I. Rasines

Excess electrical conductivity in polycrystalline Bi-Ca-Sr-Cu-O compounds and thermodynamic fluctuations of the amplitude of the superconducting order parameter

Abstract Measurements of the rounding effects on the electrical resistivity above the superconducting transition in Bi-Ca-Sr-Cu-O polycrystalline compounds are reported, to our knowledge for the first time in this HTSC system. These effects are analyzed in terms of thermodynamic fluctuations of the amplitude of the superconducting order parameter (SCOPF). In the mean-field-like region, the experimental critical exponent seems to be compatible with an order parameter of two components (2d) fluctuating in two dimensions (2D). This contrasts with previous results for A-Ba-Cu-O ( A = Y , Ln ) and Ln-M-Cu-O ( M = Ba , Sr ) superconductors, where SCOPF seem to be 2d-3D in all the different dynamic critical regions.

Neutron diffraction study of the magnetic structure of Er2BaNiO5

Abstract The magnetic ordering of R2BaNiO5 (R Y, Er) has been studied by neutron powder diffraction. The crystal structure of the Er compound was refined at T = 50 K, above the ordering temperature, in the orthorhombic space group Immm. The structure contains isolated chains of flattened NiO6 octahedra along the a axis. For Y2BaNiO5 no magnetic ordering has been detected between room temperature and 1.5 K. For Er2BaNiO5 the neutron diffraction patterns below 33 K show magnetic peaks that reach a maximum in intensity at T = 4 K. The magnetic structure is antiferromagnetic and can be described in terms of a single propagation vector k = [ 1 2 ,0, 1 2 ] . The group-theory analysis gives the possible magnetic structures compatible with the crystal symmetry. The structure that leads to a best fit of the neutron data involves only one irreducible representation for both Ni and Er sublattices. Ni atoms are antiferromagnetically coupled along the Ni-O-Ni chains parallel to the a axis direction. The coupling between chains is ferromagnetic along the b direction. The Er sublattice, constituted by two Er atoms per primitive cell, can be described as a (AxAz) mode. The magnetic moments for Ni2+ and Er3+ are 1.54(5) and 7.9(1) μB at T = 4 K, respectively. The thermal evolution of the magnetic moments suggests that the Ni-Er interactions are predominant between 33 and 16 K, and, at lower temperatures, cooperative Er-Er interactions lead to a saturation of the Er3+ ordered moments at T = 4 K. The different behaviour of the Y and Er compounds demonstrates the important role that the presence of a paramagnetic R3+ ion (e. g. Er3+) plays in the three-dimensional ordering of both Ni2+ and R3+ sublattices.

Preparation, neutron structural study and characterization of BaNbO3: a Pauli-like metallic perovskite

The cubic perovskite BaNbO{sub 3} has been prepared in sealed ampoules from mixtures of BaO, NbO{sub 2} and Nb, in different ratios. the Ba and O contents, determined by thermogravimetry, X-ray and neutron diffraction, are always close to the stoichiometric values. Resistivity and magnetic measurements indicate that BaNbO{sub 3} behaves as a metal, showing a high room-temperature resistivity and a Pauli-like behavior. This suggests an important electronic delocalization in this system. The enhanced value of the Pauli susceptibility indicates a relatively strong correlation among the 4d electrons in narrow {pi} conduction bands of T{sub 2g} parentage.

The series of spinels Co3−sAlsO4 (0 < s < 2): Study of Co2AlO4

Abstract Co(III) of Co 3 O 4 can be gradually replaced by Al to produce the series of spinels Co 3− s Al s O 4 (0 s Fd 3 m (No. 227), Z = 8. For Co 2 AlO 4 ( s = 1), a = 8.086(1)A, U = 528.7(2)A 3 , D x = 5.25 Mgm −3 , u = 0.264, and 27% of Al in 8( a ) positions, R = 0.031. The frequencies of the observed ir absorption bands of Co 2 AlO 4 are also presented.

Crystal structure of Nd2Te4O11: An example of a rare earth tellurium oxide

Abstract The preparation and growth of single crystals and crystal structure determination of Nd2Te4O11 are reported. It crystallizes in the monoclinic system, space groupC/2c,with parametersa = 12.635(6)A,b = 5.204(1)A,c = 16.277(3)A, β = 106.02(8)°, andZ = 4.An agreement factor ofR = 2.3%was obtained after refinement of positional and anisotropic thermal parameters. The structure is described as an interconnecting network of NdO8 distorted square antiprisms, which link in two dimensions through edge sharing, with TeO4 polyhedra which link in the same two dimensions through corner sharing. Sheets of (Nd2O10)∞ join with those of (Te8O20)∞ to form a three-dimensional network. The location and role of the lone pairs are discussed. The structure can also be described as a distorted hcp array of oxygen ions, neodymium ions, and lone pairs in which tellurium ions occupy selected trigonal bipyramidal sites.

