Influence of octahedral tilting and composition on electrical properties of the Li0.2 − xNaxLa0.6TiO3 (0 ≤ x ≤ 0.2) series

The Rietveld analysis of ND patterns of polycrystalline Li_0.2 − xNa_xLa_0.6TiO₃ (0 ≤ x < 0.2) samples, recorded between 300 and 1075 K, shows an orthorhombic–tetragonal transformation, in which the octahedral tilting along the b axis is eliminated at ~ 773 K, but the vacancy ordering along the c axis remains. In Li rich samples, conductivity (10^− 3 Ω^− 1 cm^− 1 at 300 K) departs from the Arrhenius behaviour, decreasing activation energies from 0.37 to 0.14 eV when octahedral tilting is eliminated. Successive Maxwell–Wagner blocking processes, detected in the real part of dielectric constant plots, have been ascribed to the Li blocking at interior domains, grain-boundary and electrode–electrolyte interfaces. The substitution of Li⁺ by Na⁺ decreases the amount of vacant A-sites, decreasing several orders of magnitude the conductivity when the amount of vacancies approaches the vacancy percolation threshold (n_p = 0.27). Below the percolation threshold, Li ions only display local mobility, remaining confined into small domains of perovskites.     

Water incorporation into the Ba2(In1 − xMx)2O5‪ (M = Sc 0 ≤ x < 0.5 and M = Y 0 ≤ x < 0.35) system and protonic conduction

Ba₂(In_1 − xM_x)₂O_{5 − y / 2}(OH)_y‪□_{1 − y / 2} (y ≤ 2; M = Sc³⁺ 0 ≤ x < 0.5 and M = Y³⁺ 0 ≤ x < 0.35) compounds were prepared by reacting Ba₂(In_1 − xM_x)₂O₅‪ phases with water vapor. This reaction is reversible. Analyses of the hydration process by TG and XRD studies show that the thermal stability of hydrated phases increases when x increases and that the incorporation of water is not a single-phase reaction inducing either a crystal system or space group modification. Fully hydrated (y = 2) and dehydrated (y = 0) samples have been stabilized at room temperature and characterized for all compositions. In wet air, all phases show a proton contribution to the total conductivity at temperatures between 350 and 600 °C. At a given temperature, proton conductivity increases with the substitution ratio and reaches at 350 °C, 5.4 10^− 3 S cm^− 1 for Ba₂(In_0.65Sc_0.35)₂O_4.2□_0.2(OH)_1.6.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

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.     

Synthesis and dielectric properties of Bi1−xNdxFeO3 perovskites

Single-phase polycrystalline samples of Bi_1−xNd_xFeO₃ were prepared by standard solid state reaction method. X-ray diffraction (XRD) patterns of the powder samples were recorded and analyzed for the confirmation of crystal structure lattice parameters. Further, these samples were characterized by IR technique to identify and understand the aspect of bonding in the present samples. The dielectric measurements were carried out on the samples as a function of frequency in the range 100 Hz to 1 MHz at room temperature and also as a function of temperature in the range 300–750 K at certain fixed frequencies. The DC and AC electrical resistivity studies were carried out in order to understand conduction mechanism in the samples.     

Oxygen nonstoichiometry and electrical conductivity of the solid solutions La2−xSrxNiOy (0≤x≤0.5)

Oxygen nonstoichiometry and electrical resistance of a series La_2−xSr_xNiO_y solid solutions, where x=0.0, 0.2 and 0.5 in argon flows at oxygen partial pressures 1.5, 10.2, 49.2, 100 and 286 Pa within the temperature range of 20–1050°C were studied. Nickelate oxygen desorption/sorption spectra when heating–cooling at constant rate demonstrated strong dependence of cation composition of the samples. Unlike La_1.5Sr_0.5NiO_y compounds those of La₂NiO_y and La_1.8Sr_0.2NiO_y have weakly bonded oxygen, capable to exchange reversibly with the gas phase at the temperatures higher than 250°C. The equilibrium values of oxygen nonstoichiometry and specific resistance for the these nickelates were determined at 300–1050°C and p_O2=1.5–286 Pa as a functions of temperature versus oxygen partial pressure. All nickelate studied appear to be p-type conductors with metal electric conductivity at equilibrium states.     

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.     

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr_1−xMn_x)O_3+δ (0.0x1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr_1−xMn_x)O_3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn⁴⁺ ion content increases monotonically in the range 0.2x1.0. The magnetic measurement of La(Cr_1−xMn_x)O_3+δ indicates that a Mn³⁺ ion is a high-spin state with (d)³(dγ)¹. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr³⁺, the Mn³⁺, the Mn⁴⁺ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr_1−xMn_x)O_3+δ is a p-type semiconductor and exhibits the hopping conductivity.     

