Axis Lattice Parameter Research Articles

Anomalous magnetic ordering inPrBa2Cu4O8andCmBa2Cu3O7

A review of temperature-dependent magnetization data for nonsuperconducting $\mathrm{Pr}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ and $\mathrm{Cm}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ suggests that the failure of each to superconduct is related to the presence of Pr and Cm on their respective Ba sites. This defect is manifested, in each case, by short $c$-axis lattice parameters and anomalous high-temperature magnetic ordering which has been incorrectly attributed to ordering of the entire magnetic sublattice. Instead, it is shown that the anomalous high-temperature ordering as seen in the magnetization data is consistent with the ordering of magnetic ions substituted on the Ba site.

Spin-charge-lattice coupling near the metal-insulator transition inCa3Ru2O7

We report x-ray scattering studies of the $c$-axis lattice parameter in ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ as a function of temperature and magnetic field. These structural studies complement published transport and magnetization data, and therefore elucidate the spin-charge-lattice coupling near the metal-insulator transition. Strong anisotropy of the structural change for field applied along orthogonal in-plane directions is observed. Competition between a spin-polarized phase that does not couple to the lattice and an antiferromagnetic metallic phase that does gives rise to a rich behavior for $\mathbf{B}\ensuremath{\Vert}b$.

Phase transition in Sr0.75Ba0.25NbO3 near the Curie temperature

Strontium barium niobate has the tungsten-bronze structure with a tetragonal unit cell, and it exhibits negative thermal expansion along the c axis between −120°C and room temperature while having a positive thermal expansion along the a axis for the same temperature range. At higher temperatures, close to the Curie temperature and above, the negative thermal expansion along the c axis changes to positive thermal expansion. The a axis lattice parameter as a function of temperature shows a change in slope at the Curie temperature. These results indicate the presence of a second-order phase transition near the Curie temperature.

Composite incommensurate K-III and a commensurate form: Study of a high-pressure phase of potassium

K-III, the high-pressure phase of potassium stable above $20\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, is reported to have a host-guest composite structure comprising the same 16-atom host structure as Rb-IV and a $C$-face-centered, rather than a body-centered, tetragonal, guest structure. At $22.0(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the incommensurate ratio of the host and guest $c$-axis lattice parameters passes through the commensurate value of $8∕5$ but, within the experimental resolution of approximately $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, no lock-in to a commensurate structure is observed.

Phase diagram in bilayered-hydrate [formula omitted] revealed by Co nuclear quadrupole resonance

We have performed Co nuclear quadrupole resonance (NQR) measurements in various bilayered-hydrate (BLH) Na x CoO 2 · y H 2 O compounds, showing superconducting (SC) and non-SC characteristics. A weak magnetic order was found in a BLH sample with a longer c -axis lattice parameter. We also found that the NQR frequency ν Q arising from the ± 7 2 ↔ ± 5 2 transition is related to the ground state of this system. A phase diagram is developed, in which the superconducting and magnetic-order temperature T c and T M are plotted with respect to ν Q .

Out-of-plane thermal conductivity of the layered thermoelectric oxideBi2−xPbxSr2Co2Oy

The out-of-plane thermal conductivity $({\ensuremath{\kappa}}_{c})$ of ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Pb}}_{x}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{y}$ is measured and analyzed. A low value of $4--8\phantom{\rule{0.3em}{0ex}}\mathrm{mW}∕\mathrm{cm}\phantom{\rule{0.2em}{0ex}}\mathrm{K}$ is observed at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, which is close to the minimum thermal conductivity. This implies the phonon mean-free path is of the order of the $c$-axis lattice parameter. The temperature dependence is also anomalous, and the positive temperature coefficient $d{\ensuremath{\kappa}}_{c}∕dT>0$ is observed. These features can be understood semiquantitatively by a phenomenological expression of $\ensuremath{\kappa}$ including the contribution of optical phonons.

