Dielectric Properties and Electrical Conductivity of (<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)TlGaSe<Subscript>2</Subscript> · <Emphasis Type="Italic">x</Emphasis>Tm Crystals

We have synthesized samples based on the layered compound TlGaSe₂ and containing thulium: (1–x)TlGaSe₂ · xTm with x = 0.001, 0.005, 0.01, and 0.02. The polycrystalline samples have been used as charges for growing crystals with the corresponding compositions by the Bridgman method. The phase composition of the (1–x)TlGaSe₂ · xTm samples has been determined by X-ray diffraction analysis. Their dielectric properties have been studied in ac electric fields at frequencies in the range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. We have identified the relaxation character of the dielectric permittivity, the nature of the dielectric loss, and the hopping mechanism of charge transport in the (1–x)TlGaSe₂ · xTm crystals. Our results demonstrate that increasing the thulium concentration in the crystals reduces the mean hop distance and time of charge carriers and increases the ac conductivity and the density of localized states near the Fermi level in the crystals.     

Phase relations and properties of alloys in the SnSe-DySe system

Phase relations in the SnSe-DySe system have been studied using differential thermal analysis, X-ray diffraction, microstructural analysis, microhardness tests, and density measurements, and its T-x phase diagram has been mapped out. The SnSe-DySe system contains a new ternary compound with the composition DySnSe₂, which crystallizes in orthorhombic symmetry with unit-cell parameters a = 5.74 ± 0.02 Å, b = 10.49 ± 0.03 Å, and c = 11.66 ± 0.05 Å (Z = 7, V = 702 ų, measured density ρ_meas = 7.02 g/cm³, X-ray density ρ_x = 7.26 g/cm³). In addition, the system contains SnSe-based solid solutions, Sn_{1 ? x} Dy _x Se (up to 4 mol % DySe). Their electrical conductivity and thermoelectric power have been measured as functions of temperature.     

Preparation and Properties of Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te

Mn _x Hg_{1 ? x}Te (x = 0.05, 0.12) single crystals were grown by solid-state recrystallization, and their axial and radial homogeneity was assessed by optical, electrical, and electron-microscopic measurements. The crystals are p-type, with a hole concentration of (4.3–5.3) × 10²² m^?3 and Hall mobility in the range (410–570) × 10^?4 m²/(V s).     

Structure and Magnetic Properties of Mn-doped CoFe 2 O 4 Nanoparticles Prepared by Solvothermal Route

Spinel Co_1?xMn_xFe₂ O ₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) nanoparticles were synthesized by a solvothermal method using polyethylene glycol (PEG) as a surfactant. X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy and vibrating sample magnetometer (VSM) techniques were used to characterize phase, morphology, valence states, and magnetic properties of the samples. XRD analysis showed single phase of spinel structure, and the decrease of lattice constant on the effective Mn substitution was investigated. The effect of the PEG on the spherical aggregates of Co_1?xMn_xFe₂ O ₄ nanoparticles was observed. The result of XANES spectra showed Mn²⁺/Mn³⁺, Fe³⁺, and Co²⁺ exist in the samples. The samples showed ferromagnetism at room temperature with a maximum saturated magnetization of 72 emu/g and the smallest coercivity of 45 Oe for x = 1.0. The origin of ferromagnetic behavior is believed to be due to the occupation of Mn²⁺/Mn³⁺ ions.     

Stability region of Gd<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3 ± δ</Subscript> solid solutions in air

