Basal Cu-O Research Articles

Metal‐Organic Niccolite: Synthesis, Structures, Phase Transition, and Magnetic Properties of [CH3NH2(CH2)2NH2CH3][M2(HCOO)6] (M=divalent Mn, Fe, Co, Ni, Cu and Zn)

We report the synthesis, crystal structures, thermal and magnetic characterizations of a family of metal-organic frameworks adopting the niccolite (NiAs) structure, [dmenH(2)(2+)][M(2)(HCOO)(6)(2-)] (dmen=N,N'-dimethylethylenediamine; M=divalent Mn, 1Mn; Fe, 2Fe; Co, 3Co; Ni, 4Ni; Cu, 5Cu; and Zn, 6Zn). The compounds could be synthesized by either a diffusion method or directly mixing reactants in methanol or methanol-water mixed solvents. The five members, 1Mn, 2Fe, 3Co, 4Ni, and 6Zn are isostructural and crystallize in the trigonal space group P31c, while 5Cu crystallizes in C2/c. In the structures, the octahedrally coordinated metal ions are connected by anti-anti formate bridges, thus forming the anionic NiAs-type frameworks of [M(2)(HCOO)(6)(2-)], with dmenH(2)(2+) located in the cavities of the frameworks. Owing to the Jahn-Teller effect of the Cu(2+) ion, the 3D framework of 5Cu consists of zigzag Cu-formate chains with Cu-OCHO-Cu connections through short basal Cu-O bonds, further linked by the long axial Cu-O bonds. 6Zn exhibits a phase transition probably as a result of the order-disorder transition of the dmenH(2)(2+) cation around 300 K, confirmed by differential scanning calorimetry and single crystal X-ray diffraction patterns under different temperatures. Magnetic investigation reveals that the four magnetic members, 1Mn, 2Fe, 3Co, and 4Ni, display spin-canted antiferromagnetism, with a Néel temperature of 8.6 K, 19.8 K, 16.4 K, and 33.7 K, respectively. The Mn, Fe, and Ni members show spin-flop transitions below 50 kOe. 2Fe possesses a large hysteresis loop with a large coercive field of 10.8 kOe. The Cu member, 5Cu, shows overall antiferromagnetism (both inter- and intra-chains) with low-dimensional characteristics.

Anomalies of conductivity behavior near the paramagnetic–antiferromagnetic transition in single-crystals [formula omitted

The temperature dependences of resistance, R ( T ) , of two single-crystals La 2 CuO 4 + δ samples have been studied with the aim to detect a possible change in the R ( T ) behavior induced by paramagnetic–antiferromagnetic (PM–AFM) transition. One of the samples with δ ≲ 0.01 , was fairly homogeneous in oxygen distribution (not phase-separated) with Néel temperature T N ≈ 266 K . Conductivity of this sample has been determined by Mott's variable-range hopping below T N . The other, far less resistive, sample with δ ≈ 0.05 , was inhomogeneous (phase-separated) showing both PM–AFM ( T N ≈ 205 K ) and superconducting ( T c ≈ 25 K ) transitions. It is found that for the homogeneous sample the resistivity decreases above T N far faster with temperature than below it (for both directions of measuring current, parallel and perpendicular to basal CuO 2 planes). A similar behavior of conductivity near PM–AFM transition is also found for the phase-separated and less resistive sample. In this case a clear kink in R ( T ) curve near T N ≈ 205 K can be seen. Furthermore, a transition to metallic ( dR / dT > 0 ) behavior occurs far enough above T N . The observed behavior of the samples studied is related to increased delocalization of charge carriers above T N . This is in accordance with decrease in the AFM correlation length and corresponding enhancement of the hole mobility above T N known for low-doped lanthanum cuprates.

Metal–Organic Perovskites: Synthesis, Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn, Fe, Co, Ni, Cu, and Zn; C(NH2)3= Guanidinium)

We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal-organic perovskite ABX(3), [C(NH(2))(3)][M(II)(HCOO)(3)], in which A = C(NH(2))(3) is guanidinium, B = M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO(-). The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2(1). In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti-anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)(3)](-) frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn-Teller effect of Cu(2+) results in a distorted anionic Cu-formate framework that can be regarded as Cu-formate chains through short basal Cu-O bonds linked by the long axial Cu-O bonds. These materials show higher thermal stability than other metal-organic perovskite series of [AmineH][M(HCOO)(3)] templated by the organic monoammonium cations (AmineH(+)) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.