High-Tcsuperconductivity in three-fluorite-layer copper oxides. II.(Cu,Mo)Sr2(Ce,Y)3Cu2O11+δ

We have synthesized high-quality samples of the $s=3$ member of the $(\mathrm{Cu},\mathrm{Mo})\text{\ensuremath{-}}12s2$ homologous series of layered copper oxides. The $(\mathrm{Cu},\mathrm{Mo})\text{\ensuremath{-}}1232$ phase forms as a single phase for the cation composition $({\mathrm{Cu}}_{0.75}{\mathrm{Mo}}_{0.25}){\mathrm{Sr}}_{2}{({\mathrm{Ce}}_{0.67}{\mathrm{Y}}_{0.33})}_{3}{\mathrm{Cu}}_{2}{\mathrm{O}}_{11+\ensuremath{\delta}}$ through solid-state synthesis in air, but the samples as synthesized are not superconductive. Superconductivity is induced in the air-synthesized samples by means of high-pressure oxygenation carried out at $5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in the presence of ${\mathrm{Ag}}_{2}{\mathrm{O}}_{2}$ as an excess oxygen source. With increasing ratio of ${\mathrm{Ag}}_{2}{\mathrm{O}}_{2}$ to the $(\mathrm{Cu},\mathrm{Mo})\text{\ensuremath{-}}1232$ phase the $c$-axis lattice parameter gradually decreases and ${T}_{c}$ increases. The highest ${T}_{c}$ value of $53\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reached for samples oxygenated in the presence of $100\phantom{\rule{0.3em}{0ex}}\mathrm{mol}%$ ${\mathrm{Ag}}_{2}{\mathrm{O}}_{2}$ exceeds the values reported for $\mathrm{Cu}$-based two-fluorite-layer compounds.

Structural origin of the low superconducting anisotropy of Bi1.7Pb0.4Sr2Ca0.9Cu2O8 crystals

The structure of ${\mathrm{Bi}}_{1.7}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{0.9}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ $({T}_{c}=93\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ was refined from x-ray single crystal diffraction data in space group $A2aa$, $a=5.3852(9)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $b=5.4286(9)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $c=30.997(6)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. For this composition, the crystal structure is free from the structural modulation connected with the presence of additional oxygen atoms in the BiO layers. The $c$-axis lattice parameter is slightly larger in (Bi,Pb)-2212 crystals than in Pb-free Bi-2212 crystals, but the complete structural refinement revealed an inhomogeneous redistribution of the inter-planar distances. In particular, the distance between the two neighboring BiO layers has significantly decreased in the modulation-free phase as compared to the Pb-free phase. We believe that this is a key point in understanding the widely observed reduction of the anisotropy in Bi-2212 by Pb-doping. The irreversibility line as well as the onset of the second magnetization peak in modulation-free (Bi,Pb)-2212 crystals were studied. Because of the strongly reduced value of the anisotropy, the 2D-3D crossover field value, ${H}_{\mathit{cr}}$, is significantly higher in (Bi,Pb)-2212 crsystals than in Bi-2212 crystals. As a direct consequence of the structural modifications induced by the removal of the modulation, the nature of the inter-layer coupling changes from electromagnetic in modulated Bi-2212 to Josephson in modulation-free (Bi,Pb)-2212. Annealing the (Bi,Pb)-2212 crystals under oxygen reintroduced the structural modulation.

ＭｇＢ２超伝導体の合成に及ぼす雰囲気の影響

MgB2 is an intermetallic compound and is a promising superconductor for practical applications due to its high TC value of 39 K. The production cost of MgB2 wires is expected to be low compared to the conventional superconducting wires, since it is composed of cheap and available elements. However, the critical current density JC in applied magnetic fields is not yet at practical level and, hence, the improvement of high field performance remains one of the most important issue to be solved urgently.Recent advances are showing that the addition of SiC is effective to improve the high field JC, while the use of MgH2 instead of Mg as raw material is enhancing MgB2 crystal growth. In both situations the mechanisms of these effects are still unclear.This paper reports the synthesis of MgB2 superconductor in high-pressure hydrogen-atmosphere, which is an extreme reducing atmosphere. The results are compared with those for inert atmosphere i.e. argon-atmosphere, in order to provide useful information for wire processing.Samples were prepared by reacting the mixture of Mg (or MgH2), B and nano-SiC powders in a high-pressure (up to 3.5 MPa) steel vessel, at temperatures below 900°C. Samples that are fabricated in the pressurized hydrogen atmosphere show higher TC, bigger a -axis lattice parameter and larger grain size than for the samples synthesized in Ar-atmosphere, in the same conditions. It turns out that the pressurized hydrogen atmosphere is useful to produce MgB2 with good crystallinity. Nevertheless, the JC in magnetic field is higher for the samples synthesized in argon atmosphere and it is considered that the lattice distortion in a -plane, smaller grain size and fine precipitates, such as MgO, are responsible for the stronger flux pinning. SiC addition is effective regardless the atmosphere and the highest JC was obtained for the sample starting from MgH2+2B+SiC powder mixture heat treated in argon atmosphere.

