Synthesis and structural characterisation of YSr2Cu3−xMxO7±y (M = Fe, Ti, Al, Co, Ga, Pb; 0.5 < x < 1), and the non-existence of the parent phase YSr2Cu3O7

The possibility of synthesising phases of general composition YSr₂Cu_3−xM_xO_7±y has been examined for a wide variety of metals M. Single phase tetragonal products (typically a = 3.83 Å, c = 11.5 Å) can be synthesised provided that the concentration of M is sufficiently large (x > 0.5), and M has a preference for the Cu(1) sites (the four-coordinate sites) of the YBa₂Cu₃O₇, structure. Neutron diffraction has demonstrated that a high degree of disorder exists in the (001) planes containing the M cations and, in particular, significant oxygen displacements are found. The non-existence of the unsubstituted variant YSr₂Cu₃O₇ is explained in terms of the high degree of compressive stress on the Cu(1) sites which could exist in such a material.     

Ba2(Cu0.7Cd0.3)O3: a new double-center perovskite structure

A new phase of Ba₂(Cu_0.7Cd_0.3)O₃ has been discovered. The composition of the phase was examined by energy dispersive X-ray spectrometry. The phase exhibits a tetragonal structure with lattice constants a=4.077 Å and c=7.970 Å, and its space group is P4/mmm. We propose a double-center-perovskite structure model, which is similar to that of (Ba, Sr)₂CuO₂(CO₃) [1, 2]. The simulated XRD is close to the observed one. The analysis results of elemental analyzer and Fourier transform infrared spectrometer reveal that the phase does not contain CO . The cuprate is an insulator, but its new cell structure and the large size of the CuO₂ planes seem to suggest a good reason for researching new superconductors. The roles of CdO in the solid state reaction and the formation of the new phase are discussed.     

Observation of superconductivity (Tc = 50 K) in a new tetragonal alkaline-earth cuprate Sr0.8Ba1.2CuO3+δ, synthesised at ambient pressure

The ambient-pressure synthesis of a new tetragonal alkaline-earth superconducting cuprate, Sr_0.8Ba_1.2CuO_3+δ, from a cupro-oxycarbonate is reported. Magnetic-susceptibility measurements show the presence of a superconducting transition 50 K in a post-annealed sample. The crystal structure, refined from time-of-flight powder neutron-diffraction data was found to have an oxygen-deficient La₂CuO₄-type tetragonal T structure (a = 3.8988(3) Å and C = 12.815(3) Å) with oxygen vacancies located within the CuO₂ planes. Ordering of these oxygen vacancies is responsible for the observation of a superlattice in both neutron- and electron-diffraction measurements. An interpretation of the electron-diffraction patterns suggests that the superlattice in Sr_0.8Ba_1.2CuO_3+δ and also in the isostructural superconductor Sr₂CuO_3+δ are of an identical nature.     

Preparation and superconducting properties of YBa2Cu4O8

Samples of YBa₂Cu₄O₈ are prepared by means of a new high oxygen pressure technique employing oxygen-HIP. Both magnetization and resistivity measurements show the superconductivity transition at 82.5 K. The lower and upper critical fields of YBa₂Cu₄O₈ are obtained from the magnetization measurements. The Ginzburg-Landau parameters, ζ(0)=17 Å, λ(0)=2400 Å, are estimated from these results.     

Synthesis and superconductivity of (Pb0.5Cd0.5)(Sr0.9R0.1)2(R′0.7Ce0.3)2Cu2Oy

New Pb based layered superconducting cuprates (Pb_0.5Cd_0.5)(Sr_0.9R_0.1)₂(R′_0.7Ce_0.3)₂Cu₂O_y with (R,R′) = (Eu,Gd), (Eu,Dy) and (Nd,Gd), have been successfully synthesized. The structure of (Pb,Cd)(Sr,Eu)₂(Gd,Ce)₂Cu₂O_y is determined using XRD data. The X-ray powder diffraction pattern can be indexed by the tetragonal structure with lattice parameters a = 3.8250 Å, C = 29.2235 Å and a space group of I4/mmm. The effects of synthesis conditions on (Pb,Cd)(Sr,Eu)₂(Gd,Ce)₂Cu₂O_y are studied and a T_c(onset) of 26 K is obtained by resistance measurements. It is isostructural to the superconductor (Pb,Cu)(Sr,Eu)₂(Eu,Ce)₂Cu₂O_y.     

