Ac conductivity, dielectric losses, permittivity behavior of BaxSr1?xFe0.8Co0.2O3?δ (x?=?0, 0.5 and 1) ceramics

In this paper, we have investigated ac conductivity, dielectric losses, permittivity of Ba _x Sr_1−x Fe_0.8Co_0.2O_3−δ (x = 0, 0.5, 1) ceramics. It has been observed that increase in barium content decreases activation energy (i.e. conductivity increases) in the higher temperature region (550–770 K). In the lower temperature region system with x = 0 shows metallic conductor behavior and system with x = 1 shows thermally activated behavior. The ferroelectric peak temperatures are observed at 700, 750 and 723 K for x = 0, 0.5 and 1, respectively. This peak temperature can be attributed to the oxygen-vacancy-related dielectric relaxation. With the increase of barium content the dielectric dispersion tend to diminishes. Partial substitution of strontium with barium (for x = 0.5) reduces dielectric losses by 50%.     

Synthesis and properties of Ca1 ? xTiSi1 ? xFe2xO5 solid solutions

We have studied the CaTiSiO₅-Fe₂TiO₅ system. Ca_{1 − x} TiSi_{1 − x} Fe_2x O₅ solid solutions were prepared by low-temperature plasma synthesis in a hydrogen-oxygen flame. The CaTiSiO₅-Fe₂TiO₅ system contains two phases of variable composition: α (monoclinic structure) and β (orthorhombic structure). We have determined the homogeneity ranges, unit-cell parameters, and electrical parameters of the synthesized solid solutions. The Ca and Si in CaTiSiO₅ can be replaced by Fe³⁺. Fe³⁺ substitution increases the conductivity of the material by up to five orders of magnitude.     

The structural and electrical property of CuCr1 − xNixO2 delafossite compounds

A series of CuCr_1 − xNi_xO₂ (0 ≤ x ≤ 0.06) polycrystalline samples was prepared. The electrical conductivity was measured in the temperature range of 160–300 K. It was found that the electrical conductivity (σ) increases rapidly with the doping of Ni²⁺ ions. At room temperature, the σ is 0.047 S cm^− 1 for the sample with x = 0.06, which is two orders of magnitude larger than that of the CuCrO₂ sample (9.49E− 4 S cm^− 1). The Seebeck coefficients are positive for all samples, which indicate p-type conducting of the samples. The experimental results imply that it is possible to get higher electrical conductivity p-type transparent conducting oxides (TCO) from CuMO₂ by doping with divalent ions.     

Growth and electrical properties of pure and Ni-doped Co3O4 single crystals

Single crystals of pure and Ni-doped Co₃O₄ spinel-type phases have been grown by C.V.T. Electrical conductivity and thermopower measurements have been performed between 25 and 950 K. Ni-doping gives rise to a significant increase of electrical conductivity at room temperature by three orders of magnitude. The obtained results suggest that the Ni atoms are located in the octahedral sites of the spinel lattice with both oxidation states 2+ and 3+, as illustrated by the formula : Co²⁺_1?yCo³⁺_y[Co³⁺_2?xNi²⁺_yNi³⁺_x?y]O₄.     

Investigations of the Spin Hamiltonian Parameters for the Cu2+ Sites in PrBa2Cu3O6+x and Pr0.5Er0.5Ba2Cu3O6+x

The spin Hamiltonian parameters (the anisotropic g factors g _∥ and g _⊥ and the hyperfine structure constants) for the Cu²⁺ sites in PrBa₂Cu₃O_6+x and Pr_0.5Er_0.5Ba₂Cu₃O_6+x are theoretically investigated using the high order perturbation formulas of these parameters for a 3d⁹ ion in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are determined from the superposition model and the local structures of the Cu²⁺ sites. The theoretical spin Hamiltonian parameters are in good agreement with the observed values for both systems, and the results show improvements as compared with those based on various adjustable covalency coefficients in the previous work. The larger hyperfine structure constants for PrBa₂Cu₃O_6+x than those for Pr_0.5Er_0.5Ba₂Cu₃O_6+x can be attributed to the weaker covalency due to the relatively longer Cu²⁺– O^[`2]\mathrm{O}^{\bar{2}} bonds in the former.     