Structure and Dynamics of NaxCoO2 and the Hydrated Superconductor

ABSTRACTThe properties of the NaxCoO2 class of materials are of interest from a number of viewpoints. These compounds are based on a triangular lattice of spin-½ ions—prototype RVB system— where a high thermoelectric-power Curie-Weiss metallic paramagnet is found for Na0.7CoO2, a charge ordered insulator at x=0.5, and a paramagnetic metal where superconductivity is induced in Na0.3CoO2 when it is intercalated with water. Here we briefly review our neutron diffraction and inelastic scattering measurements characterizing the crystal structure and lattice dynamics, and relate these to the observed physical properties. The basic structure of NaxCoO2 is hexagonal and consists of robust layers of CoO2 interspersed by Na layers with two inequivalent sites. Two special cases are x=1 where one of these sites is fully occupied and the other empty, and x=½ where both sites have equal occupancies of ¼ and the system is a charge ordered insulator. For general × the site occupancies are inequivalent and vary systematically with x. In the regime of x=0.75 we find a first-order transition from a high symmetry Na site at low T to a three-fold split site (with lower symmetry) at high T. This transition is first order and varies with x. For the Na0.3CoO2. 1.4(H/D)2O superconductor, the water forms two additional layers between the Na and CoO2, increasing the c -axis lattice parameter of the hexagonal P 63/mmc space group from 11.16 Å to 19.5 Å. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice to a good approximation. We find a strong inverse correlation between the CoO2 layer thickness and the superconducting transition temperature (TC increases with decreasing thickness). The phonon density-of-states for Na0.3CoO2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ∼100 meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.

Structure and dynamics of superconductingNaxCoO2hydrate and its unhydrated analog

Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na0.3CoO2 1.4(H/D)2O, and the parent Na0.3CoO2 material. The structure of Na0.3CoO2 consists of alternate layers of CoO2 and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and CoO2, increasing the c-axis lattice parameter of the hexagonal P63/mmc space group from 11.16 A to 19.5 A. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The CoO2 layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the CoO2 layer thickness and the superconducting transition temperature (TC increases with decreasing thickness). The phonon density-of-states for Na0.3CoO2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ~100 meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.

Wigner-crystal and bi-stripe models for the magnetic and crystallographic superstructures ofLa0.333Ca0.667MnO3

The crystallographic (charge-ordered) and magnetic superstructures of ${\mathrm{La}}_{0.333}{\mathrm{Ca}}_{0.667}{\mathrm{MnO}}_{3}$ were studied by high-resolution synchrotron x-ray and neutron powder diffraction. In the antiferromagnetic structure, which was refined using a noncollinear model, the a lattice parameter is tripled and the c lattice parameter is doubled with respect to the average crystallographic unit cell (Pnma setting). The crystallographic structure below the charge-ordering temperature ${(T}_{\mathrm{CO}}\ensuremath{\sim}260\mathrm{K})$ is characterized by ordering of the ${d}_{{z}^{2}}$ orbitals of the Jahn-Teller-distorted ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra in the orthorhombic ac plane, and the appearance of superlattice peaks in the x-ray patterns corresponding to a tripling of the a axis lattice parameter. The intensities of the superlattice peaks can be accounted for satisfactorily in terms of ordering of the ${\mathrm{Mn}}^{3+}$ cations in sites as far apart as possible in the ac plane (``Wigner-crystal'' model) and transverse displacements of the ${\mathrm{Mn}}^{4+}{\mathrm{O}}_{6}$ octahedra in the c direction. These results are not consistent with a recently proposed model based on transmission electron microscopy (TEM) data in which the ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra are arranged in a bi-stripe pattern (``bi-stripe'' model). In particular, the large longitudinal stripe modulation revealed by TEM is not observed, suggesting that the TEM data may not be representative of the bulk sample. Within the framework of the Wigner-crystal model, the magnetic structure at 1.5 K and the charge-ordered superstructures at 160 and 1.5 K were refined from the neutron data.

