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

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 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

Asymptotic Lattice Displacements about Point Defects in Cubic Metals

We have calculated the asymptotic displacements (a) about a single vacancy in Al, Na, K, and Li, (b) about a single interstitial Cu atom in a Cu host lattice, and (c) about a unit single double force along a (100) direction in A1 and Cu. These calculations were made using the asymptotic equations of the method of lattice statics which, in its full form, is based on the Fourier transformation of the direct-space force equations between the detect and the host atoms in a large supercell of the lattice. Results were also obtained for each of the defect types in A1 and Cu by means of an alternative approach proposed by Lie and Koehler. The asymptotic displacements around the spherical defects in Al, Cu, Na, and K were compared with corresponding results obtained using the exact method of lattice statics. From this comparison it appears that elasticity theory cannot be justifiably applied closer than the (4,4,4) neighbor to the defect in Al, the (5,4,4) neighbor in Cu, and the 26th or the 27th neighbors in Na and K. We also find large displacements along (110) in A1 and along (111) in Cu, Na, and K. The displacements obtained using a Green\u27s-function technique developed by Lie and Koehler agree well with our results for the spherical detect in A1 and for the unit single double-force defects in A1 and Cu. There are significant differences in the (100) directions between the two sets of results for a spherical detect in Cu. In general, the asymptotic method of lattice statics appears to be more exact and to involve fewer computational manipulations than the method of Lie and Koehler

\u3ci\u3eA Priori \u3c/i\u3ePredictions of Phase Transitions in KCaF\u3csub\u3e3\u3c/sub\u3e and RbCaF\u3csub\u3e3\u3c/sub\u3e: Existence of a New Ground State

We have made an a prior; theoretical study of the potential-energy surfaces for KCaF3 and RbCaF3, and have examined the relative stability of the various lower-symmetry structures generated from the cubic perovskite phase by rotations of the CaF6 octahedra. A completely new ground state was discovered which, when included in the sequence of energy levels, allows us to give a full account both qualitatively and, in the case of RbCaF3, quantitatively of the phase transition sequences in both systems

First-Principles Study of Structural Instabilities in Halide-Based Perovskites: Competition Between Ferroelectricity and Ferroelasticity

We have made a systematic theoretical survey of the competition between ferroelastic and ferroelectric instabilities in the family of halide-based perovskites of formula ABX3, where A is an alkali-metal ion, B is a Be, Mg, or Ca ion, and X is a halide ion. Initially we surveyed the whole series of such compounds, making a theoretical lattice-dynamical study using first-principles interionic potentials composed of a long-range pure Coulomb interaction between the spherically symmetric free ions, and a short-range component calculated by the Gordon-Kim approach from the overlapping free-ion charge densities. We then proceeded to examine in more detail three compounds, NaCaBr3, NaCaCl3, and NaCaF3, which manifested both ferroelectric (zone-center) and ferroelastic (zone-boundary) instabilities (there were no structures which showed zone-center instabilities alone). For these three systems we then proceeded to a full energy minimization. This was done by allowing all 40 ions in the lowest symmetry phase to relax independently. It was found that the most stable structure of all three compounds consisted of triply canted halide octahedra, turned through equal angles about all three Cartesian axes. In this phase all eigenfrequencies are real, implying absolute stability, and the ferroelectric instability has been removed. We also discuss the possibility of a ferroelectric, or near-ferroelectric, intermediate phase