Optical, electrical and magnetic properties of Co3O4 nanocrystallites obtained by thermal decomposition of sol–gel derived oxalates

Nanocrystallites of tricobalt tetraoxide (Co₃O₄) have been synthesized by sol–gel process using cobalt acetate tetrahydrate, oxalic acid as precursors and ethanol as a solvent. The process comprises of gel formation, drying at 80 °C for 24 h to obtain cobalt oxalate dihydrate (α-CoC₂O₄·2H₂O) followed by calcination at or above 400 °C for 2 h in air. These results combined with thermal analysis have been used to determine the scheme of oxide formation. The room temperature optical absorption spectra exhibits blue shift in both (i) ligand to metal (p(O²⁻) → e_g(Co³⁺), 3.12 eV), and (ii) metal to metal charge transfer transitions (a) t_2g(Co³⁺) → t₂(Co²⁺), 1.77 eV, (b) t₂(Co²⁺) → e_g(Co³⁺), 0.95 eV together with the d–d transitions (0.853 and 0.56 eV) within the Co²⁺ tetrahedra. The temperature dependent ac electrical and dielectric properties of these nanocrystals have been studied in the frequency range 100 Hz to 15 MHz. There are two regimes distinguishing different temperature dependences of the conductivity (70–100 K and 200–300 K). The ac conductivity in both the temperature regions is explained in terms of nearest neighbor hopping (NNH) mechanism of electrons. The carrier concentration measured from the capacitance (C)–voltage (V) measurements is found to be 1.05 × 10¹⁶ m⁻³. The temperature dependent dc magnetic susceptibility curves under zero field cooled (ZFC) and field cooled (FC) conditions exhibit irreversibilities whose blocking temperature (T_B) is centered at 35 K. The observed Néel temperature (T_N  25 K) is significantly lower than the bulk Co₃O₄ value (T_N = 40 K) possibly due to the associate finite size effects.     

Shaped single crystal growth and scintillating application of Yb:(Gd,Lu)3(Ga,Al)5O12 solid solutions

Shaped single crystals of (Yb_0.05Lu_xGd_0.95−x)Ga₅O₁₂ (0.0x0.9) and Yb_0.15Gd_0.15Lu_2.7(Al_xGa_1−x)O₁₂ (0.0x1.0) 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)₃(Ga,Al)₅O₁₂ 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.     

Synthesis and crystal structure of new U(VI) and Np(VI) benzoates, K11(AnO2)23(OOCC6H5)57(H2O)18+x

The structure of new double benzoates of hexavalent actinides, K₁₁(AnO₂)₂₃(O₂C₇H₅)₅₇(H₂O)_18+x (An = U, Np), was studied. There are five crystallographically independent actinide atoms in the crystals. The coordination polyhedra of An(1), An(3), An(4), and An(5) are distorted hexagonal bipyramids, and that of An(2) is a distorted pentagonal bipyramid. Ten crystallographically independent benzoate ions have been found in the crystals. All but one anions are bidentate chelating and form with AnO₂²⁺ cations either electrically neutral [AnO₂(O₂C₇H₅)₂(H₂O)₂][An(1)O₂²⁺ cations] or negatively charged [AnO₂(O₂C₇H₅)₃]⁻[An(3)O₂²⁺, An(4)O₂²⁺, An(5)O₂²⁺ cations] complexes. The bidentate bridging benzoate ion links two adjacent An(2)O₂²⁺ cations with the formation of peculiar electrically neutral cyclic fragments [AnO₂(O₂C₇H₅)₂(H₂O)]₆ arranged perpendicular to the c-axis. There are three independent K⁺ cations in the structure. The coordination surrounding of K(1)⁺ (coordination number CN 6) is formed by oxygen atoms of two electrically neutral [AnO₂(O₂C₇H₅)₂(H₂O)₂] fragments and of the complex anion [AnO₂(O₂C₇H₅)₃]⁻. The coordination surrounding of K(2)⁺ (CN 6) consists of oxygen atoms of three complex anions [AnO₂(O₂C₇H₅)₃]⁻. The disordered K(3)⁺ cation is arranged near the sixfold axis between the cyclic fragments [AnO₂(O₂C₇H₅)₂(H₂O)]₆. An important structure-forming factor in the crystals is hydrogen bonding involving coordinated water molecules. In the coordination polyhedra of actinides, the An-O bond lengths regularly decrease in going from U to Np owing to actinide contraction.     