Effect of Ag doping on structure andcritical temperature of Bi2Sr2CaCu2O8+δ superconductors

The effects of Ag substitution for Bi, Sr and Cu on the c -axis lattice parameter and critical temperature ( T c ) were determined for the Bi 2− x Sr 2− y CaCu 2− z O 8+ δ (BSCCO-2212) superconducting phase. Two distinct regimes of behavior were observed: fast cooling represented the regime where the uptake of excess oxygen was the controlling factor and slow cooling represented the regime where the cation content was the controlling factor on the structure and properties. Samples showed higher T c values and longer c -axes with increased cooling rate. A linear increase in T c with increase in c -axis length was observed for the faster-cooled samples. For the slow-cooled samples, increased c -axis length was observed when Ag was substituted for Bi and Sr, whereas substituting Ag for Cu caused no change in c -axis length. In addition, any deviation from the ideal 2212 stoichiometry was shown to reduce the T c values of samples that were slowly cooled and fully oxygenated. Preliminary results obtained by high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy indicated that relatively small amounts of Ag (≈1 at.%) can occupy Bi sites and a larger amount (≈10 at.%) of Ag can substitute for Cu.

The electrical and magnetic properties of the (Bi 1 − yCd y)Sr 2(Y 1 − xCa x)Cu 2O 7 − δ series

(Bi 0.33Cd 0.67)Sr 2YCu 2O 7 − δ can be synthesized single phase by XRD, between 800 and 950 °C. It has a tetragonal structure, space group P4 mmm , and lattice parameters a = 3.802 A ̊ and c = 11.96 A ̊ . The Cd site can be fully replaced with an appropriate M II ion and the Y site can be chemically substituted up to 100% by lanthanide ions with ionic radii falling between those of Nd and Gd, with a measurable shift in a and c axis lattice parameters. As synthesized, (Bi 0.33Cd 0.67)Sr 2YCu 2O 7 − δ is semiconducting and paramagnetic down to 4 K. Superconductivity can be induced by a post-synthesis annealing in high pressure oxygen to give T c = 40 K . Thermopower measurements show that the material is underdoped with S 290k = 50 μVK −1. Introduction of extra charge carriers to raise T c by doping Ca on the Y site is not chemically possible with the synthesis techniques used.

Structural inhomogeneities observed inYBa2Cu3O7−δcrystals with optimal transport properties

The high-${T}_{c}$ cuprates have exhibited a variety of unusual features in both their normal-state and superconducting properties. One of the challenges that has faced experimentalists in the field has been to determine whether the observed properties can be attributed to intrinsic behavior or whether they are a result of extrinsic factors such as sample inhomogeneity. In this paper, we present transport measurements on two ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystals which exhibit many of the features generally attributed to fully oxygenated, homogeneous samples. Nevertheless, x-ray-diffraction studies also presented in this paper reveal surface and bulk structural inhomogeneities in both crystals. Results of these studies show variations in the $c$-axis lattice parameters on the surface and throughout the interiors of both crystals. These are interpreted as variations in the oxygen content. The transport studies together with the structural measurements represent a more thorough and complete characterization of sample quality than is generally reported. Our results lead one to question the validity of using transport measurements alone to determine sample quality and whether any of the properties measured in crystals can be attributed to ``optimally doped'' or ``fully oxygenated'' samples.

Atomic disorder in low pressure rf-sputtered YBCO superconducting thin films

The epitaxial films of YBCO grown by low pressure (∼ 10 −3 Torr) rf-magnetron sputtering, exhibiting ‘c’- axis lattice parameter 11.753 Å and 11.718 Å on MgO(100) and SrTiO 3(100), respectively, were subjected to the Rutherford backscattering technique (RBS) for their compositional analysis. The backscattering-simulations performed on these films have revealed a gradient in the concentration of both Ba and Cu cations across the thickness of the film. The cation ratios (Ba:Y and Cu:Y) in the films grown on different substrates are found to be substrate dependent. In this paper, an attempt has been made to correlate the observed cation deficiencies with the employed experimental conditions. It is conjectured that the observed cation deficiency is expected to give rise to ‘Y-for-Ba’ replacement, thereby resulting in the extension of the c-axis parameter. This atomic disorder (replacement) also explains the observed lower values of T c.

