Phase Transformations in the High-Tc Superconducting Compounds,Ba2RCu3O7?δ (R = Nd,Sm, Gd,Y, Ho,and Er)

The phase transformation between the orthorhombic and tetragonal structures of six high-T_c superconductors, Ba₂RCu₃O_7−δ, where R = Nd, Sm, Gd, Y, Ho, and Er, and δ = 0 to 1, has been investigated using techniques of x-ray diffraction, differential thermal analysis/thermogravimetric analysis (DTA/TGA) and electron diffraction. The transformation from the oxygen-rich orthorhombic phase to the oxygen-deficient tetragonal phase involves two orthorhombic phases. A superlattice cell caused by oxygen ordering, with a′ = 2a, was observed for materials with smaller ionic radius (Y, Ho, and Er). For the larger lanthanide samples (Nd, Sm, and Gd), the a′ = 2a type superlattice cell was not observed.The structural phase transition temperatures, oxygen stoichiometry and characteristics of the T_c plateaus appear to correlate with the ionic radius, which varies based on the number of f electrons. Lanthanide elements with a smaller ionic radius stabilize the orthorhombic phase to higher temperatures and lower oxygen content. Also, the superconducting temperature is less sensitive to the oxygen content for materials with smaller ionic radius. The trend of dependence of the phase transformation temperature on ionic radius across the lanthanide series can be explained using a quasi-chemical approximation (QCA) whereby the strain effect plays an important role on the order-disorder transition due to the effect of oxygen content on the CuO chain sites.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Theoretical aspects of charge correlations in θ-(BEDT-TTF)2X

A review is given on the theoretical studies of charge correlations in θ-(BEDT-TTF)₂X. Various studies show that within a purely electronic model on the θ-type lattice with on-site U and nearest neighbor V_p and V_c interactions, the diagonal stripe, c-axis three-fold, and the vertical stripe charge correlations are favored in the regime V_p<V_c, V_p∼V_c, and V_p>V_c, respectively. In the realistic parameter regime of V_p∼V_c, there is competition between the c-axis three fold state and diagonal stripe state. Since these are different from the experimentally observed a-axis three fold and the horizontal stripe charge correlations, additional effects have to be included in order to understand the experiments. The electron–lattice coupling, which tends to distort the lattice into the θ_d-type, is found to favor the horizontal stripe state, suggesting that the occurrence of this stripe ordering in the actual materials may not be of purely electronic origin. On the other hand, distant electron–electron interactions have to be considered in order to understand the a-axis three fold correlation, whose wave vector is close to the nesting vector of the Fermi surface. These studies seem to suggest that the minimal model to understand the charge correlation in θ-(BEDT-TTF)₂X may be more complicated than expected. Future problems regarding the competition between different types of charge correlations are discussed.     

Structure and cation distribution in perovskites with small cations at the A site: the case of ScCoO3

We synthesize ScCoO₃ perovskite and its solid solutions, ScCo_1−xFe_xO₃ and ScCo_1−xCr_xO₃, under high pressure (6 GPa) and high temperature (1570 K) conditions. We find noticeable shifts from the stoichiometric compositions, expressed as (Sc_1−xM_x)MO₃ with x = 0.05–0.11 and M = Co, (Co, Fe) and (Co, Cr). The crystal structure of (Sc_0.95Co_0.05)CoO₃ is refined using synchrotron x-ray powder diffraction data: space group Pnma (No. 62), Z = 4 and lattice parameters a = 5.26766(1) Å, b = 7.14027(2) Å and c = 4.92231(1) Å. (Sc_0.95Co_0.05)CoO₃ crystallizes in the GdFeO₃-type structure similar to other members of the perovskite cobaltite family, ACoO₃ (A³⁺ = Y and Pr-Lu). There is evidence that (Sc_0.95Co_0.05)CoO₃ has non-magnetic low-spin Co³⁺ ions at the B site and paramagnetic high-spin Co³⁺ ions at the A site. In the iron-doped samples (Sc_1−xM_x)MO₃ with M = (Co, Fe), Fe³⁺ ions have a strong preference to occupy the A site of such perovskites at small doping levels.     

Thermal expansion of β-FeSi2 at low temperatures

We present the temperature dependence of the lattice constants (a, b, c) of β-FeSi₂ single crystals at low temperatures. a showed the largest temperature dependence of 0.14% and the relationship of a>c>b did not change at 8-300 K. The linear thermal expansion coefficients α showed remarkable anisotropy. α along the a-axis (αa) was much larger than αb and αc, and showed negative thermal expansion at temperatures below 60 K. From these results, we estimated the temperature dependence of the lattice mismatches at β-FeSi₂/Si heterojunctions.     

