Apical Oxygen Atoms Research Articles

Structural phase transition and structural properties in oxygenated La2CuO4+y; y ∼0.02

The crystal structure of the oxygenated La2CuO4+y; y ∼0.02 has been studied by X-ray diffraction. It is found that the structural phase transition from tetragonal to orthorhombic symmetry occurring at about 450 K (Tc) is of second order and shows diffuse scattering above Tc. The diffuse scattering is related to static and/or dynamic short-range order of cooperative tilts of rigid-like elongated CuO6 octahedra, which make two-dimensional networks stacked along the [001] axis. The structures at room temperature and 210K are determined using a 4-circle X-ray diffractometer and have the orthorhombic space group 10D2h-Pccn. The structure is mainly characterized by anisotropic cooperative tilts of the CuO4 octahedra around the [100]p and [010]p primitive axes in two-dimensional octahedron-networks, where suffix “p” means pseudo K2NiF4-type structure. The anisotropy of the tilting-angles results from pushing by excess oxygen atoms to apical oxygen atoms of the CuO6 octahedra. It is also found by refinement of the structure that the excess oxygen atoms are randomly sited at the atomic coordinates (1/4,1/4,0.244) and/or (1/4, 1/4,0.279).

Synthesis and characterization of triorganotin(IV) complexes of 5-[( E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids. : Crystal and molecular structures of a series of triphenyltin 5-[( E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoates (aryl=phenyl, 2-methylphenyl, 3-methylphenyl and 4-methoxyphenyl)

The triphenyltin and tri- n-butyltin complexes of some 5-[( E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoic acids have been synthesized and characterized by 1H-, 13C-, 119Sn-NMR, IR and 119mSn Mössbauer spectroscopic techniques in combination with elemental analysis. The crystal structures of triphenyltin 5-[( E)-2-(aryl)-1-diazenyl]-2-hydroxybenzoates (aryl=phenyl, 2-methylphenyl, 3-methylphenyl and 4-methoxyphenyl) are reported. Both X-ray and 119Sn Mössbauer data indicate that the triphenyltin complexes adopt a monomeric distorted tetrahedral configuration with the carboxylate ligand coordinating in a monodentate mode. By contrast, 119Sn Mössbauer spectroscopy shows that each tributyltin complex is polymeric and features a trans-trigonal bipyramidal geometry with a planar SnBu 3 unit and two apical carboxylate oxygen atoms derived from bidentate bridging carboxylate ligands. This is a solid-state effect, as both 119Sn-NMR and 1 J( 13C– 119/117Sn) coupling constant data indicate tetrahedral geometries in solution for the triphenyl and tri- n-butyl complexes.

Square-pyramidal copper(II) chelate complexes of edta-type and related ligands forming six-membered rings. Crystal structure of magnesium (ethylenediamine- N, N, N′-tri-3-propionato- N′-3-propionate)copper(II) decahydrate, Mg[Cu(edtp)]·10H 2O

Two square-pyramidal complexes, Mg[Cu(edtp)]·10H 2O ( 1) (edtp=ethylenediamine- N, N, N′, N′-tetra-3-propionate ion, acting as a pentadentate) and Mg[Cu(ed3p)] 2·6H 2O ( 2) (ed3p=ethylenediamine- N, N, N′-tri-3-propionate ion) have been prepared and characterized. A square-pyramidal geometry was established crystallographically for the complex Mg[Cu(edtp)]·10H 2O ( 1). The Cu(II) ion in the complex anion is displaced by 0.195 Å from the plane of the four in-plane (2N and 2O) ligand atoms towards the apical oxygen atom of the β-alaninate (R) ring (CuO ax=2.173 Å). The conformations of the chelate rings are those usually expected for these kinds of chelates (a five-membered diamine (E) ring in a twist(λ) conformation, two β-alaninate (G 1 and G 2) rings in a distorted twist-chair and one β-alaninate (R) ring in a distorted twist-boat conformation). The structural data correlating the square-pyramidal stereochemistry are discussed with reference to those obtained for related complexes. Electronic absorption and reflectance spectra are discussed in comparison with those of related complexes of known structures. IR data (carboxylate region) are also reported.