Catena-Poly[[pyridinecopper(II)]-μ-N-[(2-oxido-1-naphthyl)methylene]glycinato

In the title compound, [Cu(C13H9NO3)(C5H5N)], the CuII atom is coordinated in a distorted square-pyramidal geometry, with two N and two O atoms in the basal positions and one O atom in the apical position. The apical Cu—O bond [2.3520 (16) Å] is much longer than the basal Cu—O and Cu—N bonds [1.9139 (14)–2.0136 (17) Å]. The carboxyl­ate group bridges CuII atoms, forming a zigzag chain along the a axis.

Intrinsic magnetic centers and microdomains in oxygen-deficientYBa2Cu3O6.5andTmBa2Cu3O6+x

We measured the influence of magnetic centers (MC{close_quote}s) on the nuclear relaxation in 123 compounds using {sup 169}Tm NMR and {sup 63}Cu NQR in TmBa{sub 2}Cu{sub 3}O{sub 6+x} (x=0, 0.2, 0.3, 0.4, 0.5, 0.6), and {sup 89}Y NMR and {sup 63}Cu NQR in YBa{sub 2}Cu{sub 3}O{sub 6.5}. Particular attention is paid to the region 0.5{le}x{le}0.6 in order to deal with both a well-defined ortho II structure and a high enough MC concentration. The experiments reveal a two-component nuclear relaxation in the superconducting compounds at temperatures above 1 K, especially pronounced in the x=0.5 samples. The relaxation data give evidence for microphase separation due to oxygen disorder in the basal CuO{sub x} planes. In the well-annealed samples, the MC{close_quote}s predominating the relaxation have S=1/2. The concentrations of these MC{close_quote}s are of the order of 3{percent} in the disordered (nonsuperconducting) microphase and {approximately}0.3{percent} in the ordered (superconducting) one. {copyright} {ital 1997} {ital The American Physical Society}

Influence of Oxygen and Strontium Content on the Pr 2- ySr yCuO 4-δ System

Samples of the Pr 2- y Sr y CuO 4-δ system have been prepared with accurate control of the oxygen content. T'-T*-T-Sr 2CuO 3 single phases are only obtained in air for y = 0, 0.4, 1, and 2, respectively. The microstructural study indicates that oxygen vacancies are randomly disordered for y = 0.4 (Pr 1.6Sr 0.4CuO 3.94) but an incommensurable modulated structure appears for y = 1 (PrSrCuO 3.6) as a consequence of ordering of anionic vacancies along [110]. Two new materials are obtained when air-synthesized materials are annealed under oxygen atmosphere. For y = 0.45, an orthorhombic T* phase is isolated. For y = 1, a material showing ordered oxygen vacancies in the basal Cu-O planes is described.

Superconducting critical temperature, oxygen ordering and devil's staircase in YBa 2Cu 3O 6+ [formula omitted

Using the appropriate extension of the three-band Hubbard model to describe the basal CuO 2+ x subsystem of YBa 2Cu 3O 6+ x , for a certain region of parameters we obtain: a) the number of holes n H in the superconducting planes and T c as a function of x have a plateau, b) using a structural model to include the effect of interatomic OO repulsions, there is a transition from structures without first or second nearest-neighbor O atoms (HS) to order in chains (CS) at x ∼ 0.45 for increasing x . We also study the structural model for values of x for which the CS are stable.

Intermediate steps in YBa2Cu3O7?x synthesis by sol-gel method: YBaCuO oxycarbonate

High T c -superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders by sol-gel synthesis. However this synthesis involves intermediate phases formation which impedes the obtention of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800° and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure—SG P4/mmm—similar to that of the parent oxide with a stoichiometry close to YBa2Cu2.95(CO3)0.35O6.6. The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the xerogel precursor is annealed at 925°C in O2 or at 800°C in Ar, then in O2. The grain growth and microstructure development of YBaCuO have also been investigated and compared using two different powders, i.e. sol-gel route and commercial powder from Hoechst.

Synthesis of YBa 2Cu 3O 7−x by sol-gel route formation of YBaCuO oxycarbonate intermediate

High- T c superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders. However, sol-gel synthesis of this oxide involves the formation of intermediate phases which impedes the obtaining of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800 and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure — SG P4/ mmm — similar to the parent oxide with a stoichiometry close to YBa 2Cu 2.95(CO 3) 0.35O 6.6. The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the sol-gel precursor is annealed at 925°C in O 2.