Synthesis and structures of chloride thiosilicates with lanthanides Ln3Cl[SiS4]2 (Ln = La, Ce, Pr)

Single crystals of Ln₃Cl[SiS₄]₂ (Ln=La, Ce, Pr) were prepared by the reaction of lanthanide metal, sulfur, silicon and chlorine. Data collection was carried out using a STOE imaging plate detector at 293 K. The homologue compounds crystallize in the space group C2/c of the monoclinic system isotypically to Ln₃X[SiS₄]₂ (X=Br, I) and the A-type of the halide oxosilicates Ln₃X[SiO₄]₂ (X=Cl, Br; Ln=La, Ce, Pr) with four formula units in cells of dimensions:

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La₃Cl[SiS₄]₂: a=1567.2(3) pm, b=777.8(2) pm, c=1101.5(2) pm, β=96.88(2)°

•

Ce₃Cl[SiS₄]₂: a=1559.4(3) pm, b=770.2(2) pm, c=1096.9(2) pm, β=97.07(2)°

•

Pr₃Cl[SiS₄]₂: a=1555.9(3) pm, b=764.2(1) pm, c=1093.2(2) pm, β=97.40(2)°

The corresponding residuals (all data) for the refined structures are 2.28% (La), 2.15% (Ce), and 3.17% (Pr), respectively. In the crystal structures, the chloride ions form chains along [0 0 1] with trigonal coordination by the lanthanide ions.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1−xTe_x) and Fe(Se_1−xS_x). In the case of Fe(Se_1−xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1−xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1−xCo_x)AsF. The T_c of Fe(Se_1−xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.     

Hydrothermal synthesis, characterization and magnetic properties of NaVGe2O6 and LiVGe2O6

Supercritical fluids are shown to be an excellent reaction media for the synthesis of novel solid state phases at intermediate temperatures. LiVGe₂O₆ and NaVGe₂O₆ have the common pyroxene structure composed of VO₆ linear chains. NaVGe₂O₆ crystallizes in the monoclinic space group C2/c with four formula units having cell dimensions a = 9.960(4) Å, b = 8.853(10) Å, c = 5.4861(10) Å, β = 106.403(3)°. The structure was refined until R = 0.0290 and R_w = 0.0370. For LiVGe₂O₆ in space group P2₁/c: a = 9.8508(7) Å, b = 8.754(3) Å, c = 5.3948(13) Å, β = 108(3)°, R = 0.0240 and R_w = 0.0250. The compounds contain edge-shared VO₆ octahedral chains and corner-shared GeO₄ tetrahedral chains. The presence of these VO₆ chains results in spin-Peierls distortion. Structural and physical characterization of the compounds are reported.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

Ba(OH)2 Equilibria in the System Ba-O-H-F,With Application to the Formation of Ba2YCu3O6.5 + x From BaF2-Precursors

The ex situ process for fabricating Ba₂YCu₃O_{6.5 + x} superconducting tapes from BaF₂- based precursors involves a hydration/oxidation reaction at ≈730 °C to 750 °C generally written as: (2BaF2+Y+3Cu)(amorphous)+(2H2O+2.25O2)(g)→Ba2YCu3O6.5+x(s)+4HF(g).However, microscopic observations of partially processed films suggest the presence of a transient liquid phase during conversion. Alternatively, the conversion reaction can be rewritten as the sum of several intermediate steps, including the formation of a barium hydroxide liquid: (BaF2)(amorphous)+2H2O(g)→Ba(OH)2(liq)+2HF(g).To evaluate the possibility of a hydroxide liquid conversion step, thermodynamic calculations on the stability of Ba(OH)₂(liq) have been completed from 500 °C to 900 °C at 0.1 MPa p_total. Based on currently available data, the calculated phase diagrams suggest that a viable hydroxide reaction path exists in the higher part of this temperature range. The calculations indicate that Ba(OH)₂(liq) may be stable at log p_H2O (Pa) values from ≈4 to 5, provided log p_HF (Pa) values can be maintained below 0 to −1. Limited experimental confirmation is provided by results of an experiment on BaF₂(s) at 815 °C, 0.1 MPa p_H2O, in which essentially all F at the surface was replaced by O. It is therefore possible that processing routes exist for producing Ba₂YCu₃O_{6.5 + x} based on the presence of a Ba(OH)₂ liquid, which might have an effect on conversion rates and texturing in the superconducting film.     

PrBa2Cu3O7: A new superconductor

PrBa₂Cu₃O₇ superconducts, provided Pr is kept off Ba-sites — experimentally confirming the prediction of the oxygen model and indicating that superconductivity originates in the chains, not in the planes.     