Structure and a bond-valence-sum study of the 1-2-3 superconductors(CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3OyandYBa2Cu3Oy

We present a study of the crystal structure of the 1-2-3 superconductor [Ca(x)La(1-x)][Ba$(1.75-x)La(0.25+x)]Cu(3)O(y) (``CLBLCO''). Because of the presence of both La and Ba ions in the ``Ba'' layer, the local symmetry of YBa(2)Cu(3)O(y) (``YBCO'') is lost. One can no longer assume that an ``apical'' oxygen atom always lies strictly on the line joining CuI and CuII ions, and a Pauling bond-valence calculation is very useful as an adjunct to the Rietveld refinement of the positions of the La and Ba ions. When a Rietveld analysis is performed allowing for the possibility that the apical oxygen atoms may be slightly displaced along the a or b direction, the bond valence sums for both La and Ba are close to the natural oxidation states of these ions. We have also used the bond-valence-sum method to study the mean oxidation states of the CuI and CuII ions in both CLBLCO and YBCO. Our results for YBCO differ from some previously published results. Our main conclusion is that, while the method may be useful in finding the charge concentration on the CuII plane, it definitely does not yield the concentration of mobile holes, nor the concentration of superconducting charge carriers.

Syntheses and characterization of mono- and di- N-hydroxyethylated tetraaza macrocycles containing eight C-methyl groups and their nickel(II) and copper(II) complexes

New partially N-hydroxyethylated 14-membered tetraaza macrocycles 1,8-bis(2-hydroxyethyl)-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L 2) and 1-(2-hydroxyethyl))-3,5,7,7,10,12,14,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L 3) have been synthesized selectively by the one-step reaction of 2,5,5,7,9,12,12,14-octamethyl-1,4,8,11-tetraazacyclotetradecane (L 1) with 2-hydroxyethyl bromide. The complexes [NiL 3] 2+, [CuL 2] 2+, and [CuL 3] 2+ have been prepared and characterized. The complex [CuL 2](ClO 4) 2 has a square-pyramidal coordination geometry with one apical oxygen atom; only one of the two hydroxyethyl groups is coordinated to the metal ion. Electronic absorption spectra of [CuL 3](ClO 4) 2 containing one hydroxyethyl pendant arm indicate that the geometry is similar to that of [CuL 2](ClO 4) 2. Unexpectedly, the nickel(II) complex [NiL 3](ClO 4) 2 has a severely distorted trigonal bipyramidal coordination geometry with the oxygen atom of the pendant arm at the equatorial position. The NiO bond distance of the nickel(II) complex is shorter, or not longer, than the NiN bond distances. The ligand in [CuL 2] 2+ is in the RRSS ( trans-III) configuration, as usual, whereas that in [NiL 3] 2+ has the RRRR ( trans-V) conformation. The coordination geometry and properties of [NiL 3] 2+ are quite different from those reported for other related nickel(II) complexes containing one functional pendant arm.