Rietveld refinement and electronic structure studies for the Sm2FeMnO6 new complex perovskite

We report synthesis and crystalline structure study of the Sm₂FeMnO₆ new complex perovskite, by X-ray diffraction experiments and through the application of Rietveld refinement. Results revealed the crystallization of system in a structure given by Pmn21 (#31) space group and lattice parameters a=7.621(1) Å, b=5.675(3) Å and c=5.378(3) Å. Ab initio calculations of density of states (DOS) and electronic structure were carried out for this perovskite-like system by the density functional theory (DFT) and using the full-potential linearized augmented plane waves (FP-LAPW) method. All calculations were carried out using spin polarization. Material evidences a conductor-like character, predominantly due to d–xy Fe orbital of the spin down channel. Magnetic response of system has contributions of Fe and Mn spin up orientation. The calculated magnetic moment in cell was 34.48 μ_B and the magnetic moment in interstitial was 1.54 μ_B.     

Thermally activated flux dissipation in c-axis-oriented YBa2Cu3O7−δ ultrathin films

The resistive transitions of ultrathin YBa₂Cu₃O_7−δ (YBCO) films with thicknesses 75 and 200 Å were studied under magnetic fields. For the 75 Å film under a 5 T parallel magnetic field (Hbab-plane), no broadening of the resistive transition occurred. In the perpendicular magnetic field (H ab-plane), the broadening of the resistive transition of the 75 Å film is larger than that of the 200 Å thick film. The flux activation energy U was found to be linearly dependent on the temperature and logarithmically dependent on the magnetic field for both 75 and 200 Å films, which means the two samples have a two-dimensional vortex lattice. Furthermore, the activation energy U also increased with the film thickness, indicating that the magnetic correlation length in the c-axis direction l_c is larger than the 200 Å for bulk YBCO.     

LEED crystallographic analysis for the Rh(111)-(2 × 1)---O surface structure

A tensor LEED analysis is reported for the Rh(111)-(2 × 1)---O surface structure in which atoms in the O overlayer chemisorb close to the regular (fcc type) three-fold hollow sites for half-monolayer coverage. The structure shows significant relaxations: for example, a buckling of about 0.07 Å is indicated in the first metal layer and O appears to displace laterally by about 0.05 Å. The individual O---Rh bond lengths are around 2.01 and 1.92 Å to top layer Rh atoms, which bond to two and one O atoms, respectively, but the average value (1.98 Å) is close to that in bulk RhO₂ (1.96 Å). Comparison is also made with the previously determined O---Rh bond lengths in the Rh(110)-p2mg(2 × 1) surface structure.     

Synthesis and characterization of HgBa2Can−1CunO2n+2+δ (n = 1, 2, and 3)

We have successfully prepared the first three members of the mercury-based superconducting compounds Hg--- Ba₂Ca_n−1Cu_nO_2n+2+δ, namely Hg---1201, Hg---1212 and Hg---1223 with high purity and very good quality. T he influence of the synthesis parameters is studied in detail. Using the sealed quartz tube method, very simple procedures are found to ensure a 100% reproducibility of nearly 100% pure Hg---1201 and 85–90% Hg---1212 and Hg---1223. Oxygen annealing of the sample Hg---1201 at 300°C for 18 h results in an enhancement of its critical temperature up to 97 K. The symmetry of the first and second members is tetragonal with lattice parameters a = 3.8831 (1) Å, C = 9.5357 (2) Å, and A = 3.8624 (1) Å, C = 12.7045 (2) Å, respectively. X-ray diffraction lines of Hg---1223 can be indexed in a tetragonal cell with a = 3.8564 (1) Å and C = 15.8564 (9) Å as well as in an orthorhombic cell with lattice parameters a = 5.4537 (1) Å, B = 5.4247 (1) Å, and C = 15.8505 (7) Å.     

Antiferromagnetically coupled multilayers of (Co90Fe10 tF/Ru tRu)×N and (Ni81Fe19 tF/Ru tRu)×N prepared by ion beam deposition

This work describes multilayers of Co₉₀Fe₁₀ t_F/Ru t_Ru/Co₉₀Fe₁₀ t_F and Ni₈₁Fe₁₉ t_F/Ru t_Ru/Ni₈₁Fe₁₉ t_F (20 Åt_F200 Å) prepared at ambient temperature by ion beam deposition on Si/SiO₂ 3 kÅ substrates. The samples exhibited a maximum antiferromagnetic coupling with t_Ru=3.2 Å and M–H curves characterized by zero remanent magnetic moment and enhanced saturation field. Antiferromagnetic peaks were present with t_Ru17 and 30 Å.     