Structural and spectroscopic characterization of Yb doped Lu2O3 transparent ceramics

Yb³⁺ doped Lu₂O₃ transparent ceramics were fabricated by the solid-state reaction method and sintered in H₂ atmosphere. Structural and spectroscopic properties of Yb:Lu₂O₃ ceramics were studied. The Yb:Lu₂O₃ ceramic structure, and the lattice parameter are refined with the Rietveld method. Yb:Lu₂O₃ has broad absorption and emission bands with a long fluorescence lifetime (1.31 ms). The energy level diagram is calculated based on the absorption and emission spectra, Yb³⁺ in Lu₂O₃ ceramics exhibits a big splitting energy of the ²F_7/2 ground state (1023 cm⁻¹). Furthermore, the gain cross-section (σ_g) is estimated with different β values.     

Liquidus Diagram of the Ba-Y-Cu-O System in the Vicinity of the Ba2YCu3O6+x Phase Field

This paper describes the melting equilibria in the vicinity of the high T_c phase Ba₂YCu₃O_6+x, including evidence for two Ba-Y-Cu-O immiscible liquids. Melting equilibria have been investigated in purified air using a combination of differential thermal analysis (DTA), thermogravimetric analysis (TGA), powder x-ray diffraction (XRD), MgO wick entrapment of liquid for analysis, scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis (EDS), and hydrogen reduction for determination of copper oxidation state. For relatively barium-rich compositions, it was necessary to prepare the starting materials under controlled atmosphere conditions using BaO. A liquidus diagram was derived from quantitative data for the melts involved in various melting reactions. In general the 1/2(Y₂O₃) contents of the melts participating in these equilibria were low (mole fraction <4 %). The primary phase field of Ba₂YCu₃O_6+x occurs at a mole fraction of <2.0 % 1/2Y₂O₃ and lies very close along the BaO-CuO_x edge, extending from a mole fraction of ≈43 % CuO to a mole fraction of ≈76 % CuO. It is divided by a liquid miscibility gap and extends on either side about this gap. The topological sequence of melting reactions associated with the liquidus is presented as a function of temperature. Implications for the growth of Ba₂YCu₃O_6+x crystals are discussed.     

Ambiguities in Powder Indexing: the Impact of a Quaternary Lattice Metric Singularity on the Characterization of Mawsonite and Chatkalite

A lattice metric singularity occurs when unit cells defining two (or more) lattices yield the identical set of unique calculated d-spacings. The minerals Mawsonite and Chatkalite are of especial interest as both are characterized by tetragonal unit cells that correspond to the second member of a quaternary lattice metric singularity. This singularity includes lattices that are Cubic I, Tetragonal P, Orthorhombic F, and Orthorhombic P. The Mawsonite and Chatkalite lattices are unique in that they are highly specialized. In each case: (1) the determinative c/a ratio is very near 1/√2, (2) the symmetrical scalars of the reduced form [a · a : b · b : c · c = 1:2:2] have greater specialization than required for the given reduced form type, (3) the tetragonal lattice has derivative lattices of higher symmetry, and (4) the powder pattern is highly compressed. Mawsonite and Chatkalite serve as exemplar-type compounds. Their tetragonal structure has important implications in structure determination using powder diffraction data. First, any cubic I lattice — established solely on the basis of indexing procedures — may actually be tetragonal or orthorhombic. Second, in establishing the lattice of an unknown, results from powder data indexing require routine confirmation by other techniques (e.g., single crystal, optical, etc.).     

Structural Studies of the Superconducting La2.5Y0.5CaBa3(Cu1?xFex)7Oz (0 ≤ x ≤ 0.1) Perovskite System by Neutron Diffraction

We have investigated the effect of Fe substitution on the structural and superconducting properties of La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B₁), x = 0.06 (B₂), and x = 0.10 (B₃) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B₁, B₂, and B₃ are superconducting with T _c ^R=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in T _c to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured T _c values of three samples.     

Room-temperature synthesis of nanometric α-Bi2O3

Nanometric Bi₂O₃ powder was successfully synthesized by applying the method based on self-propagating room temperature reaction (SPRT) between bismuth nitrates and sodium hydroxide. X-ray powder diffraction (XRPD) and Rietveld's structure refinement method were applied to characterize prepared powder. It revealed that synthesized material is a single phase monoclinic α-Bi₂O₃ (space group P2₁/c with cell parameters a = 5.84605(4)Å, b = 8.16339(6) Å, c = 7.50788(6) Å and β = 112.9883(8)). Powder particles were of nanometric size (about 50 nm). Raman spectral studies conformed that the obtained powder is single phase α-Bi₂O₃. Specific surface area of obtained powder was measured by Brunauer-Emmet-Teller (BET) method.     