Raman microscopy as a local probe of structural defects and oxygen content in HTS thin films

Three YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films grown by c-beam evaporation have been studied using Raman microscopy. The films were grown at different temperatures and show different levels of cation disorder as determined from the 'c' axis lattice parameter and certain X-ray peak intensity ratios. We report on how the position of the 115 cm/sup -1/ and 500 cm/sup -1/ Raman peaks change as a function of oxygen, cation disorder and spatial position across the films and relate the results to structural changes which occur in the YBCO unit cell.

Microstructural properties of Bi2Sr2Can-1CunOy multilayers grown by molecular beam epitaxy.

The microstructure of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{n\ensuremath{-}1}{\mathrm{Cu}}_{n}{\mathrm{O}}_{y}$ multilayers grown by molecular beam epitaxy on atomically flat SrTi${\mathrm{O}}_{3}$ substrates has been studied by reflection high-energy electron diffraction (RHEED), atomic force microscopy, and x-ray-diffraction (XRD) techniques. The overall RHEED data, collected in situ at different $\frac{{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Cu}{\mathrm{O}}_{y}}{{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{y}}$ (2201/2212) multilayer growth stages, demonstrated a two-dimensional growth and rather a high quality of the interfaces. Following the evolution of RHEED patterns, some evidence of an increase in surface roughness after several multilayer periods, was detected. A one-dimensional x-ray-diffraction model was applied for a quantitative analysis of growth defects in the multilayers. The substitutional disorder in the lattice and stacking faults in the molecular layers were determined by an iterative comparison of simulated x-ray-diffraction spectra with the experimental XRD data. The observed changes in the $c$-axis lattice parameter of 2201 molecular layers were interpreted as being caused by ionic substitutions of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{Ca}}^{2+}$ in the lattice and governed by the growth interdiffusion. The fitting procedure also revealed that two types of growth disorder were present in the layers: (1) stacking faults randomly distributed within the layers and (2) stacking faults localized at the interfaces. The two types of growth defect are expected to influence the superconducting properties differently and this has to be considered before the transport properties of superconducting multilayers are studied.

Investigation of Cation Disorder In ‘C’ Axis Ybco 123 Thin Films Using Raman Microscopy

AbstractTwo 123 thin films grown by e-beam evaporation have been studied using Raman microscopy. The films were grown under different conditions of temperature and oxygen partial pressure and show different levels of cation disorder as determined from the ‘c’ axis lattice parameter in combination with x-ray data. The oxygen stoichiometry in each film was changed by controlled anneals using a coulometric titration apparatus. As a result we report on the sensitivity of the 500cm−1 and 585cm−1 Raman peak to the oxygen stoichiometry and how the intensity and position of these peaks can be used to detect or confirm the presence of a small amount of cation disorder.

Macroscopic length, lattice parameter and microstructural changes in neutron-irradiated aluminum nitride due to annealing

Aluminum nitride was neutron-irradiated in the Japan Materials Testing Reactor up to 5.2×10 24 n/m 2 ( E > 1.0 MeV). The specimens were isochronally annealed under a vacuum up to 1400°C. The macroscopic length and lattice parameters of both a - and c -axis of the specimen irradiated to 8.3 × 10 22 n/m 2 at about 100°C decreased concurrently with an increase in annealing temperature from 100°C, and returned to the pre-irradiated values after annealing at 1100°C. The macroscopic lengths of the specimens irradiated to 2.4×10 24 n/m 2 at 470°C and 5.2×10 24 n/m 2 at 785°C, both containing dense interstitial dislocation loops, were not significantly influenced by annealing up to 800–900°C. Then they decreased rapidly with increasing annealing temperature up to 1400°C. The change in c -axis lattice parameter by annealing was larger than that of a -axis above 900–1000°C, and finally reduced to the pre-irradiated values after annealing at 1400°C.