Dielectric properties of (1 ? x)KNbO3 · xBiZn2/3Nb1/3O3 (x ≤ 0.4) perovskite solid solutions

(1 − x)KNbO₃ · xBiZn_2/3Nb_1/3O₃ ceramic materials have been prepared by solid-state reactions. The materials with x < 0.5 have been shown to be phase-pure perovskite solid solutions. Their average lattice parameter increases linearly with x. Below 300 K, the solid solutions with 0.1 < x ≤ 0.4 are ferroelectric relaxors. M″-M′ diagrams, representing the relationship between the real and imaginary parts of the complex electric modulus, have been used to evaluate the Curie temperature of the solid solutions and the temperature of their transition to the paraelectric phase. The Arrhenius plots of dc conductivity for the solid solutions show breaks corresponding to their phase transitions. Below 400 K, the dc conductivity is low, and its contribution to the dielectric response of the solid solutions is insignificant. Original Russian Text ? I.I. Moroz, N.M. Olekhnovich, Yu.V. Radyush, A.V. Pushkarev, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 6, pp. 747–753.     

Magnetic properties in the system La1−xSrxCoO3 (0.5 ≦ x ≦ 1.0)

The cubic perovskites La_1?xSr_xCoO₃ (0.5 ≦ x ≦ 1.0) were prepared under high oxygen pressures. From the results of the magnetic measurements, it is found that Co³⁺ ions are in a high spin state and Co⁴⁺ ions in a low spin state. The superexchange interaction for Co³⁺-O-Co⁴⁺ is the strongest of all superexchange interactions in this system.     

Nanoclustering in (Nd0.7Sr0.3)1 ? xMn1 + xO3 ± δ solid solutions

The structure of (Nd_0.7Sr_0.3)_{1 − x} Mn_{1 + x} O_{3 ± δ} solid solutions with x = 0–0.2 (Nd: Sr atomic ratio maintained constant at 2.33) annealed at temperatures from 1200 to 1500°C has been studied by X-ray diffraction. The results demonstrate that, as the annealing temperature is raised, the atomic order in the stoichiometric (x = 0) materials is stable to changes in the homogeneity of the solid solution. In the case of the nonstoichiometric manganites with x = 0.15–0.20, an increase in annealing temperature is accompanied by the development of chemical and topological ordering processes in the matrix structure and cluster growth. The cluster size ranges from 100 to 250 ?, and the largest percentage of clusters (3.5–5.5%) consist of the matrix phase. The fraction of clusters of binary composition, such as MnO-MnO₂, is 0.5–2.0%.     

Crystallization of CaHf1? x Zr x Ti2O7 (0?≤ x ≤?1) zirconolite in SiO2–Al2O3–CaO–Na2O–TiO2–HfO2–ZrO2–Nd2O3 glasses

Glass-ceramics containing (Hf,Zr)-zirconolite crystals (nominally CaHf_1−x Zr _x Ti₂O₇ with 0 ≤ x ≤ 1) were envisaged to immobilize minor actinides and plutonium. Such materials were prepared in this study by controlled crystallization of glasses belonging to the SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–HfO₂–ZrO₂–Nd₂O₃ system. Neodymium was used as trivalent actinides surrogate. The effect of total or partial substitution of ZrO₂ by HfO₂ (neutron poison for fission reactions) on glass crystallization in the bulk and near the surface is presented. It appeared that Hf/Zr substitution had not significant effect on nature, structure, and composition of crystals formed both on glass surface (titanite + anorthite) and in the bulk (zirconolite). This result can be explained by the close properties of Zr⁴⁺ and Hf⁴⁺ ions and by their similar structural role in glass structure. However, strong differences were observed between the nucleation rate I_Z of zirconolite crystals in glasses containing only HfO₂ and in glasses containing only ZrO₂. Hf-zirconolite (CaHfTi₂O₇) crystals were shown to nucleate only very slowly in comparison with Zr-zirconolite (CaZrTi₂O₇) crystals. Composition changes - by increasing either HfO₂ or Al₂O₃ concentration or by introducing ZrO₂ in parent glass - were performed to increase I_Z in hafnium-rich glasses. The proportion of Nd³⁺ ions incorporated in the zirconolite phase was estimated using ESR.     