The solid-solution range of Gd_{2 ? x} Mn _x O_{3 ± δ} has been determined using X-ray diffraction analysis of samples with 0.90 ≤ x ≤ 1.20 (Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 900 and 1400°C. The results have been used to construct a partial phase diagram of the Gd-Mn-O system in air. It is shown that gadolinium manganite with the perfect metal stoichiometry, GdMnO_{3 ± δ}, does not exist. The material of this composition in air consists of two phases: a Gd_{2 ? x} Mn _x O_{3 ± δ} solid solution with an orthorhombic perovskite-like structure and the binary oxide Gd₂O₃. The solid solution extends to the composition Gd_0.96Mn_1.04O_{3 ± δ}. Over the entire temperature range studied, gadolinium oxide does not dissolve in Gd_0.96Mn_1.04O_{3 ± δ}. Mn₃O₄ exhibits significant solubility in Gd_{2 ? x} Mn _x O_{3 ± δ}. In particular, Gd_0.86Mn_1.14 O_{3 ± δ} is single-phase by X-ray diffraction in the temperature range 1185–1400°C. Below 1185°C, Gd_{2 ? x} Mn _x O_{3 ± δ} is in equilibrium with another gadolinium manganite, GdMn₂O₅. With decreasing synthesis temperature, the GdMn₂O₅ solubility in Gd_{2 ? x} Mn _x O_{3 ± δ} drops precipitously. At 900°C, the only single-phase sample was Gd_0.96Mn_1.04O_{3 ± δ}.     

Doping mechanisms and electrical properties of bismuth tantalate fluorites

Phase-pure bismuth tantalate fluorites were successfully prepared via conventional solid-state method at 900 °C in 24–48 h. The subsolidus solution was proposed with the general formula of Bi_3+x Ta_1?x O_7?x (0 ≤ x ≤ 0.184), wherein the formation mechanism involved a one-to-one replacement of Ta⁵⁺ cation by Bi³⁺ cation within ~4.6 mol% difference. These samples crystallised in a cubic symmetry, space group Fm-3 m with lattice constants, a = b = c in the range 5.4477(± 0.0037)–5.4580(± 0.0039) Å. A slight increment in the unit cell was discernible with increasing Bi₂O₃ content, and this may attribute to the incorporation of relatively larger Bi³⁺ cation in the host structure. The linear correlation between lattice parameter and composition variable showed that the Vegard’s law was obeyed. Both TGA and DTA analyses showed Bi_3+x Ta_1?x O_7?x samples to be thermally stable as neither phase transition nor weight loss was observed within ~28–1000 °C. The AC impedance study of Bi₃TaO₇ samples was performed over the frequency range 5–13 MHz. At intermediate temperatures, ~350–850 °C, Bi_3+x Ta_1?x O_7?x solid solution was a modest oxide ion conductor with conductivity, ~10^?6–10^?3 S cm^?1; the activation energy was in the range 0.98–1.08 eV.     

Mechanism of AC charge transport in TlSb<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>2</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0 and 0.03)

The dielectric properties and ac electrical conductivity of TlSb_1–xGa _x S₂ (x = 0, 0.03) single-crystals have been measured in the frequency range 5 × 10⁴ to 3.5 × 10⁷ Hz. Experimental data on the frequency dispersion of the dielectric coefficients and electrical conductivity of the TlSb_1–xGa _x S₂ (x = 0, 0.03) single crystals have allowed us to identify the nature of the dielectric loss and the mechanism of charge transport and evaluate parameters of localized states in the band gap. The incorporation of gallium atoms into the crystal lattice of TlSbS₂ crystals has been shown to lead to an increase in the Fermi-level density of states and mean hop time and distance.     

Thermodynamic properties of Ge-Ni melts

Using high-temperature isoperibol calorimetry, we have determined the partial enthalpy of mixing of nickel in Ge-Ni melts at 1800 ± 3 K in the composition range 0 < x _Ni < 0.7 (\(\Delta _{mix} \bar H^\infty \) (Ni) = ?76.0 ± 4.8 kJ/mol). The results have been used to evaluate the partial enthalpy of mixing of germanium and the integral enthalpy of mixing (Δ_mix H _min = ?37.9 ± 0.6 kJ/mol at x _Ni = 0.6). The thermochemical data and germanium activity in Ge-Ni melts have been analyzed in comparison with those reported earlier, and the most reliable thermodynamic functions of the Ge-Ni melts at 1870 K have been determined.     

