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.     

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.     

Dielectric and ferroelectric properties of the mixed crystals system (CH3NH3)5Bi2(1 − x)Sb2xCl11

Dielectric and pyroelectric properties of the mixed crystals system, (CH₃NH₃)₅Bi_{2(1 − x)}Sb_2xCl₁₁ (0 < x < 0.25) were systematically investigated. Temperature dependencies of ′_c in the vicinity of ferro-paraelectric phase transition were measured for the mixed crystals with x = 0.05, 0.07, 0.11, 0.13 and 0.25 in the frequency region 1 kHz–1 MHz. The substitution of bismuth atoms by antimony drastically reduces the magnitude of ′_c and shifts the ferro-paraelectric phase transition towards higher temperatures. The dielectric dispersion of the complex electric permittivity, _c^*, in x = 0.05 crystals was studied in the frequency range from 30 to 1000 MHz. Around 321 K phase transition, two dielectric relaxators are postulated; a low-frequency one in the megahertz region showing a critical slowing down and a high-frequency one in the gigahertz region.     

Crystal structure, superconductivity and magnetic properties of the superconducting ferromagnets Gd1.4−xDyxCe0.6Sr2RuCu2O10 (x=0–0.6)

The structural, electrical and magnetic properties of the superconducting ferromagnets, Gd_1.4−xDy_xCe_0.6Sr₂RuCu₂O₁₀ (x=0–0.6) are systematically investigated as a function of Dy doping and temperature. These compounds are characterized by high temperature superconductivity (T_c ranging from 20 to 40 K depending upon the Dy content) co-existing with weak ferromagnetism with two magnetic transitions (T_M2 ranging from 95 to 106 K and T_M1 around 120 K). Doping with Dy gives no significant structural changes except for a minor change in the c/a ratio. However the superconducting transition temperature is significantly suppressed and magnetic ordering temperature enhanced on Dy doping. These effects are described and discussed.     

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O_3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10⁻⁶ K⁻¹ at 300–800 K and (19.3–26.7)×10⁻⁶ K⁻¹ at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La_1−xSr_xFe_1−yGa_yO_3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.     

Ionic conduction properties of layer-type oxides NaxMx/2Ti1−x/2O2 (M=Ni, Co; 0.60≤x≤1.0)

Layer-type oxide Na_xM_x/2^IITi_1−x/2^IVO₂ (M=Co, Ni; 0.60≤x≤1.0) has been prepared by solid state reactions. In both series, two structural variants of type -NaFeO₂ (O3) and β-RbScO₂ (P2) have been obtained consecutively as x decreases with a borderline composition around x_c0.7. With the decrease of x, the ionic conductivity has been found to increase up to 8.4×10⁻² S cm⁻¹ at 770 K (Na_0.67Co_0.33Ti_0.67O₂). Compositions of P2 have been found to exhibit the conductivity values two to five times greater than those of O3, primarily due to the larger rectangular threshold available for the diffusion of Na⁺ ions. Such a structural effect has also been considered to depend on the polarizability of alkali ion. HT-XRD and ²³Na-NMR data of Na_0.67Co_0.33Ti_0.67O₂ strongly suggest that the diffusion of Na⁺ ion is deeply related with the local distortion of trigonal prismatic sites, leading to the change of activation energy around 430 K.     

Origin of the metal-insulator transition in the superconducting series Bi2Sr2Ca1−xYxCu2O8+δ

The metal-insulator transition in the solid solution Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ (0≤x≤1) has been investigated by TGA (oxygen content) and by X-ray absorption spectroscopy (Bi and Cu valence states). Resistivity and AC magnetic susceptibility measurements have shown that the superconducting properties and the metallic behavior vanish for x>0.55. The oxygen content δ is larger than x/2 for x≤0.3 and smaller than x/2 for x≥0.6. For x=0, the Cu K edge shows a shift towards high energy with respect to the Cu(II) oxide La₂CuO₄; this shift decreases with increasing x in agreement with the decrease of the doping hole density and the variations of the physical properties. For 0≤x≤0.3, the Bi L₃ edge shows a shift of 1 eV towards low energy with respect to the Bi(III) oxide Bi₂O₃ in agreement with the charge transfer between [CuO₂]_∞ and [BiO]_∞ planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which δ is smaller than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level and decreases the contribution of the Bi-O electron pocket to the hole density.     

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.     

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.     