Synthesis and electrical properties of Bi2V1 ? xGexO5 + y solid solutions

A continuous series of Bi₂V_{1 − x} Ge _x O_{5 + y} solid solutions has been prepared by solid-state reactions, and their polymorphism and electrical properties have been studied. The solid solutions with 0 < x ≤ 0.2 are isostructural with the monoclinic phase α-Bi₂VO_5.5, and those with 0.2 < x ≤ 0.3 are isostructural with the orthorhombic phase. In the range 0.6 ≤ x < 1, the solid solutions have the orthorhombic Bi₂GeO₅ structure. The solid solutions with 0.4 ≤ x ≤ 0.5 have a tetragonal structure. Increasing the germanium content suppresses the ferroelectric phase transition in the Bi₂VO_5.5-based solid solutions, without changing the transition temperature. With increasing vanadium content, the conductivity of the solid solutions gradually increases, from 3 × 10⁻⁵ (x = 1) to 3 × 10⁻¹ S/cm (x = 0) at 550°C. Original Russian Text ? E.P. Kharitonova, V.I. Voronkova, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 1, pp. 60–65.     

Effects of Ho2O3 addition on defects of BaTiO3

Effects of Ho₂O₃ addition on defects of BaTiO₃ ceramic have been studied in terms of electrical conductivity at 1200 °C as a function of oxygen partial pressure (P_O2°) and oxygen vacancy concentration. The substitution of Ho³⁺ for the Ti site in Ba(Ti_1−xHo_x)O_3−0.5x resulted in a significant shift of conductivity minimum toward lower oxygen pressures and showed an acceptor-doped behavior. The solubility limit of Ho on Ti sites was confirmed less than 3.0 mol% by measuring the electrical conductivity and the lattice constant. Oxygen vacancy concentrations were calculated from the positions of P_O2° in the conductivity minima and were in good agreement with theoretically estimated values within the solubility limit. The Curie point moved to lower temperatures with increasing the oxygen vacancy concentration and Ho contents.     

Crystal Structure and Magnetic Properties of (La1?xSrxMnO3)N(LaCrO3)1?N Solid Solutions

Solid solutions (La_1−x Sr _x MnO₃) _N (LaCrO₃)_1−N with (La, Sr) and (Mn, Cr) two types of substitutions have been synthesized. For all investigated compounds the valent state of Mn, probed by XPS, remains Mn³⁺, while oxygen valent state changes according to (La, Sr)-substitution. Both crystal structure analysis and magnetic properties demonstrate inhomogeneous state of investigated materials. PACS codes: 61.10.N 75.30.C 79.60     

Electrical, thermal, and dielectric properties of PbZr1?xSnxO3 (0?≤?x?≤?0.3) single crystals

PbZrO₃ and mixed PbZr_1−x Sn _x O₃ single crystals were grown by means of high temperature solution growth technique. A spontaneous crystallization was carried out in a Pt crucible, with PbO as a solvent. Electrical as well as thermal and dielectric properties were investigated in terms of Sn concentration. The measurements have been made in the large temperature range between 120 K and 800 K. The temperature dependences of c _p for investigated crystals can be approximated by the Einstein function and at high temperatures (cubic phase) c _p reaches the classical Dulong–Petit limit value ~125 J/mol K. From electrical measurements the ac and dc conductivity and the activation energies have been calculated and attributed to different types of electrical conductivity.     

Studies on electrical properties of ( x ) BaTiO3?+?(1? x ) Ni0.92Co0.03Mn0.05Fe2O4 ME composites

The (1−x) Ni_0.92Co_0.03Mn_0.05Fe₂O₄ + (x) BaTiO₃ magnetoelectric (ME) composite have been prepared using conventional double sintering ceramic process where x varies as 1.00, 0.85, 0.70, 0.55 and 0.00. The presence of both phases has been confirmed by X-ray diffraction and the microstructure study will be carried out by SEM technique. The dc resistivity and thermo-emf of the samples have been studied with variation in temperature. The variation of dielectric constant (έ) and loss tangent (tan δ) will be measured in the frequency range 100 Hz–1 MHz. The ac conductivity has been derived from dielectric constant (έ) and loss tangent (tan δ). The static value of magnetoelectric conversion factor dc (ME)_H has been studied as a function of intensity of magnetic field.     

