Possible Origins of High-Tc, Superconductivity

A new mechanism is proposed to explain high-T, superconductivity in copper-oxide-based, open perovskitelike systems. It is shown that, should the oxygen ions be moving in a double-well potential, an order-of-magnitude enhancement of the electron-lattice coupling follows automatically from a consistent treatment of this motion. Both theoretical and experimental evidence for the presence of such double wells is cited

Possible Origins of High-\u3ci\u3eT\u3csub\u3ec\u3c/sub\u3e\u3c/i\u3e, Superconductivity

A new mechanism is proposed to explain high-Tc superconductivity in copper-oxide-based, open perovskitelike systems. It is shown that, should the oxygen ions be moving in a double-well potential, an order-of-magnitude enhancement of the electron-lattice coupling follows automatically from a consistent treatment of this motion. Both theoretical and experimental evidence for the presence of such double wells is cited

Molecular-Dynamics Simulations of Some BaXF4 Compounds

We have carried out molecular-dynamics simulations on BaXF4 compounds, where X is Mg, Mn, or Zn. Ab initio potentials, with no adjustable parameters, were used to obtain short-range interactions between ion pairs. We found a polar ground-state structure which is in agreement with the A21am space group reported experimentally. We were able to reverse polarization in BaMgF4 at high temperatures, using large fields, but were unable to reverse polarization in the other compounds. The second-order phase transition in the Mn compound at 250 K was reproduced. We believe this to be the first extension of molecular dynamics to materials consisting of chains of F octahedra

Ferroelectricity in Perovskitelike NaCaF3 Predicted Ab Initio

The ability of zero-stress simulations, using Gordon-Kim pair potentials, to describe the structures and transformations of known fluoride-based perovskites is demonstrated for the case of KCaF3. When K+ is replaced by Na+ a new ferroelectric crystal isomorphous with LiNbO3 is predicted. The equivalent relationships of the ferroelectric lithium niobate structure with the perovskite and antiperovskite structures are examined. A polarization of 21 jµC/cm2 at room temperature and a transition temperature of 550 K are predicted for NaCaF3. Surface effects are examined in simulations of a 1080-ion cluster

Calculations of the Intensity of X-Ray Diffuse Scattering Produced by Point Defects in Cubic Metals

We have calculated isointensity profiles for the diffuse x-ray scattering associated with certain types of defects in Cu, Al, Na, K, Li, and a theoretical model lattice. These profiles were computed for high-symmetry planes very close to reciprocal-lattice points of the (S, 0,0), (S,S,0), and (S,S,S) type. Both cubic and double-force defects were treated. The calculations were done using a technique presented by Kanzaki for the theoretical model lattice. Kanzaki\u27s general conclusion that cubic defects produce leminiscate profiles and that double-force defects produce ellipsoidal profiles is confirmed for all the material studied. Our profiles for the model lattice agree with those obtained by Kanzaki, except for the profiles due to a double-force defect near an (S,S,S) reciprocal-lattice point

Application of the Method of Lattice Statics to Interstitial Cu Atoms in Cu

We have calculated the lattice distortion produced by a body-centered interstitial Cu atom in a Cu host lattice. The calculations have been carried out consistently on the basis of discrete lattice theory, using the technique of lattice statics which is based on the Fourier transformation of the direct-space equilibrium equations. The force constants for the perfect lattice have been taken from measured phonon-dispersion curves, and we have used Huntington\u27s Born-Mayer potential to describe the interaction between the interstitial atom and the atoms of the host lattice. The comparison of our results with those obtained by earlier workers, using semidiscrete matching techniques in which a continuum displacement solution is matched to the displacements of a few close neighbors of the defect, indicates that this latter technique is very unreliable. Similarly, the activation volumes estimated by semidiscrete techniques are also unreliable. We have also used the technique of lattice statics to calculate the strain-field interaction between two body-centered interstitial Cu atoms as a function of their separation. As in the case of the displacement fields, we have made these calculations for two different models which differ in the input elastic constants. For what we believe to be the most realistic of our models, we find a repulsive energy of 0.40 eV for two nearest-neighbor interstitials and a repulsive interaction of 0.0975 eV between two second-neighbor interstitials, For the same model, the calculated formation volume per interstitial is 1.12 atomic volumes

Application of the Method of Lattice Statics to Vacancies in Na, K, Rb, and Cs

We have calculated the lattice distortion produced by a single vacancy in Na, K, Rb, and Cs. The calculations have been carried out using the technique of lattice statics, which is based on the Fourier transformation of the direct-space equilibrium equations, making consistent use of discrete lattice theory. Three distinct types of potential have been used to describe the interactions between the host atoms. The first of these applies only to sodium, and contains an ion-electron-ion term derived from the measured phonon dispersion curves. The second applies only to potassium, and has been similarly obtained. The third is based on a model pseudopotential and applies to all four metals. Comparison has been made between our displacements due to a single vacancy in Na, using the first of these potentials, and analogous results obtained by a semidiscrete method in which only the atoms in the first five shells are allowed to relax. The agreement is reasonable for atoms in the first two neighbor shells about the vacancy, but poor for atoms farther away. The calculated displacements have been used to calculate the dilatations and relaxation energies associated with single vacancies in alkali metals. There is a large discrepancy between the magnitudes of these quantities calculated using the first Na potential and those obtained using the second Na potential, and a similar discrepancy exists between the two sets of K results. We have also used the method of lattice statics to determine the strain-field interaction energies between several types of vacancy pairs in these metals. In every case we find the next-nearest-neighbor configuration to be the most stable, whereas in the nearest-neighbor configuration, the two vacancies repel one another. The magnitudes of these binding energies depend strongly on which model potential is used