Magnetic structures ofY2SrCu0.6Co1.4O6.5andY2SrCuFeO6.5

The nuclear and magnetic structures of two layered mixed copper transition-metal oxides ${\mathrm{Y}}_{2}{\mathrm{SrCu}}_{0.6}{\mathrm{Co}}_{1.4}{\mathrm{O}}_{6.5}$ and ${\mathrm{Y}}_{2}{\mathrm{SrCuFeO}}_{6.5}$ have been determined from time-of-flight neutron powder diffraction. Both compounds consist of apex-linked pyramidal Cu/M-${\mathrm{O}}_{5}$ double layers, which alternate with oxygen defective ${\mathrm{Y}}_{2}{\mathrm{O}}_{1.5}$ fluorite layers. The Co-containing compound has a simple magnetic structure with a Shubnikov group ${\mathrm{Ib}}^{\ensuremath{'}}{a}^{\ensuremath{'}}m.$ The magnetic moments $(2.2{\ensuremath{\mu}}_{B})$ of this compound are aligned along the crystallographic b axis with antiferromagnetic order between neighboring Cu/Co ions. In contrast, the Fe-analog possesses a complicated noncolinear magnetic structure ${(Pc}^{\ensuremath{'}}{c}^{\ensuremath{'}}n,$ with moments $m\ensuremath{\perp}c),$ which can also be considered as comprising two components with ${I}^{\ensuremath{'}}{\mathrm{bam}}^{\ensuremath{'}}$ and ${\mathrm{Ib}}^{\ensuremath{'}}{a}^{\ensuremath{'}}m$ symmetry. The combination of the two components results in an $82\ifmmode^\circ\else\textdegree\fi{}$ angle between the spins of Cu/Fe ions in neighboring Cu/Fe-${\mathrm{O}}_{5}$ planes within a given double layer. The approximate $90\ifmmode^\circ\else\textdegree\fi{}$ rotation of the moments results from a ferromagnetic interaction between the neighboring Cu/Fe ions along the c axis through the apical oxygen atoms. The ferromagnetic component may originate from the $\mathrm{Cu}{d}_{{z}^{2}}^{2}$-$\mathrm{O}2p$-$\mathrm{Fe}{d}_{{z}^{2}}^{1}$ type exchange interaction. The implication of such an interaction to related layered cuprates is also discussed. By symmetry analysis, the magnetic structure of the Co compound can be classified as a ${G}_{y}g$ type and the appearance of weak ferromagnetism can be attributed to the Dzyaloshinsky-Moria interaction which couples the ${G}_{y}g$ mode and ${F}_{z}f$ mode in the same representation of the $\mathrm{Ibam}$ space group.

XAS STUDY OF (BaCuO2)2/(CaCuO2)n SUPERLATTICES

EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-Ray Absorption Near Edge Structure) spectroscopies have been used to probe the local bonding of the artificially layered superconducting (BaCuO2)2/(CaCuO2)n (n = 2, 3, 4) superlattices. Fluorescence-yield measurements have been performed above the Cu and Ba K-edges. This study shows that the charge reservoir (CR) block of the Ba/Ca superlattice contains oxygen vacancies randomly distributed in the CuO2 planes and that the two apical oxygen atoms are displaced out of the Ba plane giving rise to buckled BaO planes. These oxygen atoms have a distorted pyramidal and octahedral coordination around Cu at Ba/Ca interface and in the (BaCuO2) block, respectively.

Crystal Growth and Structure of AlSr2YCu2O7

Millimeter-size crystals of AlSr2YCu2O7 were synthesized by the flux method in alumina crucibles. Different mixtures of SrO:CuO (ranging from 25:75 to 12:88 mol ratio) were used as flux. No aluminum was added to the reagents. Depending on the flux composition and cooling rate, crystals different in morphology and size were obtained. The crystals appear to be tetragonal, space group P4/mmm. However, electron diffraction photographs, in agreement with Ramı́rez-Castellanos et al. (1), reveal that the symmetry is othorhombic with a cell a=2at, b=4at, and c=2ct (the subscript t refers to the YBCO tetragonal cell). The structural refinement was based on X-ray data collected with a KCCD Nonius diffractometer equipped with a graphite monochromator and AgKα radiation. One hundred and ninety-seven independent reflections with respect to the space symmetry quoted above, with I>3σ, were used. The arrangement is that of a YBCO structure in which the chain Cu cations have been replaced by Al. This substitution induces the movement of the oxygen atoms in the basal layer in such a way as to form, together with the apical oxygen atoms, corner-sharing zig-zag chains of oxygen tetrahedra centered around the Al cations and running parallel to the a-axis either tetragonally or orthorhombically. The b- and c-axes are quadrupled and doubled, respectively, because the zig-zag chains are out of phase perpendicularly to these directions. The additional distortion results in the shift of Al to xx0 (x≈0.056) from 000, O1 to xxz (x≈0.073) from 00z, and O3 to (0.5, y, 0) (y≈0.116) from (0.5, 0, 0).

