Oxygen-deficient Planes Research Articles

Oxygen vacancy order-disorder transition at high temperature in Bi-Sr-Fe-based perovskite-type oxides

${\mathrm{Bi}}_{1\ensuremath{-}\mathit{x}}{\mathrm{Sr}}_{\mathit{x}}{\mathrm{FeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ (BSFs) are well known not only as multiferroic materials but also as mixed oxide-ion and electronic conductors. BSFs show remarkable ionic conductivity despite their limited electrochemical activity due to their low electronic conductivity. In this study, the discontinuous change in the electrical conductivity of ${\mathrm{Bi}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{FeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ (BSF30) at around 770 ${}^{\ensuremath{\circ}}\mathrm{C}$ was investigated in the context of their unique defect order-disorder transition. Based on the electrical conductivity and the Seebeck coefficient of BSF30, both functions of temperature and oxygen partial pressure, the discontinuous change in both hole and oxide-ion conductivities was the result of a reduction in their mobility but unrelated to their concentration term. High-temperature x-ray diffraction and scanning transmission electron microscopy revealed that periodic oxygen-deficient planes with fivefold $d$-spacing of ${001}$ emerge in an ordered low-temperature phase while the ordered defect structure disappears in a high-temperature phase. The discontinuous change of electrical conductivity of BSF30 was then found to be the result of the order-disorder transition of its oxygen sublattice, i.e., oxygen vacancies. Because there was no such transition in the cation sublattice, the cation did not affect the electrical conductivity of BSF30. This study suggests that BSFs with the desired electrochemical and multiferroic functionalities can be designed by controlling their order-disorder transition.

Low-Dimensional Oxygen Vacancy Ordering and Diffusion in SrCrO3-δ.

We investigate the formation mechanisms of vacancy-ordered phase and collective mass transport in epitaxial SrCrO3-δ films using ab initio simulations within the density functional theory formalism. We show that as the concentration of oxygen vacancies (VO) increases, they form 1D chains that feature Cr-centered tetrahedra. Aggregation of these 1D VO chains results in the formation of (111)-oriented oxygen-deficient planes and an extended vacancy-ordered phase observed in recent experiments. We discuss atomic-scale mechanisms enabling the quasi-2D VO aggregates to expand along and translate across (111) planes. The corresponding lowest activation energy pathways necessarily involve rotation of Cr-centered tetrahedra, which emerges as a universal feature of fast ionic conduction in complex oxides. These findings explain reversible oxidation and reduction in SrCrO3-δ at low temperatures and provide insights into transient behavior necessary to harness ionic conductive oxides for high-performance and low-temperature electrochemical reactors.

Oxygen vacancy ordering in epitaxial La0.5Sr0.5CoO3−∂ thin films on (001) LaAlO3

High-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and electron diffraction were used to study the microstructure of epitaxial La0.5Sr0.5CoO3−∂ thin films grown on (001) oriented LaAlO3 substrates. Films were characterized before and after annealing treatments under different oxygen partial pressures. EELS shows that annealing reduces the valence state of cobalt due to loss of oxygen. HRTEM image simulations show that the superstructure contrast observed in HRTEM can be explained by shifts of cations in planes containing ordered oxygen vacancies. The as-deposited film showed weak, short-range ordering of oxygen vacancies within nanometer-sized domains. The annealed film showed long-range order and a strong anisotropy in ordering, with the oxygen-deficient planes aligned parallel to the film/substrate interface. We propose that the anisotropy in ordering is a mechanism of stress relief in these films. Implications of the observed microstructure on the oxygen transport and surface oxygen exchange properties are discussed.

Low-energy spin dynamics of CuO chains in YBa 2Cu 3O 6+ x

We study the spin fluctuation dynamics of Cu–O chains in the oxygen deficient planes of YBa 2Cu 3O 6+ x . The chains are described by a model including antiferromagnetic interactions between the spins and Kondo-like scattering of the oxygen holes by the copper spins. There are incommensurate spin fluctuations along the direction of the chains. The dynamic structure factor of this system is qualitatively different from that of a quasi one-dimensional localized antiferromagnet due to the presence of itinerant holes. We compute the dynamic structure factor that could be measured in neutron scattering experiments.

Quasi-one-dimensional spin fluctuations inYBa2Cu3O6+x

We study the spin fluctuation of the oxygen deficient planes of YBa(2)Cu(3)O(6+x). The Cu-O chains that constitute these planes are described by a model that includes antiferromagnetic interactions between spins and Kondo-like scattering of oxygen holes. The spectrum of magnetic excitations shows the presence of incommensurate dynamic fluctuations along the direction of the chains. The presence of itinerant holes is responsible for the existence of important differences between the spin dynamics of this system and that of a quasi-one-dimensional localized antiferromagnet. We comment on the possibility of experimental observation of these fluctuations.

Theoretical study of Cu-O chain fragments in the oxygen-deficient planes of YBa2Cu3O6+x. I. Microscopic properties.