XPS study of the dependence on stoichiometry and interaction with water of copper and oxygen valence states in the YBa2Cu3O7−x compound

Abstract In order to elucidate the valence states of both copper and oxygen in YBa 2 Cu 3 O 7− x as a function of the oxygen content, their O 1 s and Cu 2 p core-level X-ray photoelectron spectra were studied at room temperature for 0.9 ≥ x ≥ 0.1. No evidence of the Cu 3+ (i.e., 3 d 8 ) configuration was found for the ground state of a superconducting sample ( x ⋍ 0.1 ). Rather, the ground state for this composition can be described as a mixture of two configurations: mainly 3 d 9 KL (i.e., Cu 2+ O − hybridization), where K represents an electron of the conduction band and L stands for a hole in the oxygen bonded to a virtually divalent copper, and some 3 d 10 (i.e., Cu + ). The amount of monovalent copper was found to increase with x , as oxygen (O − species) from Cu 1 O chains parallel to the crystallographic b axis leave the lattice and electrons are transferred to the adjacent Cu 2+ ions. Simultaneously, the concentration of holes delocalized in the oxygen valence band decreases, the Fermi level goes upward, and the materials behavior at room temperature changes from quasi-metallic (degenerated p -type semiconductor) to p -type semiconducting. For x ⋍ 0.9 the amount of Cu + predominates over that of Cu 2+ . The high reactivity of the superconducting material with water is evidenced by the special characteristics of its O 1 s core-level spectrum. The presence of OH − ions indicates dissociative adsorption of water molecules from the air. The XPS signal due to OH − species is higher than that of the O 2− lattice ions, even when the sample was preserved from exposure to air. Moreover, when the superconducting sample was contaminated by prolonged exposition to air, the O 2− signal could hardly be observed. These results are consistent with the existence of delocalized holes in O 2 p orbitals. In fact, the O − lattice species strongly react with water molecules to produce more stable OH 0 radicals, which further recombine to generate H 2 O 2 whose decomposition is catalyzed by Cu 2+ ions. As a result, molecular oxygen from lattice O − species is evolved. Charge neutrality in the superconductor lattice is preserved by filling O − vacancies with OH − groups, which became coordinated to Ba 2+ ions as inferred from the analysis of the Ba 3 d XPS spectrum.

A new kind of B cations 1 : 3 ordering in cubic perovskites: The oxides Ba(M0.25Sb0.75)O3 (M = Li, Na)

The perovskites Ba(M0.25Sb0.75)O3 (M = Li, Na) have been obtained as white powders by heating in air mixtures of BaCO3, M2CO3 (M = Li, Na) and Sb2O3 up to 1343 K. They are cubic (a = 2ao, with a/A values of 8.224 (1) (M = Li) and 8.2806 (2) (M = Na), and are described in the space group Im3m (No. 229), Z = 8, where the M and Sb atoms have been found to be ordered. Both oxides are the first examples of cubic perovskites exhibiting 1 : 3 long-range order at the octahedral cations sublattice.

Sr9Ni6.64O21: A New Member (n= 2) of the Perovskite-RelatedA3n+3An′B3+nO9+6nFamily

Crystals of a new phase Sr{sub 9}Ni{sub 6.64}O{sub 21} were grown. This compound in the n = 2 member of the A{sub 3n+3}A{sub n}{prime}B{sub n+3}O{sub 6n+9} series. The composition and the crystal structure have been established form X-ray single crystal diffraction data. The structure contains face-shared chains of NiO{sub 6} polyhedra parallel to the c axis and is related with hexagonal 2H polytype perovskite. Sr{sub 9}Ni{sub 6.64}O{sub 21} crystallizes in the space group R3c (No. 167) with a = 9.467(2) {angstrom}, c = 35.87(5) {angstrom}, V = 2784.(4) {angstrom}{sup 3}, and Z = 6. A comparison is made between the structure of the title compound and the other members of the series.

The new pyrochlores Pb2(M0 · 5Sb1 · 5)O6 · 5 (M = Al, Sc, Cr, Fe, Ga, Rh)

From mixtures of PbO, M2O3 (M = Al, Sc, Cr, Fe, Ga, Rh), and Sb2O3, the new oxides Pb2(M0 · 5Sb1 · 5)O6 · 5 have been obtained as coloured powders giving X-ray diffraction patterns typical of cubic pyrochlores, S.G. Fd3m (No. 227), and aA values from 10.3964(1) (M = Al) to 10.5558(1) (M = Sc). The best R factors were obtained for Pb in 16(c) positions, M and Sb (1:3) randomly distributed in 16(d), oxygen in 48(f) and one half of the 8(a) sites, and x values for the oxygen positional parameter (origin at center, 3m) from 0.430 (M = Sc) to 0.436 (M = Al). Apparent interatomic distances compare with those calculated. The angles of the coordination polyhedra around the metals are reported for the compounds of Al and Sc. Seven-coordination of Pb(II) is accounted for by the non-bonded electron pair effect.

Single-crystal growth of superconducting Bi2Sr2CaCu2O8 using rotary crucibles

Crystals grown using static and rotary crucibles from mixtures of analytical grade Bi2O3, Sr(OH)2·8H2O, CaCO3 and CuO had nominal compositions Bi2SrmCanCupOq (1.5 < m ≤ 2.0, 0.2 < n ≤ 2.0, 1.8 < p < 3.0), were characterized by X-ray powder diffraction and scanning electron microscopy, and contained intergrown crystals of Bi2Sr2CaCu2O8 (2212) and Bi2Sr2CuO6 (2201) in various relative amounts, together with crystals of CuO, (Sr, Ca)2CuO3 and Sr(Cu, Bi)O2. The intergrowths of 2212 and 2201 with highest 2212 contents were obtained from melts at 980°C of mixtures with m = p = 2 and n = 1, using rotary crucibles and a cooling rate of 0.5°C h-1. After being recrystallized, crystals of 2212 appeared free of even traces of 2201, were orthorhombic, had cell dimensions of a = 5.404(2) A, b = 5.418(4) A and c = 30.80(3) A, and showed a superconducting transition at 90 K as indicated by magnetization measurements. The crystal structure of one orthorhombic Sr(Cu, Bi)O2 crystal of unit-cell constants a = rm3.4865(5) A, b = rm16.176(2) A and c = rm3.8818(4) A was refined in the space group Cmcm arriving at the composition Sr(Cuin0.85Bi0.15)O 2.225, R = 0.037, and the main distances and angles which are given.