present paper is to clarify that if one calculates instead the ``proper'' piezoelectric response, the branch dependence is eliminated. From this analysis, a simplified recipe for the direct finite-difference computation of the proper piezoelectric coefficients emerges naturally.

cond-mat.mtrl-sci↗

arXiv1998

Structure and oxidation kinetics of the Si(100)-SiO2 interface

We present first-principles calculations of the structural and electronic properties of Si(001)-SiO2 interfaces. We first arrive at reasonable structures for the c-Si/a-SiO2 interface via a Monte-Carlo simulated annealing applied to an empirical interatomic potential, and then relax these structures using first-principles calculations within the framework of density-functional theory. We find a transition region at the interface, having a thickness on the order of 20Å, in which there is some oxygen deficiency and a corresponding presence of sub-oxide Si species (mostly Si^+2 and Si^+3). Distributions of bond lengths and bond angles, and the nature of the electronic states at the interface, are investigated and discussed. The behavior of atomic oxygen in a-SiO2 is also investigated. The peroxyl linkage configuration is found to be lower in energy than interstitial or threefold configurations. Based on these results, we suggest a possible mechanism for oxygen diffusion in a-SiO2 that may be relevant to the oxidation process.

cond-mat.mtrl-sci↗

arXiv1998

Heterovalent and A-atom effects in A(B'B'')O3 perovskite alloys

Using first-principles supercell calculations, we have investigated energetic, structural and dielectric properties of three different A(B'B'')O_3 perovskite alloys: Ba(Zn_{1/3}Nb_{2/3})O_3 (BZN), Pb(Zn_{1/3}Nb_{2/3})O_3 (PZN), and Pb(Zr_{1/3}Ti_{2/3})O_3 (PZT). In the homovalent alloy PZT, the energetics are found to be mainly driven by atomic relaxations. In the heterovalent alloys BZN and PZN, however, electrostatic interactions among B' and B'' atoms are found to be very important. These electrostatic interactions are responsible for the stabilization of the observed compositional long-range order in BZN. On the other hand, cell relaxations and the formation of short Pb--O bonds could lead to a destabilization of the same ordered structure in PZN. Finally, comparing the dielectric properties of homovalent and heterovalent alloys, the most dramatic difference arises in connection with the effective charges of the B' atom. We find that the effective charge of Zr in PZT is anomalous, while in BZN and PZN the effective charge of Zn is close to its nominal ionic value.

cond-mat.mtrl-sci↗

arXiv1998

Properties of a continuous-random-network model for amorphous systems

We use a Monte Carlo bond-switching method to study systematically the thermodynamic properties of a "continuous random network" model, the canonical model for such amorphous systems as a-Si and a-SiO$_2$. Simulations show first-order "melting" into an amorphous state, and clear evidence for a glass transition in the supercooled liquid. The random-network model is also extended to study heterogeneous structures, such as the interface between amorphous and crystalline Si.

cond-mat.stat-mech↗

arXiv1998

Core reconstruction of the 90-degree partial dislocation in non-polar semiconductors

We investigate the energetics of the single-period and double-period core reconstructions of the 90-degree partial dislocation in the homopolar semiconductors C, Si, and Ge. The double-period geometry is found to be lower in energy in all three materials, and the energy difference between the two geometries is shown to follow the same trends as the energy gap and the stiffness. Both structures are fully reconstructed, consisting entirely of fourfold coordinated atoms. They differ primarily in the detail of the local strains introduced by the the two reconstructions in the core region. The double-period structure is shown to introduce smaller average bond-length deviations, at the expense of slightly larger average bond-angle bending distortions, with respect to the single-period core. The balance between these two strain components leads to the lower energy of the double-period reconstruction.

cond-mat.mtrl-sci↗

arXiv1998

Electrostatic model of atomic ordering in complex perovskite alloys

We present a simple ionic model which successfully reproduces the various types of compositional long-range order observed in a large class of complex insulating perovskite alloys. The model assumes that the driving mechanism responsible for the ordering is simply the electrostatic interaction between the different ionic species. A possible new explanation for the anomalous long-range order observed in some Pb relaxor alloys, involving the proposed existence of a small amount of Pb^4+ on the B sublattice, is suggested by an analysis of the model.