Crystal structure of the tetragonal superconductor CaLaBaCu 3O x (6.69⩽ x⩽6.94)

The room-temperature crystalline structure of CaLaBaCu 3O x has been studied by high-resolution neutron powder diffraction. The profile analysis on specimens with oxygen contents x ranging from 6.94 (80 K superconductor) to 6.69 (non-superconductor) indicates that these compounds are isostructural with the non-superconducting tetragonal YBa 2Cu 3O y ( y<6.4, space group P4/mmm), and that Ca and La atoms occupy the Y and Ba sites of the YBCO structure almost randomly. The changes in bond lengths with x are essentially the same as those of YBa 2Cu 3O y . The superconductivity of the system can be understood in terms of the charge transfer between the superconducting CuO 2 plane and the basal CuO plane, which would explain the variation of T c with the change in oxygen content. The similarities and differences between this system and YBa 2Cu 3O y are discussed.

Monte Carlo study of oxygen thermal desorption from YBa 2Cu 3O 6 + x compound

A Monte Carlo study of oxygen thermal desorption from the free edges of a basal CuO plane of YBa 2Cu 3 O 6 + x compound is performed. Perfectly ordered copper-oxygen chains with the imposed nearest and next-nearest neighbour oxygen-oxygen interactions and energy barriers for oxygen diffusion and desorption are the starting conditions of the simulation. The characteristic peak related to the destruction of an ordered OI structure is found. The experimental data can be described if the surface reaction is considered as rate-limiting and readsorption of oxygen is introduced in the calculation.

Observation of superstructures on the surfaces of sputtered YBa2Cu3O7−x thin films by low-energy electron diffraction

The surface structures of sputtered c-axis-oriented YBa2Cu3O7−x (YBCO) thin films were studied by means of low-energy diffraction (LEED). The in situ analysis, without exposing the films to air, showed the formation of superstructures of the YBCO (001) 2×2 and YBCO (001) 4×1 type. Air-exposed surfaces showed no LEED pattern but could be transformed into the YBCO (001) 2×2 structure by annealing at 500 °C. The observed superstructures are interpreted as being due to oxygen loss of the YBCO surface region, causing an oxygen vacancy ordering in the basal CuO planes.

Oxygen ordering and superconducting temperature of YBa 2Cu 3O 6+x

An analysis of the relation between the superconducting temperature T c and the oxygen ordering in the basal CuO x plane of the high-temperature superconductor YBa 2Cu 3O 6+ x is presented. Superconductivity in the CuO 2 planes of a non-stoichiometric compound is assumed to appear locally, just in the neighbourhood of the regions with O-Cu-O chains. Bulk superconductivity occurs when these regions form a percolation network characterized by the minimal width d of a percolation path with T c depending on d. The evaluation of d by the cluster field method allows one to obtain quantitative agreement of the calculated and experimental T c( x) dependences.

LATTICE GAS SIMULATION OF OXYGEN ORDERING IN YBa2Cu3O6+x SHOWING DYNAMICAL SCALING

A 2-dimensional anisotropic lattice model for the oxygen ordering in the high Tc superconductor of the YBa 2 Cu 3 O 7+x type is shown to exhibit an ordering dynamics that obey algebraic growth laws which depend on whether it is an Ortho-I or Ortho-II phase. It is possible to relate this dynamical scaling behavior to a similar scaling in the experimentally observed temporal variation of the superconductivity transition temperature and hence suggesting a specific coupling between the coherence of oxygen order in the basal Cu-O planes and the superconducting state. Furthermore it is possible to explain the variation in the transition temperature with the oxygen density x by a phase mixing model of Ortho-II/Ortho-I domains and an assumption about the charge transfer between the basal and superconducting plane.

Relation between superconducting transition temperature and oxygen ordering in YBa2Cu306+x

THE superconducting transition temperature, Tc, of the ceramic high-temperature superconductor YBa2Cu3O6+xis known to depend not only on the oxygen stoichiometry x (0 < x < 1) but also on the specific ordering of the oxygen atoms in the basal CuO planes1–7. Here we present computer simulations of the formation of oxygen-ordered domains of orthorhombic structure in the basal CuO plane using a microscopic model of the oxygen ordering. Together with a minimal-model assumption for the charge transfer, our calculations strongly suggest that it is these domains that are responsible for the characteristic variation of Tc(x). Our results lead to a theoretical prediction of Tc(x) that is in close quantitative agreement with experiments.

