Polarization Dependence of Born Effective Charge and Dielectric Constant in KNbO3_3

The Born effective charge Z^{*} and dielectric tensor \epsilon_{\infty} of KNbO_3 are found to be very sensitive to the atomic geometry, changing by as much as 27% between the paraelectric cubic and ferroelectric tetragonal and rhombohedral phases. Subtracting the bare ionic contribution reveals changes of the dynamic component of Z^{*} as large as 50%, for atomic displacements that are typically only a few percent of the lattice constant. Z^{*}, \epsilon_{\infty} and all phonon frequencies at the Brillouin zone center were calculated using the {\it ab initio} linearized augmented plane-wave linear response method with respect to the reference cubic, experimental tetragonal, and theoretically determined rhombohedral ground state structures. The ground state rhombohedral structure of KNbO_3 was determined by minimizing the forces on the relaxed atoms. By contrast with the cubic structure, all zone center phonon modes of the rhombohedral structure are stable and their frequencies are in good agreement with experiment. In the tetragonal phase, one of the soft zone center modes in the cubic phase is stablized. In view of the small atomic displacements involved in the ferroelectric transitions, it is evident that not only the soft mode frequencies but also the Born effective charge and dielectric constants are very sensitive to the atomic geometry.Comment: 26 pages, revtex, no figures; to appear in Phys. Rev. B15 (Oct.), 199

Pressure Dependence of Born Effective Charges, Dielectric Constant and Lattice Dynamics in SiC

The pressure dependence of the Born effective charge, dielectric constant and zone-center LO and TO phonons have been determined for $3C$-SiC by a linear response method based on the linearized augmented plane wave calculations within the local density approximation. The Born effective charges are found to increase nearly linearly with decreasing volume down to the smallest volume studied, $V/V_0=0.78$, corresponding to a pressure of about 0.8 Mbar. This seems to be in contradiction with the conclusion of the turnover behavior recently reported by Liu and Vohra [Phys.\ Rev.\ Lett.\ {\bf 72}, 4105 (1994)] for $6H$-SiC. Reanalyzing their procedure to extract the pressure dependence of the Born effective charges, we suggest that the turnover behavior they obtained is due to approximations in the assumed pressure dependence of the dielectric constant $\varepsilon_\infty$, the use of a singular set of experimental data for the equation of state, and the uncertainty in measured phonon frequencies, especially at high pressure.Comment: 25 pages, revtex, 5 postscript figures appended, to be published in Phys. Rev.

Theoretical study of the interaction of hydrogen with metals

Typescript.Thesis (Ph. D.)--University of Hawaii at Manoa, 1989.Includes bibliographical references.Microfiche.x, 134 leaves, bound ill. 29 cmElectronic structure calculations are carried out for simple metal hydrides and hydrogen adsorption on simple metal surfaces. The studies are based on the ab initio pseudopotential method and the local-density approximation. Structural and electronic properties of magnesium hydride are studied. The calculated structural properties include equilibrium lattice parameters, cohesive energy, elastic constants, bulk modulus, and phonon frequency. The electronic band structure, density of states, charge density distribution are also obtained. Superconductivity in doped magnesium hydride is proposed. Lithium beryllium hydride is studied because of its proposed metallic behavior. The relative stability of two perovskite structures" of lithium beryllium hydride is investigated. Band structures are calculated under ambient and high pressure conditions to determine whether the structures are metallic, The adsorption of hydrogen on the close-packed surface of Be and Mg are studied. The equilibrium distance between hydrogen and the surfaces, the adsorption energy, and the hydrogen oscillation frequency are calculated for hydrogen in the high-symmetry adsorption sites. The energetics for hydrogen to go into a subsurface site is also investigated. Charge density distribution, work function and potential are studied for the clean and hydrogen-covered surfaces. Electronic band structure and H-induced surface states are determined for hydrogen adsorption on Be. Surface energy of the clean surfaces is also calculated

Dynamic local distortions in KNbO<SUB>3</SUB>

Molecular dynamics simulations of the perovskite oxide KNbO<SUB>3</SUB> are performed with a first-principles effective Hamiltonian. They reveal the prevalence of local polar distortions with short-range chain-like correlations, present even in the paraelectric phase far above T<SUB>c</SUB>. The ordering of these dynamically fluctuating distortions yields the observed temperature sequence of ferroelectric phases. The simulations also reproduce the essential features of diffuse X-ray scattering measurements and the weak temperature dependence of diffuse streak patterns observed by Comes et al. These local distortions suggest an order-disorder character for the transitions. Softening of optical phonon branches is observed in the same simulations not only near q=0, suggesting a displacive character for the transition, but also over large regions of the Brillouin zone. Dynamic real-space chains thus provide a unified framework for understanding both the order-disorder and displacive characteristics of these phase transitions

Effective Hamiltonian for the ferroelectric phase transitions in KNbO<SUB>3</SUB>

An effective Hamiltonian for the ferroelectric transitions in KNbO<SUB>3</SUB> is constructed from first-principles density-functional-theory total-energy and linear-response calculations through the use of a localized, symmetrized basis set of "lattice Wannier functions". The detailed description provided here provides the foundation for the use of this effective Hamiltonian in classical Monte Carlo, classical molecular dynamics and quantum mechanical simulations, as well as guidance for the construction of effective Hamiltonians for related systems