New Bi-based high- Tc superconducting phases obtained by low-temperature fluorination

A new superconducting phase, Bi 2Sr 2Ca 2Cu 3O 8F 4, was obtained by fluorination of standard Bi(2223) at moderate temperatures (250–300°C), using as fluorine source ammonium hydrogen difluoride, NH 4HF 2. The presence of fluorine in this new phase was confirmed by energy-dispersive X-ray analysis. X-ray and neutron diffraction experiments furthermore showed that fluorine atoms replace oxygen atoms in the BiO layers in the ratio 2:1. The additional anions form a square-mesh layer between neighboring BiF layers. The incorporation of fluorine increases the crystallographic c-axis parameter by ∼1.8 Å ( a=5.409 Å, b=5.407 Å, c=38.792 Å). The CuO 2 layers remain undistorted but the distance from the Cu atom to the apical oxygen atom of the square pyramids is decreased to 2.27 Å, with respect to 2.42 Å in the original phase. The superconducting transition temperature, determined from magnetic susceptibility measurements, was found to be 75 K. When applied to Bi(2212), the same fluorination process produced the new phase Bi 2Sr 2CaCu 2O 6F 4 with similar structural features.

Lattice and Polarizability Effects on Singlet and Triplet States in an Effective Hamiltonian for High-Tc Cuprates

From various theoretical approaches it seems to be rather well established that the Zhang–Rice (ZR) singlet state should play an important role in doped high-temperature superconducting (HTSC) oxides. Yet the conclusion that a one-band model is capable of describing many of the unusual physics observed in HTSC compounds has been questioned because the pz states of the oxygen atoms in the planes as well as those of the apical oxygen atoms hybridize with the Cu d3z2-r2 orbitals and induce a mixing of the in-plane states with the out-of-plane bands. It is shown that electron–lattice coupling and polarizability effects modify the electronic structure of the effective Hamiltonian considered here.

COORDINATION OF (4S, 9S)-4,9-DIETHYL-2,11-DIOXA-5, 8-DIAZA-1 δ5- PHOSPHATRICYCLO[6.3.0.01.5]UNDECANE WITH RETENTION OF HYDROPHOSPHORANE STRUCTURE: OBTAINING AND CHARACTERISTIC FEATURES OF ADDUCTS WITH BF3 AND ZnCl2

Reactions of tricyclic hydrophosphorane, (4s, 9S)-4, 9-diethyl-2, 11-dioxa-5, 8-diaza-1 δ5-phospha-tricyclo[6.3.0.01.5] undecane (1), with BF3×Et2O and with ZnCl2, under mild conditions have been investigated. The phosphorane ligand in the complexes obtained (2 and 3, respectively) was found to maintain its tricyclic structure, while the BF3 and ZnCl2 groups coordinate to the apical nitrogen and oxygen atoms of the trigonal bipyramid of phosphorane. The structure of the complexes has been evaluated by the following set of methods: IR and NMR 11B, 13C, 19F, 31P spectroscopy, mass-spectrometry, X-ray photoelectron spectroscopy, ultracentrifugation and elemental analysis. Spectral peculiarities and dynamic behavior in different solvents of compound 2 have been examined in detail.