Dynamical scaling of oxygen ordering in YBa2Cu3O7- delta.

Comptuer simulation on a two-dimensional anisotropic lattice-gas model of oxygen ordering in high-${\mathit{T}}_{\mathit{c}}$ supeconductors of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ -type shows that the ordering dynamics obey algebraic growth laws which are different in the ortho-I and ortho-II phases. It is possible to relate this dynamical scaling behavior to a similar scaling in the experimentally observed temporal variation of the superconductivity transition temperature, hence suggesting a specific coupling between the coherence of oxygen order in the basal Cu-O planes and the superconducting state.

Low temperature internal friction spectrum of polycrystalline Y-Ba-Cu-O

The internal friction (IF) and the elastic modulus of polycrystalline YBa 2Cu 3O x specimens have been measured as a function of the temperature between 80K and 300K in the kilohertz frequency range. IF spectrum shows three reproducible maxima at 90K, 115K and 220K respectively. The 90K IF peak is not related to the superconducting transition because it is still observed after high temperature annealings in vacuum, which lead to the disappearance of the superconductivity. The 115K peak is due to a relaxation mechanism, which can be interpreted by oxygen atom jumps within the basal Cu-O plane. The broad 220K maximum, which was also observed by ultrasonic measurements, seems to be affected by the sample preparation: sintering conditions. Finally, a frequency hysteresis has been observed in every superconducting sample, which accounts for an anomalous behaviour of the elastic modulus.

Structural phase relations in the La2−x−yLnxSryCuO4−δ, the Ln2−xCexCuO4−δ and the Ln2−ySryCuO4−δ systems: A powder neutron diffraction study

High resolution powder neutron diffraction is used to study the structural phase diagram of the systems Ln 2− x Ce x CuO 4−δ Ln 2− y Sr x CuO 4−δ and La 2− x − y Ln x Sr y CuO 4−δ , in particular the structural variants T′ (N CuO 4 structure), T (K 2 NiF 4 structure) and O (orthorhombically distorted T structure). The T′ phase which is characterised by flat (CuO 2 ) ∞ sheets is stable to both Ce 4+ and Sr 2+ doping and undergoes no structural phase transitions above 1.5 K, in contrast to the tilting mechanism operative in the T structure. This has important implications for the mechanism of antiferromagnetic ordering of Cu spins in the T′ phase because of the lack of static structural distortions. The value of the tolerance factor is within the stability range of the T′ phase and the basal plane Cu-O bond length is not compressed as in the T phase. Transport measurements indicate that the undoped and the hole doped T′ phases have much higher resistivities and activation energies than their O/T analogues. In contrast, electron doping of the T′ phase leads to reduced resistivities and activation energies, similar to hole doped O phases. The O phase of La 2 CuO 4 transforms to the tetragonal T′ phase at a Pr 3+ doping level of x = 0.6 . Doping of the critical composition La 1.4 Pr 0.6 Cu 4 with Sr 2+ induces a transition back to the T phase with abrupt changes in the lattice dimensions. The composition La 1.2 Pr 0.6 Sr 0.2 CuO 4 shows a T→O phase transition upon lowering the temperature [( a − b )/( a + b )] = 2.07(2) x 10 −3 at 1.5 K ], compared with La 1.8 Sr 0.2 CuO 4 and La 1.15 Pr 0.6 Sr 0.25 CuO 4 which are tetragonal down to liquid helium temperatures. Furthermore, La 1.2 Pr 0.6 Sr 0.2 CuO 4 is superconducting with a transition temperature of T c = 29.5 K . The effect of Sr 2+ doping on La 2− x Ln x CuO 4−δ is t increase the critical x c value for the O→T′ phase transition. Furthermore, the critical T d value for the T→O phase transition also increases compared with the La 2− y Sr y CuO 4−δ system, extending the stability range of the O phase into higher values of the formal Cu oxidation state (up to an effective value of +2.25).

Band structure of YBa 2Cu 3O x in relation with the oxygen vacancy distribution

The electronic band structure and the density of states of the YBa 2Cu 3O x superconductor are computed for x = 7 using the extended Hückel scheme. Two ordered arrangements of the oxygen vacancies in the basal Cu-O planes have been considered. It is shown that the oxygen vacancy distribution plays a major role in the electronic structure of this compound and deeply influences the density of states at the Fermi level.