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We describe a strategy to design ferroelectric heteroanionic materials based on the coupling of anion order and octahedral tilts in $n=1$ Ruddlesden-Popper structures, which allows noncentrosymmetric and polar compounds to arise from centrosymmetric anion-ordered structures. We investigate the relative phase stabilities of the polymorphs of ${\mathrm{Sr}}_{2}{\mathrm{ScO}}_{3}\mathrm{F}$ and ${\mathrm{Ca}}_{2}{\mathrm{ScO}}_{3}\mathrm{F}$ derived from this coupling using electronic structure calculations. We find that large degrees of octahedral tilting can stabilize different anion orders derived from assembly of $[{\mathrm{ScO}}_{5}{\mathrm{F}]}^{8\ensuremath{-}}$ octahedra. To further understand the link between octahedral tilting and anion order, we quantitatively separate the contributions of electrostatics and covalent interactions to the stability of tilts. We find that the tilts are driven primarily by covalent interactions and that local out-of-plane polar displacements, induced from the anion order, further stabilize the octahedral tilts through the pseudo-Jahn-Teller effect.

The electronic structure and properties of LaYBa{sub 2}Cu{sub 2}Ti{sub 2}O{sub 11}, as a typical member of the {ital L}{sup {prime}}{ital L}{sup {double_prime}}Ba{sub 2}Cu{sub 2}Ti{sub 2}O{sub 11} family, were determined by means of the full-potential linear muffin-tin method. From the results obtained it appears that {ital L}{sup {prime}}{ital L}{sup {double_prime}}Ba{sub 2}Cu{sub 2}Ti{sub 2}O{sub 11} may be a good candidate for high-{ital T}{sub {ital c}} superconductivity since its band structure at the Fermi energy is dominated by a quasi-two-dimensional Cu-O {ital dp}{sigma} band with a prominent van Hove singularity. We suggest that hole doping of about 0.6 holes per unit cell (or 0.3 holes per Cu-O layer) through mono- and/or divalent metal substitution for lanthanides may lead to an optimum {ital T}{sub {ital c}} in these materials. The role of different structural imperfections on the {ital T}{sub {ital c}} that apparently could be created in the process of chemical doping {emdash} oxygen impurities between the Cu-O layers and oxygen vacancies in the Ti-O plane {emdash} was also determined. We found that these likely structural imperfections are unfavorable for the creation of the superconducting state in these materials. {copyright} {ital 1996 The American Physical Society.}

The microscopic cause of conductivity in transparent conducting oxides like ZnO, In{2}O{3}, and SnO{2} is generally considered to be a point defect mechanism in the bulk, involving intrinsic lattice defects, extrinsic dopants, or unintentional impurities like hydrogen. We confirm here that the defect theory for O-vacancies can quantitatively account for the rather moderate conductivity and off-stoichiometry observed in bulk In{2}O{3} samples under high-temperature equilibrium conditions. However, nominally undoped thin-films of In{2}O{3} can exhibit surprisingly high conductivities exceeding by 4-5 orders of magnitude that of bulk samples under identical conditions (temperature and O{2} partial pressure). Employing surface calculations and thickness-dependent Hall measurements, we demonstrate that surface donors rather than bulk defects dominate the conductivity of In{2}O{3} thin films.

In order to address the growing need for $p$-type transparent conducting oxides (TCOs), we present a materials design approach that allows to search for materials with desired properties. We put forward a set of design principles (DPs) that a material must meet in order to qualify as a $p$-type TCO. We then start from two prototype $p$-type binary oxides, i.e., Cu${}_{2}$O and Ag${}_{2}$O, and define a large group of compounds in which to search for unique candidate materials. From this set of compounds, we extracted two oxovanadates, Cu${}_{3}$VO${}_{4}$ and Ag${}_{3}$VO${}_{4}$, which serve as a case study to show the application of the proposed materials selection procedure driven by the DPs. Polycrystalline Ag${}_{3}$VO${}_{4}$ was synthesized by a water-based hydrothermal technique, whereas Cu${}_{3}$VO${}_{4}$ was prepared by a solid-state reaction. The theoretical study of the thermochemistry, based on first-principles electronic structure methods, demonstrates that Cu${}_{3}$VO${}_{4}$ and $\ensuremath{\alpha}$-Ag${}_{3}$VO${}_{4}$ are $p$-type materials that show intrinsic hole-producing defects along with a low concentration of ``hole-killing'' defects. Owing to its near-perfect stoichiometry, Ag${}_{3}$VO${}_{4}$ has a rather low hole concentration, which coincides with the experimentally determined conductivity limit of 0.002 S/cm. In contrast, Cu${}_{3}$VO${}_{4}$ is highly off stoichiometric, Cu${}_{3\ensuremath{-}x}$VO${}_{4}$ ($x=0.15$), which raises the amount of holes, but due to its black color, it does not fulfill the requirements for a $p$-type TCO. The onset of optical absorption in $\ensuremath{\alpha}$-Ag${}_{3}$VO${}_{4}$ is calculated to be 2.6 eV, compared to the experimentally determined value of 2.1 eV, which brings it to the verge of transparency.

$^{63}\mathrm{Cu}$ NMR Knight shifts were measured on ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ for x=0.10, 0.16, and 0.20 in the normal and superconducting states. These results, combined with magnetic susceptibility measurements, allow the separation of the carrier spin and orbital contributons to the susceptibility. At temperatures just above the superconducting transition, the conduction electron spin susceptibility increases dramatially with strontium doping. The analysis of the Knight shift at low temperature suggests that the spin susceptibility is zero and the superconducting state is a spin singlet.

