Preparation, characterization and oxide ion conductivity in U-substituted Bi4V2O11

A Bi₂V_{1 − x − y}U_xBi_yO_{5.5 + 0.5x − y} solid solution derived from Bi₄V₂O₁₁ has been prepared and characterized with x up to 0.125 for y = 0. Partial substitution of U⁶⁺ for V⁵⁺ in Bi₄V₂O₁₁ leads to the stabilization at room temperature of the high-oxide ion conducting γ-phase, in contrast with other M⁶⁺ dopants which stabilize the β-phase. The lower conductivity in U substituted system compared with BICUVOX.10 is attributed to its higher activation energy. Conductivity values and activation energies of the U substituted phases compare well with Bi₂UO₆.     

Shaped crystal growth and scintillating properties of Yb:(Gd,Lu)3Ga5O12 solid solutions

Shaped single crystals of (Lu_xGd_1−x)₃Ga₅O₁₂ (0.0x1.0) and (Yb_0.05Lu_xGd_0.95−x)₃Ga₅O₁₂ (0.0x0.9) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃Ga₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect-host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Material design of new lithium ionic conductor, thio-LISICON, in the Li2S–P2S5 system

A new lithium ionic conductor of the thio-LISICON (LIthium SuperIonic CONductor) family was found in the binary Li₂S–P₂S₅ system; the new solid solution with the composition range 0.0≤x≤0.27 in Li_3+5xP_1−xS₄ was synthesized at 700 °C and characterized by X-ray diffraction measurements. Its electrical and electrochemical properties were studied by ac impedance and cyclic voltammetry measurements, respectively. The solid solution member at x=0.065 in Li_3+5xP_1−xS₄ showed the highest conductivity value of 1.5×10⁻⁴ S cm⁻¹ at 27 °C with negligible electronic conductivity and the activation energy of 22 kJ mol⁻¹ which is characteristic of high ionic conduction state. The extra lithium ions in Li₃PS₄ created by partial substitution of P⁵⁺ for Li⁺ led to the large increase in ionic conductivity. In the solid solution range examined, the minimum conductivity was obtained for the compositions, Li₃PS₄ (x=0.0 in Li_3+5xP_1−xS₄) and Li₄P_0.8S₄ (x=0.2 in Li_3+5xP_1−xS₄); this conductivity behavior is similar to other thio-LISICON family with the general formula, Li_xM_1−yM_y′S₄ (M=Si, Ge, and M′=P, Al, Zn, Ga, Sb). Conduction mechanism and the material design concepts are discussed based on the conduction behavior and the structure considerations.     

Crystal structure of YBa2Cu3O7−x thin films prepared by pulsed laser deposition with substrate bias voltage

The crystal structure of YBa₂Cu₃O_7−x thin films has been investigated by cross-section transmission electron microscopy. The samples were deposited on MgO (100) substrates at 670°C with substrate bias voltages of ±300 V. For the unbiased case, c-axis, a-axis and (103) oriented domains normal to the substrate surface were observed. In this film, the c-axis oriented domains are dominant, but the crystal often exhibits a longer c-lattice constant than that of the YBa₂Cu₃O_7−x system, so extra cationic layers are inserted in the YBa₂Cu₃O_7−x intrinsic stacking sequence. For the case of −300 V, rotated domains were dominant in the entire film; however, c-axis oriented domains also grow from the substrate surface. Small-angle semicoherent grain boundaries between them were observed. In the case of +300 V, all the grains show c-axis oriented YBa₂Cu₃O_7−x. The degree of preferential orientation of the grains is reduced at negative bias voltage of −300 V and the structure defects are reduced by applying a positive bias of +300 V.     

Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Phase evolution in the Bi---Sr---Ca---Cu---Al---O system was studied. Two Al-containing phases BiSr_1.5Ca_0.5Al₂O_z and (Sr_1−xCa_x)₃Al₂O₆ (x = 0.4 − 0.45) were determined to be chemically compatible with Bi_2.18Sr₂CaCu₂O_8+x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr_1.5Ca_0.5Al₂O_z was found to be in equilibrium only with the melt and the (Sr_1−xCa_x)₃Al₂O₆ phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr_1.5Ca_0.5Al₂O_z ↔ (Sr_1−xCa_x)₃Al₂O₆ + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + nBiSr_1.5Ca_0.5Al₂O_z and Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr_1.5Ca_0.5Al₂O_z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr_1−xCa_x)₃Al₂O₆ imbedded in Bi-2212 lamellas. The Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.     