Structural properties and Raman spectroscopy of orthorhombic (Eu1−xPrx)0.6Sr0.4MnO3 (0≤x≤1.0)

The effect of Pr doping on structural properties and room temperature Raman spectroscopy measurements is investigated in manganites (Eu_1−xPr_x)_0.6Sr_0.4MnO₃ (0≤x≤1.0) with fixed carrier concentration. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in an orthorhombic distorted structure with a space group Pnma. It is evident that, with increasing Pr substitution, three types of orthorhombic structures can be distinguished. The phonon frequencies of the three main peaks, in room temperature Raman-scattering measurements, have been discussed together with their structural characteristics, such as bond-length, bond-angles, and the change of orthorhombic structure type. With the increase of Pr content, the mode at 491  cm⁻¹ also shows a corresponding change. A step effect becomes evident, which seems to indicate the close dependence between the frequency shift of this mode and the change in crystal symmetry. This further supports the notion that the mode at 491  cm⁻¹ is closely correlated with the Jahn–Teller distortion. Moreover, we have found that the lowest frequency peak (assigned as an A_1g phonon mode) depends linearly on the tolerance factor t.     

New observation of the , 1Δg, and 2Πg states and molecular constants with all Li2, Li2, and LiLi data

This paper reports our new observation of the , 1³Δ_g (v = 2–4), and 2³Π_g (v = 2–8) states of ⁶Li⁷Li by continuous wave perturbation facilitated optical–optical double resonance spectroscopy. Combining our new experimental term values of ⁶Li⁷Li with the available experimental data of ⁶Li₂ and ⁷Li₂, molecular constants and potential energy curves by Rydberg–Klein–Rees and direct-potential-fit techniques have been determined. Born-Oppenheimer breakdown parameters of the Li₂ 1³Δ_g and 2³Π_g states are calculated.     

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.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3–x(Na0.5La0.5)TiO3 ceramic system

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–x(Na_0.5La_0.5)TiO₃ ceramic system were investigated. Two-phase system was confirmed by the XRD patterns and the EDX analysis. A co-existed second phase (Mg_0.95Co_0.05)Ti₂O₅ was also detected. The microwave dielectric properties are strongly related to the density and the matrix of the specimen. A new microwave dielectric material 0.88(Mg_0.95Co_0.05)TiO₃–0.12(Na_0.5La_0.5)TiO₃, possessing an excellent combination of dielectric properties: ε_r  22.36, Q × f  110,000 GHz (at 9 GHz), τ_f  2.9 ppm/°C), is proposed as a candidate dielectric for GPS patch antennas.     

Lithium ion conductivity of Li5+xBaxLa3?xTa2O12 (x?=?0–2) with garnet-related structure in dependence of the barium content

The stoichiometry range and lithium ion conductivity of Li_5+x Ba _x La_3−x Ta₂O₁₂ (x = 0, 0.25, 0.50, 1.00, 1.25, 1.50, 1.75, 2.00) with garnet-like structure were studied. The powder X-ray diffraction data of Li_5+x Ba _x La_3−x Ta₂O₁₂ indicated that single phase oxides with garnet-like structure exist over the compositional range 0 ≤ x ≤ 1.25; while for x = 1.5, 1.75 and 2.00, the presence of second phase in addition to the major garnet like phase was observed. The cubic lattice parameter increases with increasing x and reaches a maximum at x = 1.25 then decreases slightly with further increase in x in Li_5+x Ba _x La_3−x Ta₂O₁₂. The impedance plots of Li_5+x Ba _x La_3−x Ta₂O₁₂ samples obtained at 33 °C indicated a minimum grain-boundary resistance (R _gb) contribution to the total resistance (R _b + R _gb) at x = 1.0. The total (bulk + grain boundary) ionic conductivity increases with increasing lithium and barium content and reaches a maximum at x = 1.25 and then decreases with further increase in x in Li_5+x Ba _x La_3−x Ta₂O₁₂. Scanning electron microscope investigations revealed that Li_6.25Ba_1.25La_1.75Ta₂O₁₂ is much more dense, and the grains are more regular in shape. Among the investigated compounds, Li_6.25Ba_1.25La_1.75Ta₂O₁₂ exhibits the highest total (bulk + grain boundary) and bulk ionic conductivity of 5.0 × 10⁻⁵ and 7.4 × 10⁻⁵ S/cm at 33 °C, respectively.     

Defect chemical and statistical thermodynamic studies on oxygen nonstoichiometric Nd2 − xSrxNiO4 + δ

In order to elucidate how oxygen content changes in Nd_2 − xSr_xNiO_4 + δ (x = 0, 0.2, 0.4), defect chemical and statistical thermodynamic analyses were carried out. The relationship among δ, P(O₂), and T were analyzed by a defect equilibrium model. Since Nd_2 − xSr_xNiO_4 + δ shows metal like band conduction at high temperatures, chemical potential of hole is expressed by the integration of the Fermi-Dirac distribution function and the density of state. The nonstoichiometric variation of oxygen content in Nd_2 − xSr_xNiO_4 + δ can be explained by the defect equilibrium model with a regular solution approximation. Partial molar entropy and partial molar enthalpy of oxygen are calculated from the nonstoichiometric data and Gibbs–Helmholtz equation. The relationship among defect structure, defect equilibrium, and thermodynamic quantities is elucidated by the statistical thermodynamic model. Thermodynamic quantities are calculated by the statistical thermodynamic model with the results of defect chemical analysis and compared with those obtained from experimental results. Thermodynamic quantities calculated by the statistical thermodynamic model can explain rough tendency of those obtained from the δ–T–P(O₂) relationship.     