We present a Raman-scattering study of Y[sub 1[minus][ital x]]Ca[sub [ital x]]Sr[sub 2]Cu[sub 2]GaO[sub 7] for both the [ital x]=0 parent compound and doped compositions with [ital x]=0.25 and [ital x]=0.40. Extrapolation from YBa[sub 2]Cu[sub 3]O[sub 7[minus][ital d]] and other cuprates allows us to assign many of the Raman-active phonon modes in the [ital x]=0 material, as well as identify a two-magnon scattering peak, a second-order phonon scattering peak, and a Raman continuum out to 4000 cm[sup [minus]1]. Despite compositional inhomogeneities, the doped superconducting samples show some of the same low-energy phonon features as the [ital x]=0 material. There is, however, a doping-dependent shift in the positions of features in the 500--700 cm[sup [minus]1] range, possibly due to Ca locating on Sr sites instead of Y sites. The relative intensities of the phonon peaks in the doped material are also changed from the insulator ([ital x]=0), suggesting that a resonant Raman phenomenon is occurring. The temperature-dependent spectra show what appears to be a superconducting dip in the background intensity, but the low superconducting fractions in these samples make this difficult to verify. The doped material also has a Raman continuum out to 4000 cm[sup [minus]1] just as in the [ital x]=0more » samples, but with the two-magnon and second-order phonon scattering peaks significantly reduced in intensity. This may be a result of changes in the long-range ordering, or another manifestation of the same resonance phenomena occurring at lower energies. Single-crystal samples of the doped material Y[sub 1[minus][ital x]]Ca[sub [ital x]]Sr[sub 2]Cu[sub 2]GaO[sub 7] are necessary for a more conclusive Raman study.« less

The c(6×2) is a reconstruction of the SrTiO3(001) surface that is formed between 1050 and 1100 °C in oxidizing annealing conditions. This work proposes a model for the atomic structure for the c(6×2) obtained through a combination of results from transmission electron diffraction, surface x-ray diffraction, direct methods analysis, computational combinational screening, and density functional theory. As it is formed at high temperatures, the surface is complex and can be described as a short-range-ordered phase featuring microscopic domains composed of four main structural motifs. Additionally, nonperiodic TiO2 units are present on the surface. Simulated scanning tunneling microscopy images based on the electronic structure calculations are consistent with experimental images.

The structure of the $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.6\ifmmode^\circ\else\textdegree\fi{}$ reconstruction of ${\mathrm{LaAlO}}_{3}$ (001) has been determined using transmission electron diffraction combined with direct methods. It has a lanthanum oxide termination with one lanthanum vacancy per surface unit cell. Density functional calculations indicate that charge compensation occurs by a fractional number of highly delocalized holes, and that the surface contains no oxygen vacancies and the holes are not filled with hydrogen. The reconstruction can be understood in terms of expulsion of the more electropositive cation from the surface and increased covalency.

Polycrystalline samples of ${\mathrm{La}}_{1.83}$${\mathrm{Sr}}_{0.17}$Cu${\mathrm{O}}_{4}$ were studied by pulsed nuclear-magnetic-resonance methods on the copper nucleus over a temperature range which included both normal and superconducting phases. A Korringa constant 14\ifmmode\pm\else\textpm\fi{}1 K sec and a minimum value for the energy gap $\frac{2\ensuremath{\Delta}}{{k}_{B}{T}_{c}}=1.3\ifmmode\pm\else\textpm\fi{}0.2$ were found. A comparison of these results with the larger gap reported from infrared absorption and from tunneling experiments indicates that there is substantial anisotropy in the pairing interaction. Homogeneous line narrowing in the superconducting state was observed.

We report ultrasonic attenuation measurements on sinter-forged YBa/sub 2/Cu/sub 2/O/sub 7-//sub delta/ material, which differs from ordinary sintered material in that the crystallites are preferentially oriented to form a uniaxial sample. Three peaks in attenuation, at temperatures of 250, 180, and 70 K, were observed for longitudinal waves propagating perpendicular to the forging axis, which is similar to that reported in ordinary (isotropic) polycrystalline samples. However, for both transverse and longitudinal sound propagated along the forging axis we have a different behavior, with only one peak at 180 K, showing a strong anisotropy. It is suggested that sound waves traveling parallel to and normal to the Cu-O planes may account for the anisotropic effect, and a relaxation mechanism may explain the increase in shear wave attenuation which was seen with decreasing temperature.

The velocity of ultrasound was measured on sinter-forged polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ samples with a superconducting transition temperature of 91 K. The forging process results in crystallites which are preferentially aligned with their c axis aligned parallel to the forging axis as confirmed from optical and x-ray measurements. Sound-velocity measurements show that the material is elastically anisotropic. The temperature dependence of the sound velocity shows distinct anomalies. For propagation parallel to the forging axis, both longitudinal and shear waves displayed a thermal hysteresis between 65 and 260 K; however the longitudinal sound velocity with q-circumflex in the basal plane showed a much smaller thermal hysteresis. No effect of a magnetic field was observed on sweeping the magnetic field to 7.2 T at temperatures of 4 and 86 K.

High-${T}_{c}$ superconductivity is investigated in the Y-Ba-Cu-O system. Regions of possible compound formation are ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{x}/2+\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{1+\mathrm{x}}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{6\mathrm{\ensuremath{-}}\mathrm{x}/2+\mathrm{\ensuremath{\delta}}}$. ${T}_{c}$ values g90 K are found for the former with x=0.8 (the Chu material) and for the latter with x=0.2 and may imply a common superconductivity phase in both samples. Critical fields determined by resistance and low-field ${d}_{c}$ susceptibility measurements are reported.