Synthesis and investigation of the complex copper oxides Bi2Sr2R2−xCexCu2O10+δ (R=rare earth)

Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi₂Sr₂R_1.33Ce_0.67Cu₂O_10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi₂Sr₂Cu₂O_10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi₂Sr₂Eu_2−xCe_xCu₂O_10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (<3%) were detected for some of these samples.     

Influence of Nd substitution on the superconducting properties of ceramics in the 2212 system Bi2−wPbwSr2−xCa1−yNdx+yCu2O8+z

In the system Bi_2−wPb_wSr_2−xCa_1−yNd_x+yCu₂O_8+z different fractions of Nd are substituted on either Sr of Ca sites in order to introduce intrinsic insulating pinning centres. It is shown that a Nd concentration around x or y = 0.2 is likewise favourable with an average Nd---Nd distance in the range of the coherence length in the a, b-plane. However, clear evidence of flux pinning is only present for charge compensation with Pb²⁺. A simultaneous substitution of the Bi-based 2212 superconductor with moderate amounts of Nd³⁺ and Pb²⁺ improves the superconducting properties by strengthening the flux pinning forces.     

Characterization of Ce1−xZrxO2 thin films prepared by pyrolytic spray technique

It was demonstrated that spray pyrolysis can be used to prepare Ce_1−xZr_xO₂ thin films with x between 0 and 1. The composition of these films was determined by electron probe microanalysis (EPMA), and the crystalline structure by X-ray diffraction (XRD) and Raman spectroscopy (RS). Cyclic voltammetry (CV) was performed in an electrolyte of propylene carbonate with 1 M LiClO₄. Films with high Zr content were incapable of charge exchange of Li⁺ ions. In the contrast, films with high Ce content were found to be able to insert/extract large charge densities of Li⁺ ions. They also remained transparent during Li⁺ intercalation.     

Tunneling studies on single crystals of superconducting Bi2Ca1−xYxSr2Cu2O8+δ

Tunneling studies have been carried out on single crystals of Bi₂Ca_1−xSr_xCu₂O_8+δ over a wide range of compositions wherein the hole concentration varies by a factor of 2.5. The 2Δ value varies between 25 meV and 75 meV over the composition range studied, but scales with 2Δ/k_BT_c≈9.5 throughout.     

Trapping mechanism of superconductivity suppression induced by Pr substitution in the Ho1−xPrxBa2Cu3O7−δ system

Pr concentration dependence of the superconducting transition temperature T_c in the Ho_1−xPr_xBa₂Cu₃O_7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y_1−xPr_xBa₂Cu₃O_7−δ reference system. Both systems have the same value of the critical concentration x_c=0.58, in accordance with nearly equal ionic radii of Ho³⁺ and Y³⁺ ions. It has been shown that the T_c(x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr^IV-ions, if one takes also into account that the number of these ions changes with x.     

Effect of Hg addition on synthesis of Bi-based superconductors

We have investigated the effect of Hg addition on the superconducting properties of BiSrCaCuO system. Polycrystalline samples with nominal composition Bi₂Sr_2−xHg_xCa₁Cu₂O_y and Bi₂Sr_2−xHg_xCa₂Cu₃O_y (x=0.3) were synthesized and used to investigate the phase evolution by XRD, superconducting behaviour by R–T measurement and the structural grain boundary effects by SEM. From these measurements, it has been noticed that the phases obtained with both types of compositions are the same as Bi2212 but the T_c values are different. With additional annealing, T_{c zero} values were raised from 60 to 72 K in Bi₂Sr_2−xHg_xCa₁Cu₂O_y and 64 to 92 K in Bi₂Sr_2−xHg_xCa₂Cu₃O_y. Also, an improved grain boundary linkage has been observed by SEM for the 92 K sample.     

Spectroscopic study of BixEu1−xVO4 and BiyGd1−yVO4 mixed oxides

The mixed oxides Bi_xEu_1−xVO₄ and Bi_yGd_1−yVO₄ crystallize in a zircon-type structure, for 0 <x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6s² of Bi³⁺ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi_xEu_1−xVO₄ spectra are ascribed to intra-configurational 4f – 4ftransitions of the Eu³⁺ ion. The broad absorption shift in BiLnVO₄ (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO₄ lattice, is ascribed to charge transfer processes in a Bi-VO₄ center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi³⁺. The concept of an internal pressure of Bi³⁺ ions is also used to explain the broad A-band shifts.     