Enhancing the oxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O5 + δ membranes by coating RBaCo2O5 + δ(R= Pr, Nd, Sm, Gd) layers

The effects of a thin RBaCo₂O_5 + δ (R= Pr, Nd, Sm, Gd) layer coating on the oxygen permeation flux through Ba_0.5Sr_0.5Co_0.8Fe_0.2O_5 + δ(BSCF) membrane were investigated. Due to the high oxygen adsorption and desorption rate constants k_a and k_d of the RBaCo₂O_5 + δ (R112) materials and the porous structure of the coating layers, the oxygen permeation flux through the BSCF membranes can be enhanced remarkably. It was found that the reaction between Pr112 and BSCF also has significant influence on the oxygen permeation flux. The reaction between Pr112 and BSCF forms impurities which may block oxygen permeation flux. However, Nd112, Sm112, and Gd112 do not react seriously with BSCF, therefore, coating layers of these materials can significantly enhance the oxygen permeation flux of BSCF membranes.     

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa₂Cu₃O₇(YBCO) on (100) LaAlO₃ were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T _c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J _c) at 77 K has an average value of 2×10⁵ A cm^–2, with a tendency for decreasingJ _c with increasing film thickness (0.2 to 0.8m). Variations inJ _c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ _c. If the film composition is enriched in Y relative to Ba and Cu,T _c decreases by several degrees.     

Polymorphism of Bi1−xLnxO1.5 phases (0<x<0.40): Characterization of a new compound Bi4Ln2O9 (x=0.33; Ln=La, Pr, Nd)

The Bi_1−xLn_xO_1.5 solid solutions (Ln=La, Pr, Nd), of the β₂/β₁/ε (Bi-Sr-O) structural type, have been investigated in their Ln-rich domains. For Ln=La, Pr, and Nd, the upper limits are 0.35, 0.35 and 0.33, respectively. The Bi₄Ln₂O₉ ε phase (x=0.33) appears to be the single definite compound. For Bi₄La₂O₉, Bi₄Pr₂O₉ and Bi₄Nd₂O₉, the ε-type cells are respectively: a=9.484(4) Å, b=3.982(2) Å, c=7.030(3) Å, β=104.75(3)°; a=9.470(5) Å, b=3.945(2) Å, c=6.968(4) Å, β=104.73(3)° and a=9.439(3) Å, b=3.944(2) Å, c=6.923(2) Å, β=105.03(3)°. Upon heating, each monoclinic (ε) compound transforms successively into rhombohedral phases (β₂/β₁) and finally into a cubic fluorite-type phase. For La- and Pr-based compounds, all transitions are reversible; for Nd, depending on the thermal treatment, the reversibility of ε→β₂ can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi₄(Ln, Ln′)₂O₉ samples allows to correlate the evolution of the thermal behavior and of the unit cell parameters, to the lanthanide size. A partial plot of the (Bi₂O₃)_1−x-(La₂O₃)_x phase diagram (0≤x≤0.40) is proposed.     

Standard Reference Material (SRM 1990) For Single Crystal Diffractometer Alignment

An international project was successfully completed which involved two major undertakings: (1) a round-robin to demonstrate the viability of the selected standard and (2) the certification of the lattice parameters of the SRM 1990, a Standard Reference Material^® for single crystal diffractometer alignment. This SRM is a set of ≈3500 units of Cr-doped Al₂O₃, or ruby spheres [(0.420.011 mole fraction % Cr (expanded uncertainty)]. The round-robin consisted of determination of lattice parameters of a pair of crystals: the ruby sphere as a standard, and a zeolite reference to serve as an unknown. Fifty pairs of crystals were dispatched from Hauptman-Woodward Medical Research Institute to volunteers in x-ray laboratories world-wide. A total of 45 sets of data was received from 32 laboratories. The mean unit cell parameters of the ruby spheres was found to be a=4.7608 ű0.0062 Å, and c=12.9979 ű0.020 Å (95 % intervals of the laboratory means). The source of errors of outlier data was identified. The SRM project involved the certification of lattice parameters using four well-aligned single crystal diffractometers at (Bell Laboratories) Lucent Technologies and at NRC of Canada (39 ruby spheres), the quantification of the Cr content using a combined microprobe and SEM/EDS technique, and the evaluation of the mosaicity of the ruby spheres using a double-crystal spectrometry method. A confirmation of the lattice parameters was also conducted using a Guinier-Hägg camera. Systematic corrections of thermal expansion and refraction corrections were applied. These rubies_– are rhombohedral, with space group R3¯c. The certified mean unit cell parameters are a=4.76080±0.00029 Å, and c=12.99568 ű0.00087 Å (expanded uncertainty). These certified lattice parameters fall well within the results of those obtained from the international round-robin study. The Guinier-Hägg transmission measurements on five samples of powdered rubies (a=4.7610 ű0.0013 Å, and c = 12.9954 ű0.0034 Å) agreed well with the values obtained from the single crystal spheres.