Submicron-sized anatase,TiO2 with high photocatalytic activity,and (Ti,Sn)O2 nanocrystals formed via hydrothermal technique

The compositional dependence of the crystalline phase and properties of precipitates (Ti _x Sn_1?x O₂) in the TiO₂–SnO₂ system, which were hydrothermally formed at 100–200 °C from the precursor solutions of TiCl₄ and SnCl₄ under weakly basic conditions in the presence of tetramethylammonium hydroxide (TMAH) was investigated. Rutile-type (Ti, Sn)O₂ solid solutions with nano-sized crystallite were directly formed at 180 °C in the composition range of x = 0–0.8. Nanoparticles with anatase crystallite around 10 nm as a main crystalline phase of precipitates that were formed in the compositions x = 0.9 and 1.0 showed similar photocatalytic activity. As the hydrothermal treatment temperature rose from 100 to 200 °C, the crystallite size of rutile solid solution, Ti_0.5Sn_0.5O_2, increased from 2.5 to 8.0 nm. The optical band gap of the samples changed in the range of 2.93–3.25 eV depending on their composition in the system. At the composition of x = 1.0, submicron-sized anatase-type pure TiO₂ particles (sizes of cuboid sides are around 100–120 nm) with pretty high crystallinity and superior photocatalytic activity were formed from the aqueous solution of TiCl₄ under basic hydrothermal condition at 180 °C in the presence of TMAH with concentration as 1.3 times high as the condition in the case of the nano-sized anatase.     

Fabrication and characterization of compositionally graded Bi1−x Gd x FeO3 thin films

An undoped BiFeO₃ thin film, Gd doped Bi_0.95Gd_0.05FeO₃ thin film with a constant composition, Gd up-graded doped Bi_1?x Gd _x FeO₃ and Gd down-graded doped Bi_1?x Gd _x FeO₃ thin films were successfully grown on Pt (111)/Ti/SiO₂/Si (100) substrates using a sol-gel and spin coating technique. The crystal structure, ferroelectric and dielectric characteristics as well as the leakage currents of these samples were thoroughly investigated. The XRD (X-Ray Diffraction) patterns indicate that all these thin films consist of solely perovskite phase with polycrystalline structure. No other secondary phases have been detected. Clear polarization-electric field (P-E) hysteresis loops of all these thin films demonstrate that the incorporation of Gd³⁺ into the Bi site of BFO thin film have enhanced the ferroelectric performance of pure BiFeO₃ thin film, and the Gd down-graded doped Bi_1?x Gd _x FeO₃ thin film has the best ferroelectric properties. Compared to other thin films, the optimal ferroelectric behavior of the Gd down-graded doped Bi_1?x Gd _x FeO₃ thin film results from its large dielectric constant, low dissipation factor and low leakage current.     

Synthesis and investigation of superconducting properties of heterosubstituted magnesium diborides

The properties of magnesium diborides with the composition Mg_{1 ? x} M _x B₂, where M = Al, Ga, In, and Tl, and Mg (B_{1 ? x} E _x ), where E = N, P, and Si (0.05 ≤ x ≤ 0.3), made by sintering of calculated amounts of boron, magnesium, and a substituent element or by a solid-phase exchange reaction of a corresponding halogenide of a metal with MgB₂ at 1070–1170 K are studied. Using RFA technique, it is shown that, from all employed heterosubstituents, only the atoms of aluminum, gallium, and silicon in the amount x ≤ 0.2 are incorporated into magnesium and boron sublatticies. Here, the temperature of the superconducting transition T _c = 39 ± 1K determined for pure magnesium diboride hardly changes.     

Growth of Sillenite-Structure Single Crystals

The main processes for preparing bulk single crystals and films of photorefractive and piezoelectric Bi₁₂M _x O_20±δ (M = Group II–VIII elements) sillenite compounds are considered. Experimental data are summarized on the crystal growth of Bi₁₂M _x O_20±δ from the melt and under hydrothermal conditions, and the key morphological features of sillenites are analyzed. Various types of macroscopic growth defects in sillenite-type crystals are described, and their origin is discussed. The compositions of second-phase inclusions in undoped and doped (Group I–VIII elements) Bi₁₂SiO₂₀, Bi₁₂GeO₂₀, and Bi₁₂TiO₂₀ single crystals are presented, and the main physicochemical properties of various Bi₁₂M _x O_20±δ crystals are summarized.     