Effect of Cu doping in MgB2 superconductor at various processing temperatures

The effects of Cu doping in MgB₂ superconductor has been studied at different processing temperatures. The polycrystalline samples of Mg_1−xCu_xB₂ with x = 0.05 were synthesized through the in-situ solid sate reaction method in argon atmosphere at different temperature range between 800–900 °C. The samples were characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and low temperature R–T measurement techniques for the phase verification, microstructure and superconducting transition temperature, respectively. The XRD patterns of Mg_1−xCu_xB₂ (x = 0.05) do not exhibit any impurity traces of MgB₄ or MgB₆ and they show the sharp transition in the samples prepared at 850 °C. The onset transition temperature of the prepared samples is around 39 K, which is almost the same as that for the pure MgB₂. This indicates that Cu doping in MgB₂ does not affect the transition temperature. The SEM micrograph of Mg_0.95Cu_0.05B₂ has shown that the sample is dense with grain size smaller than 1 μm.     

Disruption of antiferromagnetic order in Zn-doped La2CuO4

The magnetic phase diagram of La₂(Cu_1−xZn_x)O₄ has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T_N(x) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T_N. Extrapolation of our results yields T_N → 0 K at x = 0.11 for a single (Cu_1−xZn_x)O₂ plane, and comparison with YBa₂(Cu_1−xZn_x)₃O₆ implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.     

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.     

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.     

Optical and electrical properties of binary WO3–Pb3O4 glasses

A glass system of the composition xWO₃+(100−x)Pb₃O₄, with x=5, 10, 20 and 30 mol.% was prepared. The optical absorption, ac and dc conductivities are the subject of the present work. The optical absorption indicates that the electronic transition is indirect and is associated with phonon assisted transition. The exponential dependence of the absorption coefficient as a function of the incident photon energy suggests that the Urbach rule is obeyed, and indicates the formation of a band tail. On the other hand, ac conductivity measurements are performed in the frequency range 0.1–100 kHz, and in the temperature range 300–600 K. The results of the electrical conductivity are discussed on the basis of electronic glass conduction models. Correlated narrow-band limit for random sites and single polaron hopping model are found to describe the experimental results effectively. The dielectric constant was correlated to the optical band gap and a satisfactory relation was found. It was also possible to calculate the thermochromic properties from independent ac and dc measurements, and it was possible to evaluate the optical gap at 0 K by extrapolation.     

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.     

Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB_2−xC_x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB₂ lattice. The superconducting transition temperature, T_c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ_D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T_c and θ_D, is seen to increase monotonically from 0.8 in MgB₂ to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T_c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T_c is argued to mainly arise due to large decrease in θ_D with C concentration and a decrease in the hole density of states at N(E_F).     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni).: V. Oxygen permeability of Mg-doped La(Ga, Co)O3−δ perovskites

The LaGa_1−x−yCo_xMg_yO_3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤y≤0.10 at x=0.60 and 0≤y≤0.20 at x=0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa_1−x−yCo_xMg_yO_3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10⁻⁶ K⁻¹ at 300–1100 K. Doping La(Ga,Co)O_3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa_1−x−yCo_xMg_yO_3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.     

The relationship between crystal structure and electrical conductivity in the LaY1−xInxO3 (x=0.0–0.7) system

The electrical conductivity of the LaY_1−xIn_xO₃ (x=0.0–0.7) system has been studied from the viewpoint of crystal chemistry. The high temperature form of LaYO₃ (x=0.0) was ascertained to be the Sm₂O₃-type (B-type rare earth) structure, not perovskite-type one. The X-ray diffraction (XRD) experiments revealed that the samples with x=0.05 and 0.10 were the mixed phase of Sm₂O₃-type and perovskite-type structure, and changed to perovskite phase in the range of x0.20. From oxygen partial pressure dependence of the electrical conductivity, it was found that both the Sm₂O₃-type and the perovskite-type single phases showed hole conduction, but the mixed phase did oxide-ion one. The electrical conductivity of the LaY_1−xIn_xO₃ (x=0.0–0.7) system increased with increasing x, and showed the maximum value in the range of x=0.05–0.10, and then decreased with increasing x. The occurrence of oxide-ion conduction was discussed from the viewpoint of lattice distortion in the mixed phase.     

Ionic conductivity and phase stabilization in Cu- and Tl-substituted CsAg2I3

Samples of general formula CsAg_2−xM_xI₃, x=0–0.4 and M=Cu and Tl, have been prepared and studied by powder X-ray diffraction, DSC and electrical conductivity measurements. X-ray diffractograms and DSC curves showed the possibility of stabilizing the high-temperature -phase at lower temperatures in Tl-substituted samples, while such results were not obtained in Cu-substituted samples. Ionic conductivity measurements showed two regions corresponding to the low- and high-temperature phases. The transition temperature between the two phases was found to remain unchanged with the addition of Cu⁺ and decreased gradually with increasing Tl⁺. The ionic conductivity decreased in Cu-substituted samples and enhanced with the incorporation of Tl⁺ ion in the lattice of CsAg₂I₃. Dielectric constant was found to show behaviour similar to that of the ionic conductivity, and this is an evidence of the predominant effect of ion hopping on this property.