Structural Transformations in the Fluorinated T* Phase

LaHo0.75Sr0.25CuO3.9, representing the intergrowth of rock salt, fluorite-type slabs, and (CuO2)-layers have been fluorinated using XeF2 as a soft fluorinating agent at temperatures ranging from 200 to 400°C. The resulting structures were studied by a combination of X-ray diffraction, electron diffraction, and high resolution electron microscopy. The samples show a complex microstructure with alternating bands of two fluorinated phases with different structures. Narrow bands (60–100 Å) of a monoclinic phase (am=a√2≈5.5 Å, bm=a√2≈5.5 Å, cm=c=12.84 Å, β=91.4°) are formed in an orthorhombic matrix. The formation of the monoclinic superstructure (A phase) is accompanied by an equal anion arrangement within the initially different rock salt and fluorite-type slabs and by cooperative displacements of the A cations. In the structure of the less fluorinated B phase, the fluorite slabs are not affected by fluorination. Fluorine atoms occupy part of the interstitial positions in the rock salt slabs with partial removal of apical oxygen atoms; this lies most probably at the origin of the incommensurate modulation of the B phase.

Effect of zinc substitution on internal friction in YBCO superconductors

The temperature spectra of internal friction for Zn doped ceramic YBa2(Cu1-xZnx)3O6+ are presented with zinc content x varying from 0 to 0.1. Two thermally activated peaks (P1, P2) near the superconducting transition temperature Tc were found to show different behaviours upon zinc doping: the peak at 110 K (P1) decreases rapidly with increasing of x while another one at 120 K (P2) has no significant change. The relaxation mechanism of P1 is interpreted in terms of jumps of apical oxygen atoms between off-centred positions produced by the Jahn-Teller effect. The effect of zinc substitution on the depression of superconductivity is also discussed.

First-principles calculations of phonon dispersion and lattice dynamics inLa2CuO4

First-principles density-functional linearized augmented plane-wave linear-response calculations are presented of the lattice dynamics of tetragonal ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}.$ Phonon frequencies and eigenvectors are obtained throughout the Brillouin zone. Generally good agreement is achieved with experiment, but frequencies of the lowest-lying branches, which involve anharmonic motion of the apical oxygen atoms parallel to the ${\mathrm{CuO}}_{2}$ planes, are underestimated. The octahedral tilt mode at the X point is found to be the most unstable mode throughout the Brillouin zone, consistent with the observed phase transition to the orthorhombic structure at low temperature. The calculated dispersion of the highest frequency ${\ensuremath{\Sigma}}_{3}$ branch is in good agreement with experiment, showing that a proposed large renormalization of the phonon spectrum by a Jahn-Teller electron-phonon interaction is unlikely.

Structural Transition in the La2−xNdxCuO4System

The structural transition in the La2−xNdxCuO4system is studied through thex=0.45, 0.5 compositions, using neutron powder diffraction. Both compositions could be refined as biphasic systems represented by theI4/mmmand theAbmaspace groups. The presence of T and T′ phases could be observed in both compositions. The apical oxygen atom in the copper coordination octahedron can be regarded as the key for structural distortion causing the transition from T to T′ structures.

Oxygen-isotope effect on the high-frequency Raman phonons inBi2Sr2CaCu2O8+δ

Raman spectra have been obtained from single-crystal samples of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}\mathrm{}}$ with varying degrees of oxygen-isotope exchange. A comparison of the high-energy $(g~400{\mathrm{cm}}^{\ensuremath{-}1})$ vibrational spectra obtained from an unexchanged sample, a partially exchanged sample and a fully exchanged crystal has been carried out to obtain an unambiguous assignment of the ${A}_{1g}$ modes at 465 and $630 {\mathrm{cm}}^{\ensuremath{-}1}.$ The 465 mode is assigned to c-axis vibrations of O(3) atoms in the Bi-O layers and the 630 mode to the O(2) apical oxygen atoms in the Sr-O layers.

Site occupancy and Tc degradation in iron substituted YBa 2(Cu 1− xFe x) 4O 8+ δ

A series of YBa 2(Cu 1− x Fe x ) 4O 8+ δ samples with x=0.00625, 0.0125, 0.01875, 0.025 and 0.05, were synthesized and characterized by resistance vs. temperature measurements, X-ray diffraction and by Mössbauer spectroscopy. In order to make the site assignment for the Fe atoms, a Rietveld refinement of the X-ray spectra was performed and a point charge calculation of the quadrupole splitting for all the possible oxygen environments around the two Cu sites was carried out, taking into account the two possible ionic states of the iron atoms that occupy those sites. We found that, for low level iron concentrations, the Fe atoms occupy only the Cu(1) sites of the structure in a fivefold pyramidal coordination, with the apical oxygen atom placed along the a-axis between the double chains of the unit cell. The presence of these extra oxygen atoms in the crystal structure could be related to the rapid T c reduction as iron concentration increases.