Ionic conductivity of BaF<Subscript>2</Subscript> + ZnF<Subscript>2</Subscript> + CdF<Subscript>2</Subscript> + YbF<Subscript>3</Subscript> optical fluoride ceramic

The electrical conductivity of an optical fluoride ceramic in the quaternary system BaF₂ + ZnF₂ + CdF₂ + YbF₃ has been determined in the temperature range 338–722 K using impedance spectroscopy (5 to 5 × 10⁵ Hz). The 500-K ionic conductivity of the ceramic is σ = 3.3 × 10^–4 S/cm, which corresponds to the electrical characteristics of single crystals of the best conducting nonstoichiometric M_1–x R _x F_{2 + x} (M = Sr, Ba; R = La–Nd; x = 0.3–0.5) fluorite phases. We have observed nonmonotonic variation (breaks) in temperature-dependent σ, which is due to competing fluoride ion transport processes in different parts of the ceramic sample. The highly conductive state of the BaF₂ + ZnF₂ + CdF₂ + YbF₃ fluoride ceramic seems to be due to the formation of structural regions corresponding to a Ba_1–x Yb _x F_{2 + x} solid solution.     

Fabrication of perovskite-type Ba(Sn1−x Ta x )O3 ceramics and their power factors

Perovskite-type Ba(Sn_1?x Ta _x )O₃ (0.01 ≤ x ≤ 0.06) ceramics with high relative densities (92.7–94.4 %) were fabricated using the hot isostatic pressing (HIP) method at 1273 K and 196 MPa for 4 h in an atmosphere of argon gas. The lattice parameter decreased slightly with increasing x. From the XPS measurement, the Ta⁵⁺ ion was stable in Ba(Sn_1?x Ta _x )O₃ ceramics and the broad peak of the Ta4f level was the overlap between the Ta⁵⁺4f_5/2 and Ta⁵⁺4f_7/2 levels. Ba(Sn_1?x Ta _x )O₃ ceramics were n-type semiconductors, and their electrical resistivities increased with increasing x. The increase in the electrical resistivity was explained by impurity scattering due to the presence of the Ta ions. The absolute value of the Seebeck coefficient (S) increased with increasing temperature and x. The power factor (S ² σ), which was calculated from electrical conductivity (σ) and the Seebeck coefficient, was ca. 1.0 × 10^?5 W m^?1 K^?2 at x = 0.01.     

New NaSrPO<Subscript>4</Subscript>:Sm<Superscript>3+</Superscript> phosphor as orange-red emitting material

Sm³⁺-activated NaSrPO₄ phosphors could be efficiently excited at 403 nm, and exhibited a bright red emission mainly including four wavelength peaks of 565, 600, 646 and 710 nm. The highest emission intensity was found for NaSr _1?x PO₄: xSm³⁺ with a composition of x = 0.007. Concentration quenching was observed as the composition of x exceeds 0.007. The decay time values of NaSr_1?x PO ₄ : xSm³⁺ phosphors range from around 2.55 to 3.49 ms. NaSr_1?x PO₄: xSm³⁺ phosphor shows a higher thermally stable luminescence and its thermal quenching temperature T ₅₀ was found to be 350°C, which is higher than that of commercial YAG:Ce³⁺ phosphor and ZnS:(Al, Ag) phosphor. Because NaSr_1?x PO₄: xSm³⁺ phosphor features a high colour-rendering index and chemical stability, it is potentially useful as a new scintillation material for white light-emitting diodes.     