Superconductivity at 27 K in Nb-1222 cuprate: NbSr2(GdCe)2Cu2Oy

Samples of NbSr₂Gd_2−xCe_xCu₂O_y have been prepared by solid state reaction. The X-ray powder diffraction patterns show that a single phase can be formed in the composition range of 0.49<x<0.51 that has TaSr₂ (NdCe)₂Cu₂O_y structure [1], the cell parameters being a=3.8669±0.0046 Å and c=28.742±0.0048 Å. Superconductivity was found by resistance measurements in the sample of NbSr₂Gd_1.5Ce_0.5Cu₂O_y: the onset transition temperature is 27.4 K, and zero resistance appears at approximately 13 K.     

A new layered cuprate superconductor: GaSr2(Y, Ce)2Cu2O9−δ

A new layered cuprate compound with a nominal composition of GaSr₂Y_2−xCe_xCu₂O_9−δ has been prepared. It crystallizes in a tetragonal lattice with cell parameters: a = 3.812 Å, c = 28.16 Å. The structure of the compound belongs to the same family of 1222 phase and is derived from that of GaSr₂LnCu₂O₇ by replacing the single Ln³⁺ layer with a double fluorite (Y, Ce)₂O₂ layer. Like other parent cuprate compounds of superconductors, the as-prepared samples showed antiferromagnetic and semiconducting behavior. After treatment under high oxygen pressure, the samples exhibited bulk superconductivity with transition temperatures between 12–14 K.     

A 62 K superconductor with an original structure: Sr4−xBaxTlCu2CO3O7

A new superconductor with an original structure, Sr_4−xBa_xTlCu₂CO₃O₇ has been isolated for 1≤x≤2. It crystallizes in a A-type space group with a=3.84 Å≈a_p, b≈8 × a_p, c≈17.0 Å. The HREM study of this new curprate shows that it derives from the Tl_0.5Pb_0.5Sr₄Cu₂CO₃O₇ structure by a shearing mechanism. Indeed, it can be described as ribbons of the latter structure, four octahedra thick, shifted c/2 with respect to each other. It results in infinite single perovskite layers waving along the (001) plane and connected through flat mixed layers [(TIO)₄(CO)₄]∞. This oxycarbonate can also be described as a 1201-type structure in which the [TIO]∞ layers are replaced by mixed [(TIO)₄(CO)₄]∞ layers. The susceptibility measurements show that this cuprate exhibits a significantly higher critical temperature than the 1201-cuprates. The T_{c onset} of the synthesized phase is 56 K and it can be increased up to 62 K by hydrogen annealing; in the same way, one observes a large Meissner volume fraction of 35% at 4.2 K.     

Duree de vie et depolarisation par collisions avec l''helium des niveaux 5P3/2 Et 5P1/2 de l''ion Cd

The lifetimes of the Cd⁺ 5²P_3/2 and 5²P_1/2 states have been measured by the Hanle effect. The Cd⁺ ions are produced in a d.c. discharge in cadmium vapor, with helium as buffer gas. The results are: τ(5²P_3/2) = (2.60±0.20) ×10⁻⁹sec, and τ(5²P_1/2) = (3.05 ± 0.13) × 10⁻⁹sec.

We measured also the cross sections for the destruction of the orientation in the 5²P_1/2Cd⁺ state (<5Ų), of the orientation (18±10Ų) and of the alignment (46±10Ų) in the 5²P_3/2 state due to collisions with the helium atoms.     

A new family of Pb based 1222 cuprates (Pb,Mg)(Sr,R)2(R′,Ce)2Cu2Oy (R,R′:rare-earth element)

A series of new Pb based 1222 cuprates, (Pb,Mg)(Sr,R)₂(R′,Ce)₂Cu₂O_y, has been reported in this paper. The structure of the compounds has been determined by the powder X-ray diffraction method. It crystallizes in a tetragonal lattice with space group I4/mmm and lattice parameters a = 3.818 Å and C = 29.085 Å for R,R′=Nd,Dy. XRD results show that the structure is isostructural with (Pb,Cu)(Sr,R)₂(R′,Ce)₂Cu₂O_y with the (Pb,Mg)O layer replacing the (Pb,Cu)O layer. Resistivity measurements show that the compounds are semiconductors under the present experimental conditions.     

LEED crystallographic analysis for the structure formed by 2 ML of O at the Zr(0001) surface

A tensor LEED analysis is reported for the Zr(0001)-(1 × 1)-O surface which involves oxygen at a total coverage of 2 monolayers. The structure is indicated to have two layers of O: one forms an overlayer in which the O atoms bond to hollow sites of three-fold coordination on the regular metal surface, while the other layer has the O atoms in tetrahedral hole sites between the first and second metal layers. The stacking sequence, designated as (C)B(A)AB... corresponds to the first three layers of anion-terminated cubic ZrO₂, although some lateral compression is needed for superposition on the regular hcp Zr structure. The absorption of O in the tetrahedral holes results in a significant expansion in the first-to-second Zr---Zr interlayer spacing to about 3.44 Å from the bulk vaue of 2.57 Å. The O---Zr bond lengths are estimated to equal 2.07 Å for the overlayer O atoms, and 2.21 Å for the O atoms in tetrahedral hole sites. Comparisons are made with the structures of the corresponding 0.5 and 1 ML surfaces formed by the O/Zr(0001) system.     