cond-mat.mtrl-sci↗

arXiv1998

Maximally-localized Wannier functions for disordered systems: application to amorphous silicon

We use the maximally-localized Wannier function method to study bonding properties in amorphous silicon. This study represents, to our knowledge, the first application of the Wannier-function analysis to a disordered system. Our results show that, in the presence of disorder, this method is extremely helpful in providing an unambiguous picture of the bond distribution. In particular, defect configurations can be studied and characterized with a novel degree of accuracy that was not available before.

cond-mat.mtrl-sci↗

arXiv1998

Ab-initio study of SrTiO3 surfaces

We present first-principles total-energy calculations of (001) surfaces of SrTiO3. Both SrO-terminated and TiO2-terminated surfaces are considered, and the results are compared with previous calculations for BaTiO3 surfaces. The major differences are in the details of the relaxed surface structures. Our calculations argue against the existence of a large ferroelectric relaxation in the surface layer, as had been previously proposed. We do find some indications of a weak surface ferroelectric instability, but so weak as to be easily destroyed by thermal fluctuations except perhaps at quite low temperatures. We also compute surface relaxation energies and surface electronic band structures, obtaining results that are generally similar to those for BaTiO3.

cond-mat.mtrl-sci↗

arXiv1998

Maximally-localized Wannier functions in perovskites: Cubic BaTiO3

The electronic ground state of a periodic crystalline solid is usually described in terms of extended Bloch orbitals; localized Wannier functions can alternatively be used. These two representations are connected by families of unitary transformations, carrying a large degree of arbitrariness. We have developed a localization algorithm that allows one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set of maximally localized Wannier functions. We apply this formalism here to the case of cubic BaTiO3. The purpose is twofold. First, a localized-orbital picture allows a meaningful band-by-band decomposition of the whole Bloch band complex. In perovskites, these Wannier functions are centered on the atomic sites and display clearly a s, p, d, or hybrid character. Second, since the centers of the Wannier functions map the polarization field onto localized point charges, the ground state dielectric properties become readily available. We study the Born effective charges of the paraelectric phase of BaTiO3. We are able to identify not only the contributions that come from a given group of bands, but also the individual contributions from the ``atomic'' Wannier functions that comprise each of these groups.

cond-mat.mtrl-sci↗

arXiv1998

Temperature-dependent dielectric response of BaTiO3 from first principles

Monte Carlo simulations with an effective Hamiltonian parametrized from first principles are performed to study the dielectric response of BaTiO3 as a function of temperature, with particular emphasis on the behavior of the dielectric constant near the transition from the ferroelectric tetragonal phase to the paraelectric cubic phase.

cond-mat.mtrl-sci↗

arXiv1998

Ferroelectric effects in PZT

First-principles calculations are performed to investigate alloying and ferroelectric effects in lead zirconate titanate (PZT) with high Ti composition. We find that the main effect of alloying in the paraelectric phase of PZT is the existence of two sets of B-O bonds, i.e., shorter Ti-O bonds vs. longer Zr-O bonds. On the other hand, ferroelectricity leads to the formation of very short covalent Ti-O bonds and to the formation of covalent chains of Pb-O bonds. The covalency in the ferroelectric phase is mainly induced by an enhancement of hybridization between Ti 3d and O 2p, and between Pb 6s and O 2p. These hybridizations induce a striking decrease of the effective charges when going from the paraelectric to the ferroelectric phase of PZT.

cond-mat.mtrl-sci↗

arXiv1998

Electronic polarization in the ultrasoft pseudopotential formalism

An expression is given for the electronic polarization of an insulating crystal within the ultrasoft pseudopotential scheme. The pseudopotential charge-augmentation terms modify the usual Berry-phase expression, and also give rise to a second term that takes the form of a conventional expectation value.

cond-mat.mtrl-sci↗

arXiv1997

Electromechanical behavior of BaTiO3 from first principles

Using an effective Hamiltonian parametrized from first principles, Monte Carlo simulations are performed in order to study the piezoelectric response of BaTiO3 in the ferroelectric tetragonal phase as a function of temperature. The effect of an electric field on the phase behavior is also illustrated by a simulation of the transformation of a rhombohedral domain into a tetragonal one under a strong field.

cond-mat.mtrl-sci↗