Optical and transport properties of Fe-doped Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se (<Emphasis Type="Italic">x</Emphasis> = 0.35)

The optical and transport properties of Fe²⁺-doped Cd _x Hg_1?x Se crystals with a midgap Fe²⁺ level have been studied. The results demonstrate that Fe²⁺ ions influence both the optical and transport properties of Cd _x Hg_1?x Se〈Fe²⁺〉. The observed optical absorption bands are due to a donor Fe²⁺ level in the band gap, with a depth E _Fe = 0.21 eV, and to band-band transitions. Thermal anneals in Hg and Se vapors have different effects on the carrier concentration and mobility in the crystals. The effect of annealing on the transport properties of the Fe²⁺-doped crystals differs from that for undoped crystals and is governed by the state of point defects.     

Kinetics of gas-phase conversion of U<Subscript>3</Subscript>O<Subscript>8</Subscript> into water-soluble compounds in nitrating atmosphere at 25–30°С

The kinetics of the gas-phase conversion of U₃O₈ in NO_x–H₂O (vapor)–air and HNO₃ (vapor)–H₂O (vapor)–air atmospheres was fitted by the Kazeev–Kolmogorov–Erofeev equation. The following parameters n and K were obtained: for experiments in NO_x–H₂O (vapor)–air atmosphere, n = 0.2 ± 0.1 and K = 0.2 ± 0.2 h^–1; for experiments in HNO₃ (vapor)–H₂O (vapor)–air atmosphere, n = 0.3 ± 0.2 and K = 0.03 ± 0.02 h^–1 (confidence probability p = 0.95). For the U₃O₈ conversion in both media, n < 0.5, which suggests the diffusion control of the U₃O₈ conversion under the action of both HNO₃ and NO_x.     

Thermodynamics of oxygen uptake for the YBa2Cu3Ox superconductor

Thermogravimetric analysis and pressure (p) monitoring in known volume systems are used to give phase diagram information (e.g. vant Hoff plots) for roughly 6<x<7 in YBa₂Cu₃O_x. As an example we find for O₂ desorption for the important composition x = 6.8, beyond which superconductivity with transition temperature, T_s > 90 K becomes essentially independent of x, ΔH^o = 43.4 ±1 kcal/mo (O₂) and ΔS^o = 48.6 ±1 cal/Kmo (O₂). ΔS^o stays roughly constant for x > 6.7 but decreases for x < 6.7 indicating increased disorder on the O substructure. p (x) curves do not show phase separation which corresponds to above critical behavior, but given indications for limiting compositions near x = 6 and x = 7. Accordingly a model is proposed in which these compositions correspond to valencies of Cu⁺ and Cu³⁺ respectively for the symmetric Cu site (Ba coordinated) while the asymmetric Cu (Ba and Y coordinated) is taken to be 2+ over this whole range in x.     

Growth and properties of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript>-NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

Platelike Li_{1 ? x} Na _x Cu₂O₂ single crystals up to 2 × 10 × 10 mm in dimensions have been grown by slowly cooling (1 ? x)Li₂CO₃·xNa₂O₂·4CuO melts in alundum crucibles in air. Li_{1 ? x} Na _x Cu₂O₂ solid solutions in the LiCu₂O₂-NaCu₂O₂ system have been shown to exist in the composition range 0.78 < x < 1. The temperature stability ranges of NaCu₂O₂ and LiCu₂O₂ are 780–930 and 890–1050°C, respectively. The Mössbauer spectra and electrical conductivity of the crystals have been measured.     

Structure,dielectric and ferroelectric properties of lead free (K,Na)(Nb)O<Subscript>3</Subscript>-xBiErO<Subscript>3</Subscript> piezoelectric ceramics

The effect of BiErO₃ (BE) as a doping material on the structural, dielectric and ferroelectric properties of (KNa)NbO₃ ceramics was explored in this research. Co-existence of two phase regions was confirmed in the composition range at x?=?0.5% and x?=?1.0%. The addition of BE content led to a decrease of the grain size and the ceramics became denser. Bulk P–E hysteresis loops were obtained with a maximum polarization of P _max = 30.56 µC/cm² and a remnant polarization of P _r = 25.10 µC/cm², along with a coercive field of E _c  ~ 11.26 kV/cm. The results revealed that a field strain value of ~?0.26 for x?=?0.5% of BE substitution was attained. This presents outstanding piezoelectric and dielectric properties.