Limitations of interface sharpness in a-Si:H/a-SiC:H multilayers

The goal of this work is to study the sharpness of interfaces in amorphous silicon based compositional multilayers (superlattices) by a number of different techniques and to discuss their limitations. From monitoring plasma transients during glow discharge deposition of a-Si:H/a-SiC:H multilayers, a lower limit of 3 Å for interface sharpness is estimated. Transmission electron microscopy (TEM) images yield an upper limit of 5–10 Å. These images directly show the increase in undulation from the substrate towards the film surface. From the comparison of simulated X-ray diffraction (XRD) spectra with measurements the interface sharpness is found to be between 5 and 8 Å. Using a series of multilayers with increasing number of interfaces, structural characteristics of the interfacial region can be extracted. For example, infrared absorption spectroscopy (FTIR) and elastic recoil detection (ERD) lead to an estimate of 1.1 × 10¹⁴ cm^-2 additional hydrogen atoms per single interface.     

Surface and interface properties of epitaxial Fe3O4 films studied by Mössbauer spectroscopy

Stoichiometric Fe₃O₄ films have formed epitaxially on -Al₂O₃ and MgO single-crystal substrates by a reactive vapor deposition method. In order to apply conversion electron Mössbauer spectroscopy depth-selectively, a 5–7 Åthick probe layer containing ⁵⁷Fe was formed at various depths in inactive ⁵⁶Fe₃O₄ matrix films. At the topmost surfaces and also at the interfaces, the essential electronic features of bulk Fe₃O₄ are retained, including a rapid electron hopping between the Fe²⁺ and Fe³⁺ ions at B sites. Minor depth-dependent changes are confined to a few outermost atomic layers, and the changes depend on the orientation and the lattice mismatch with the substrate. For (111) growth on -Al₂O₃, the surface layer seems to be strongly relaxed to reduced the electric polarization, while a high density of defects seems to be concentrated at the interface with -Al₂O₃. For (001) growth on MgO, the surface retains the spinel lattice though slightly oxidized, while the interface with MgO has good crystallinity and stoichiometry. An enhanced thermal fluctuation of the Fe³⁺-spins in contact with the MgO substrate and in the topmost surface layer can be seen in their reduced magnetic hyperfine field at 300 K.     

Strain effects in epitaxially grown La0.7Sr0.3MnO3 thin films

We report here on an X-ray absorption study of La_0.7Sr_0.3MnO₃ films epitaxially grown on SrTiO₃ substrate. The local organization around Mn in oriented films with 600 Å in thickness was investigated by polarized Extended X-ray Absorption Fine Structure. The angle between electric field vector and film surface was set equal to 5° and 70° to investigate almost independently the contribution of the manganese neighbors situated in and out of the film plane. The first neighboring shell oxygen is found to be the same in both geometries, but small changes in the next neighboring contribution are observed. These changes are associated with variation in the Mn–Mn bond length. A small in-plane elongation (3%) is observed in the constrained films with respect to the unconstrained case.     

Charge ordering in Eu3S4 determined by the valence-difference contrast of synchrotron X-ray diffraction

Synchrotron X-ray diffraction study for single crystals of Eu₃S₄ has revealed that a Th₃P₄-type structure transforms to a charge-ordered one at T_c=188.5 K. The crystal structures of Eu₃S₄ at T=300, 180 and 160 K were determined in the least-squares refinements with the Mo K intensity data. The valence-difference contrast method was applied at the L_II absorption edge of Eu, utilizing a large difference in anomalous scattering factors between Eu²⁺ and Eu³⁺. The cation distribution of Eu²⁺ and Eu³⁺ was determined by crystal-structure analyses based on the intensity data collected at two wavelengths of λ=1.6312 and 1.6298 Å.The least-squares structural refinements suggest that the most plausible atomic arrangement is [Eu³⁺]^4a[Eu²⁺Eu³⁺]^8dS₄. The charge-ordering scheme is that a half of Eu³⁺ ions occupy the whole 4a sites in the crystal structure, while the remaining half of Eu³⁺ ions mix with Eu²⁺ in the 8d sites. The scheme is also supported by the energy dependence of Bragg intensities for 400 and 004 reflections.