Effects of cation- or oxide ion-defect on conductivities of apatite-type La–Ge–O system ceramics

A series of apatite-type La–Ge–O ceramics were prepared and their cation-defect at the 4f+6h sites and oxide ion-defect at 2a site were investigated. In La_xGe₆O_12+1.5x ceramics of x=6–12, the higher conductivities were obtained in the region of apatite composition, La_x(GeO₄)₆O_1.5x−12 (x=8–9.33), and the highest conductivity was achieved for La₉(GeO₄)₆O_1.5 (x=9), where the number of cation (La³⁺) occupying the 4f+6h sites is 9 and the number of oxide ion occupying the 2a site is 1.5. The ceramics with cation- and oxide ion-defects were La_9−0.66xSr_x(GeO₄)₆O_1.5 (x=0–1), La_9−1.33xZr_x(GeO₄)₆O_1.5 (x=0–1), La_9−xSr_x(GeO₄)₆O_1.5−0.5x (x=0–3), La_9−xZr_x(GeO₄)₆O_1.5+0.5x (x=0–1), La_x(GeO₄)_3x−21(AsO₄)_27−3xO_1.5 (x=0–3), La_x(GeO₄)_33−3x(AlO₄)_3x−27O_1.5 (x=0–3), La₉(GeO₄)_6−x (AlO₄)_xO_1.5−0.5x (x=0–3), La₉(GeO₄)_6−x(AsO₄)_xO_1.5+0.5x (x=0–1), La_9.33−xSr_x(GeO₄)₆O_2−0.5x (x=0–1.2) and La_x(GeO₄)_4.5(AlO₄)_1.5O_1.5x−12.75 (x=8.8–9.83), which were prepared by the partial substitution of La³⁺and GeO₄⁴⁻of the basic apatite La₉(GeO₄)₆O_1.5 with Sr²⁺ or Zr⁴⁺ and AlO₄⁵⁻ or AsO₄³⁻. Such substitutions lowered the conductivity of La₉(GeO₄)₆O_1.5. These results were discussed by the electrostatic interaction between Sr²⁺, Zr⁴⁺, AlO₄⁵⁻ or AsO₄³⁻ and oxide ion as a conductive species.     

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La₂Cu_1−xCo_xO_4+δ solid solutions with orthorhombic K₂NiF₄-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La₂Cu_1−xCo_xO_4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10⁻⁶ K⁻¹. Studying the dependence of oxygen permeation fluxes through La₂Cu(Co)O_4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La₂Cu_1−xCo_xO_4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.     

Tuning the multiferroic properties of Pb(Fe1/2Nb1/2)O3 by cationic substitution

A series of substituted lead iron niobate compounds with the general formula Pb²⁺_(1−x)A^Z_x(Fe_{{(1−(2−Z)x)/2}}Nb_{{(1+(2−Z)x)/2}})O₃ (0<x<0.6 and A=La³⁺, K⁺ or Sr²⁺) were prepared by a modified solid-state synthesis. The relative concentrations of Fe³⁺ and Nb⁵⁺ were adjusted to compensate the charge imbalance due to the aliovalent substitution. The dielectric constant and magnetic susceptibilities were studied as a function of temperature. The temperature of the dielectric maximum, T_M, of the substituted compounds decreased linearly with increasing concentration of the substituent ions. The magnetic measurements showed an antiferromagnetic transition at temperatures T_N1 due to the superexchange interactions mediated by Fe–O–Fe and an additional antiferromagnetic-type transition at T_N2. T_N1 linearly increased with the increasing concentration of Fe³⁺ ion at the B-site of ABO₃-type substituted compounds. T_M is shown to be directly dependent on the concentration of the ferroactive Nb⁵⁺ ions at the B-site and Pb²⁺ ions at the A-site.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Lithium insertion to ReO3-type metastable phase in the Nb2O5–WO3 system

Lithium insertion to distorted ReO₃-type metastable solid solution Nb_xW_1−xO_3−x/2 (0≤x<0.25) has been studied by chemical and electrochemical methods. In the course of lithium insertion into tetragonal compounds, transition to a cubic phase was found to occur in the region where values of y (in Li_yNb_xW_1−xO_3−x/2) fall between 0.2 and 0.3, and the phase transition was found to depend on the conditions of the reaction. Changes in OCV and lattice parameters in tetragonal region (y<0.2) were discussed from the viewpoint of the ordering of lithium ions. Also, the component diffusion coefficient of lithium in tetragonal compounds Li_0.1Nb_xW_1−xO_3−x/2 (0≤x≤0.23) was found to increase with niobium content when x≤0.10, and to saturate at 4×10⁻⁹ cm²/s.