We examine some models of the strongly correlated \ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}Cu-O\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot} chain fragments that are the main components of the oxygen-deficient planes of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. We use quantum Monte Carlo methods, exact numerical diagonalization as well as an analytical approach in order to study the thermodynamic properties of the chain fragments and the charge and magnetic correlations within them as functions of the concentration of carriers and of the temperature. Only the fermionic degrees of freedom of the chain fragments are considered. \textcopyright{} 1996 The American Physical Society.

Theoretical study of Cu-O chain fragments in the oxygen-deficient planes of YBa2Cu3O6+x. II. Intrachain interactions and the chain fragment's length distribution.

We study some models of the strongly correlated \ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}Cu-O\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot} chain fragments that form on the oxygen-deficient planes of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. We use analytical and numerical methods to investigate the stability of the chain fragments as a function of their size. We discuss also the size and x dependence of the O-O intrachain interactions that enter in the phenomenological descriptions of the ${\mathrm{CuO}}_{\mathit{x}}$ planes. Only the fermionic degrees of freedom of the chain fragments are considered. \textcopyright{} 1996 The American Physical Society.

The role of Cu−O chain fragments in the REBa2Cu3O6+x high-Tc superconductors

We study the properties of the ensemble of Cu−O chain fragments that form on the oxygen deficient planes of the REBa2Cu3O6+x (REBCO) compounds. The chain fragments have an important role in the hole doping of the superconducting planes. The physics of the Cu−O chain fragments themselves is interesting and can be analyzed in terms of an effective ASYNNNI model derived microscopically.

Physical properties of CuO chain fragments within oxygen deficient planes in YBa 2Cu 3O 6+ x: a theoretical approach

We study by exact numerical diagonalization and by Quantum Monte Carlo simulations the microscopic properties of the … CuO … chain fragments that are the constituents of the oxygen deficient planes of YBa 2Cu 3O 6+ x and that play an important role in both the antiferromagnetism and the superconductivity of this compound. The information about the electronic and magnetic properties of chain fragments thus obtained is then used as input of a statistical mechanics calculation of the behaviour of an ensemble of such chain fragments.

Herringbone configuration of chain fragments in YBa 2Cu 3O 6+x: Myth or reality? (theoretical view on the problem)

Existence of the superstructure formed by very short CuOCu chain fragments, which are dominant elements of oxygen deficient planes of YBCO with a rather low oxygen content x, can be theoretically warranted by invoking simple electrostatic and charge transfer mechanisms.

SELECTED PROPERTIES OF Y-Ba-Cu-O IN CONNECTION WITH THE PROPERTIES OF Cu-O CHAINS

Here we discuss some experimental evidence which supports a significant role of …-Cu-O…chain fragments, situated in oxygen deficient planes, in the doping of carriers into CuO 2 planes, in structural phase transitions and in many other normal and superconducting properties of Y-Ba-Cu-O . The relevant experiments are associated with the structural, magnetic, XAS, NMR/NQR investigations. The well-established phase diagram of YBa 2 Cu 3 O 6+x, i-e., the Néel, T N , and superconducting, T c , transition temperatures versus the oxygen content x, is the basis of the theory which should explain the structural phase transitions as well as the charge transfer mechanism. It appears that lattice-gas models do not answer all the requirements, particularly, concerned with the intrinsic chain fragments. Although the theory of the oxygen deficient plane, sharing quantum mechanics of finite chain fragments, statistical mechanics of the chain fragment ensemble as well as Monte Carlo simulations, is in its formative stage yet, it can readily apply to quantitative interpretation of selected experiments.

Antiferromagnetic ordering of Cu ions in NdBa2Cu

Neutron-scattering experiments performed on single crystals ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.1}$ reveal antiferromagnetic order for T${T}_{N}$=385\ifmmode\pm\else\textpm\fi{}2 K in which the structure is characterized by the magnetic wave vector ((1/2,1)/2,(1/2); this had previously been observed only at low temperature in single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ [the Y(1:2:3) compound]. The magnetic intensities are accounted for by ${\mathrm{Cu}}^{2+}$ spins coupled antiferromagnetically in the ${\mathrm{CuO}}_{2}$ planes as in the yttrium compounds ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ (x0.5). However, unlike the Y(1:2:3) compound case, the oxygen-deficient planes in the Nd(1:2:3) compound exhibit small staggered magnetization \ensuremath{\epsilon}S which induces the spin ordering -S-\ensuremath{\epsilon}S-S+S+\ensuremath{\epsilon}S+S, along the tetragonal axis. The average staggered magnetization evaluated at room temperature is about 0.40${\ensuremath{\mu}}_{B}$ in the ${\mathrm{CuO}}_{2}$ planes while in the oxygen deficient (Cu-chain) layers it is about 0.04${\ensuremath{\mu}}_{B}$. The exchange coupling between the Cu atoms in the oxygen deficient layer containing Cu chains and its nearest-neighbor ${\mathrm{CuO}}_{2}$ layers (Cu planes) is thus ferromagnetic; this may be readily understood by consideration of the relevant ${\mathrm{Cu}}^{2+}$ orbitals.