Zur Kenntnis eines Vanadyl-Oxocuprat-Phosphats CaCu(VO)(PO4)2 mit Cu2+ auf Positionen der VO2+-Ionen und einer kupferreicheren Phase CaCu1,13(VO)0,87(PO4)2 / On an Vanadyl Oxocuprate Phosphate CaCu(VO)(PO4)2 Containing Cu2+ at Positions of VO2+ Ions and a Copper Rich Phase CaCu1.13(VO)0.87(PO4)2

Abstract Single crystals of CaCu(VO)(PO4)2 (I) and CaCu1.13(VO)0.87(PO4)2 (II) have been prepared using solid state reactions in closed and evacuated quartz tubes. X-ray characterization led to orthorhombic symmetry, space group D16 2h-Pnma, lattice constants a, b, c (I): 9.708( 1), 9.603( 1), 6.913(1) Å and (II); 9.700(1), 9.614(1), 6.896(1) Å, Z = 4. Strongly deformed CaO8 cubes are connected by planar CuO4 polygons or square VO1+4 pyramids. Short V-O distances are features of VO2+ groups. Two point positions are occupied in a disordered manner by Cu2+ and VO2+. The replacement of Cu2+ by VO2+ at one position depends on the ratio VO2+ : Cu2+ at the neighbouring position, since otherwise there would be collisions of the apical oxygen atoms of neighbouring VO1+4 pyramids.

Structural effects of Sr substitution inLa2−xSrxNiO4+δ

Structures within the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4+\mathrm{\ensuremath{\delta}}}$ system $(0<x<~1)$ were refined by Rietveld analysis of powder x-ray-diffraction data. Our results reveal three distinct regimes that parallel energetic transformations within the series. For $0<~x<0.2,$ enthalpies of oxidation are strongly exothermic and the compounds are monophasic and orthorhombic. Hole saturation occurs for $0.2<x<0.6,$ and the symmetry decreases from orthorhombic to monoclinic. For $0.6<x<1.0,$ diffraction patterns suggest phase immiscibility between a Sr-poor, hole-saturated compound and a Sr-rich phase for which oxidation is strongly favored. Fourier synthesis maps for all compositions revealed evidence for microtwinning and positional disorder among apical oxygen atoms.

Low-energy electron spectrum in copper oxides in the multiband p-d model

An exact diagonalization of the Hamiltonian in the p-d model of a CuO6 cluster was used to obtain dependences on the model parameters of the lowest-energy two-hole terms: the energy difference between the 2p orbitals of planar and apical oxygen Δ(apex)=e(2p)−e[2p(apex)], the crystal field parameter \(\Delta _d = \varepsilon _{3z^2 - r^2 } - \varepsilon _{x^2 - y^2 } \), and the ratio of the distances between the copper atom and the apical and planar oxygen atoms d(apex)/d(pl). In the limit of large d(apex)/d(pl) and Δd, our model is equivalent to the three-band p-d model and, in this case, large singlet-triplet splitting Δe⩾1 eV is also observed. As the parameters decrease, a singlet-triplet crossover is observed. Two mechanisms are identified for stabilization of the triplet term 3B1g(0) as the ground state. It is shown that for realistic values of the parameters, reduction of the p-d model to the three-band model is limited by the low energies of the current excitations because of the presence of the lower excited 3B1g and 1A1g cluster states. Intercluster hopping causes strong mixing of singlet and triplet states far from the G point. The results of the calculations are compared with data obtained by angle-resolved photoelectron emission in Sr2CuO2Cl2.