Structure, ferroelectric and piezoelectric properties of (Bi1−x−yNa0.925−x−yLi0.075)0.5BaxSryTiO3 lead-free piezoelectric ceramics

Lead-free multi-component ceramics (Bi_1−x−yNa_{0.925−x−y}Li_0.075)_0.5Ba_xSr_yTiO₃ have been prepared by an ordinary sintering technique and their structure and electrical properties have been studied. All the ceramics can be well-sintered at 1100 °C. X-ray diffraction patterns shows that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) is formed at 0.04 < x < 0.08. As compared to pure Bi_0.5Na_0.5TiO₃ ceramic, the coercive field E_C of the ceramics decreases greatly and the remanent polarization P_r of the ceramics increases significantly after the formation of the multi-component solid solution. Due to the MPB, lower E_C and higher P_r, the piezoelectricity of the ceramics is greatly improved. For the ceramics with the compositions near the MPB (x = 0.04–0.08 and y = 0.02–0.04), piezoelectric coefficient d₃₃ = 133–193 pC/N and planar electromechanical coupling factor k_P = 16.2–32.1%. The depolarization temperature T_d reaches a minimum value near the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T_d.     

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.     

Preparation and characterization of superionic materials in a new mixed system (Cu(1−x)AgxI)–(Ag2O)–(P2O5), (0.05≤x≤0.25)

In this paper attempts have been made to prepare superionic glassy electrolytes in the mixed system 30(Cu_(1−x)Ag_xI)–46.66(Ag₂O)–23.33(P₂O₅), where x=0.05, 0.1, 0.15, 0.2 and 0.25, respectively, using the melt quenching process. The solid samples were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) spectroscopy and silver ionic transport number studies. XRD analysis and FT-IR spectroscopic results have provided details regarding the various phases present in the new system and also indicated the formation of composite materials consisting of glassy and crystalline phases. The transport number measurements have indicated the formation of superionic solids having Ag⁺ ions as the mobile species in the present system.     

Defect chemistry and oxygen transport of (La0.6Sr0.4 − xMx)0.99Co0.2Fe0.8O3 − δ, M = Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2), Sr: Part I: Defect chemistry

This paper is the first part of a two part series, where the effects of varying the A-site dopant on the defect chemistry, the diffusion coefficient and the surface catalytic properties of the materials (La_0.6Sr_0.4 − xM_x)_0.99Co_0.2Fe_0.8O_3 − δ, M = Sr, Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2) (LSMFC) have been investigated. In part I, the findings on the defect chemistry are reported, while the transport properties are reported in part II. Substitution of Sr²⁺ ions with Ca²⁺ ions (smaller ionic radius) and Ba²⁺ ions (larger ionic radius) strains the crystal structure differently for each composition while keeping the average valence of the cations constant. The Ba²⁺ containing materials show the largest oxygen loss at elevated temperatures, while the purely Sr²⁺ doped material showed the smallest oxygen loss. This was reflected in the partial oxidation entropy of the materials. The measured oxygen loss was modelled with point defect chemistry models. Measurements at very low p_O2 showed several phase transitions.     

Electronic structure studies of Mg0.95Mn0.05Fe2−2xTi2xO4 (0x0.8)

The electronic structure of Mg_0.95Mn_0.05Fe_2−2xTi_2xO₄ (0x0.8) compound is investigated using near edge X-ray absorption fine structure, (NEXAFS) spectroscopy measurements, carried out at O K, Fe and Ti L_3,2-edges at room temperature. The O K-edge spectra indicate that the Fe 3d orbitals have been considerably modified and a new spectral feature start dominating in the pre-edge region at higher Ti doping. The Fe 2p NEXAFS spectra exhibit a mixed valent Fe²⁺/Fe³⁺ states apart from the conversion of Fe³⁺ to Fe²⁺ with the substitution of Ti ions. The Ti L_3,2-edge spectra indicate that Ti ions remain unchanged at 4+ state. These variations in the host electronic structure due to Ti substitution are consistent with the dielectric and transport properties of the material.     

First-principles study of electronic structure and optical properties of barium strontium titanates (BaxSr1−xTiO3)

Using first-principles calculations based on density-functional theory in its local-density approximation, we investigate the energy band structures and total density of states (TDOS) of barium strontium titanates (BSTs). Direct band gaps of 1.89 and 1.87 eV at the Γ point in the Brillouin zone are calculated for Ba_xSr_1−xTiO₃ (x = 2/3 and 1/3), respectively. The optical properties of the above perovskites in the core-level spectra are investigated by the first-principles under scissor approximation. The real and imaginary parts of the complex dielectric function and derive optical constants, viz., the refractive index, extinction coefficient, reflectivity, and the electron energy-loss spectrum are calculated.