Magnetic susceptibility and effective magnetic moment of the Nd3+ and Co3+ ions in NdCo1 ? x Ga x O3

The magnetic susceptibility of NdCo_{1 − x} Ga _x O₃ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.8, 0.9, 1) has been measured at temperatures from 80 to 950 K. The effective magnetic moments (μ_eff) due to the magnetic moments of the Co³⁺ and Nd³⁺ ions have been determined in the temperature ranges of Curie-Weiss behavior, 130–370 and 600–940 K, and have then been used, together with the effective magnetic moment of Nd³⁺ (3.62μ_B or 4.20μ_B), to evaluate the effective magnetic moment of Co³⁺ in NdCo_{1 − x} Ga _x O₃. For the solid solutions with < 2.83μ_B, we have determined the fractions of intermediate-and low-spin Co³⁺ ions. In the range 2.83μ_B < < 4.90μ_B, we have determined the fraction of high-spin Co³⁺ ions. The results indicate that, in the temperature range 130–370 K, the Co³⁺ ions in NdCo_{1 − x} Ga _x O₃ with x = 0, 0.5, 0.8, and 0.9 are in the intermediate-and high-spin states, and the fraction of high-spin Co³⁺ ions gradually increases from 10% at x = 0 to 67% at x = 0.9. In the solid solutions with x = 0.1, 0.2, 0.3, and 0.7, more than half of the Co³⁺ ions are in the low-spin state, and the rest are in the intermediate-spin state. In the temperature range 600–940 K, the Nd³⁺ ions are in the ground and excited states, with theoretically predicted of of 3.62μ_B and 5.52μ_B, respectively. Because of the significant uncertainty in in this temperature range, has been determined less accurately compared to the range 130–370 K. Original Russian Text ? N.N. Lubinskii, L.A. Bashkirov, A.I. Galyas, S.V. Shevchenko, G.S. Petrov, I.M. Sirota, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 9, pp. 1137–1143.     

New ionic conductors Ln2 + xTi2 ? xO7 ? x /2 (Ln = Dy?Lu, x = 0.132)

New oxygen-ion conductors Ln_{2 + x}Ti_{2 – x}O_{7 – x/2} (Ln = Dy–Lu, x = 0.132) with a disordered pyrochlore structure are obtained. Their ionic conductivity attains 10^-3 S/cm at 740°C, owing to the presence of defects in both the cation and anion sublattices. The materials contain ~5% Ln_Ti antisite defects. Ln_2.132Ti_1.868O_6.934 ceramics are shown to be stable in the temperature range 1600–1700°C.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1501–1504.Original Russian Text Copyright © 2004 by Shlyakhtina, Mosunov, Stefanovich, Karyagina, Shcherbakova.     

High-pressure Bi(Mg1 ? xZnx)1/2Ti1/2O3 perovskite solid solutions

We report the preparation of Bi(Mg_{1 − x} Zn _x )_1/2Ti_1/2O₃ ceramics at a pressure of 6 GPa and temperatures from 1370 to 1570 K. In the composition ranges 0 < x < 0.5 and 0.8 < x < 1, the ceramics consist of perovskite solid solutions. The solid solutions with x < 0.5 have an orthorhombic structure (sp. gr. Pnnm), and those with x > 0.7 have a tetragonal structure (sp. gr. P4mn). On heating at atmospheric pressure, the solid solutions with 0.1 < x < 0.5 undergo an irreversible orthorhombic-to-rhombohedral (sp. gr. R3c) phase transition, accompanied by heat absorption and an increase in specific volume. The transition temperature decreases with increasing x. The reduced perovskite cell parameter of the solid solutions increases linearly with x. The Curie temperature of the rhombohedral solid solutions exceeds the decomposition temperature of the perovskite phase.