Crystal structure and hydrogen occupation in H x V2O5(x=0.0–3.9)

Changes in the crystal structure and the lattice constants of powdered crystalline V₂O₅ after absorption of hydrogen were measured, as a function of the concentration, by X-ray diffractometry. The diffraction spectra obtained showed that three kinds of phases, (x<0.4 in H _x V₂O₅), (co-existing with either terminal phase in the range of 0.4x<3.2) and (3.2x3.9), appear up tox=3.9, which was found to be the upper limit of the absorption at 70C under a hydrogen pressure of 5 MPa. The crystal system of the three phases was determined to be orthorhombic, although the lattice constants differed. The diffraction lines were broadened, and the background of the X-ray spectra grew with hydrogenation, especially in the -region, which indicates the growth of the lattice distortion and a quasi-amorphous state. The occupation of hydrogen atoms in each phase is discussed on the basis of the experimental results.     

Electrical conductivity of CaZr1 − x (x = 0.01–0.20) in dry and humid air

The total conductivity of CaZr_{1 ? x} Sc _x O_{3 ? α} (x = 0.01–0.20) materials has been measured by the four-probe dc method at temperatures from 600 to 900°C in humid air (pH₂O = 40–2500 Pa), and CaZr_0.95Sc_0.05O_{3 ? α} has been characterized by impedance measurements. The ion and proton transport numbers in CaZr_0.95Sc_0.05O_{3 ? α} have been determined as functions of air humidity. The observed anomalies in the temperature dependences of electrical conductivity for the materials studied have been compared to dilatometry data and interpreted in terms of structural changes in their oxygen sublattice.     

Superconducting and Normal-State Properties of (Y1?x Bi x )Ba2Cu3O7?δ (x=0.00–0.30)

The effect of Bi in (Y_1−x Bi _x )Ba₂Cu₃O_7−δ for x=0.00–0.30 has been investigated by powder X-ray diffraction method, dc electrical resistance measurements and scanning electron microscopy. The resistance versus temperature curves showed metallic behavior for samples with x≤0.20 and semiconductor-like behavior for x=0.25 and 0.30. The resistance versus temperature curves showed onset transition temperature (T _{c onset}) between 89 and 93 K and a broadening of the superconducting-transition width with increasing Bi content for all samples. T _{c onset} decreased linearly indicating a pair-breaking mechanism in T _c suppression for x=0 to 0.3. The non-systematic broadening of the transition width may be due to the inhomogeneities in the samples as Bi content was increased. No substitution of Bi for Y was observed.     

Electrical properties of (Ni1−x )Li x )O (x=0.1 and 0.2) under various relative humidities

Rocksalt-type (Ni_1–x Li _x )O (x=0.1 and 0.2) was synthesized at 1350° C in air and its electrical resistivity (R) was measured under various relative humidities (H). R increases with increasing H in the range 0H79%, reaches a maximum value, then decreases in the range 79%<H100%. The increase in R is explained by an electron boundary layer model. On the other hand, the decrease in R is explained by ionic conductivity.     

Grain orientation and anisotropy in the physical properties of SrBi2(Nb1 − x V x )2O9 (0 ≤ x ≤ 0.3) ceramics

SrBi₂(Nb_{1 – x} V _x )₂O₉ (0 x 0.3 in molar ratio) ceramics have been fabricated via conventional sintering at elevated temperatures. Interestingly sintering the pellets in the 1320–1470 K temperature range yielded partially grain oriented ceramics. The orientation factor (f) monitored via X-ray powder diffraction studies was found to increase with increasing V₂O₅ content and reached 83% for x = 0.3. The increment in (f) was not that significant with increase in sintering temperature and its duration. The microstructural studies suggest that V₂O₅ has a truncating effect on the abnormal platy growth of SBN grains. The dielectric constant (_r) and loss (D) measurements as functions of both temperature and V₂O₅ content have been carried out along the directions parallel (_rp) and perpendicular (_rn) to the cold pressing axis of the pellet. The anisotropy (_rn/_rp) associated with _r was found to be 1.11 at 300 K and 2.1 at the Curie temperature, (T _c) respectively. Different dielectric mixture formulae that were employed to analyze the effective dielectric constants of these samples with varying porosity confirmed that the experimental value of _r was comparable with that obtained using Wiener's formula. Impedance spectroscopy was employed to rationalize the electrical behavior of these ceramics. The pyroelectric coefficients along the directions parallel and perpendicular to the pressing axis of the grain oriented (83%) SBN ceramic at 300 K were 0.13145 mC/m²K and 0.26291 mC/m²K respectively. The ferroelectric properties of these grain-oriented ceramics were better in the direction perpendicular to the pressing axis than those in the parallel direction.     

Sintering and properties of SrBi2(Ta1−x V x )2O9 ceramics

The effects of vanadium doping on the sintering, microstructure, dielectric properties, and ferroelectric properties of SrBi₂(Ta_1–x V _x )₂O₉ ceramics were investigated. The densification and grain-growth processes of the vanadium doped ceramics were shifted to a lower temperature range. For the ceramics with relative density 90%, the dielectric constant is 120–125 and 100–130 for the undoped and doped ceramics, respectively, and the dielectric loss tangent is below 1%. As compared with the undoped ceramics, the ferroelectric properties can be significantly improved by doping with an appropriate amount of vanadium and sintering at 1000°C. The variations of dielectric and ferroelectric properties are influenced by the incorporation of vanadium into crystal lattice and several microstructural factors.     

Electrical conductivity and creep of polycrystalline α-Al2O3 doped with titanium or iron plus titanium

Creep and conductivity were measured for polycrystalline -Al₂O₃ doped with titanium or iron plus titanium. Both series of samples show a transition from acceptor domination to donor domination at the point where the concentrations of titanium and acceptors are approximately equal. Parameters in the expression for the creep rate S ⁿd-^PP^r _{O 2} exp (-QlkT) (whereS is the applied stress,d the grain size,Q an activation energy andn, p, r are constants characterizing the creep mechanism) indicate limitation by bulk diffusion in the acceptor-dominated samples and in the donor-dominated samples at low [Ti]. In the latter samples the ratelimiting species isO ⁱ at highP _{O 2}, VA ^Al at lowP _{O 2}, with involvement of e at mediumP _{O 2}. At higher titanium concentrations the creep rate is limited by generation/recombination of defects at grain boundaries. Iron appears to increase the rate of these grain-boundary reactions. When second-phase particles of Al₂TiO₅ are present, is decreased but ionic conductivity is increased.     

Electrical Conductivity of H2V12 – y W y O31 + δ · nH2O and H x V x W1 – x O3 · nH2O

The electrical conductivity of H₂V_{12 – y} W _y O_{31 +} · nH₂O (0 y 3) and H _x V _x W_{1 – x} O₃ · nH₂O (x = 0, 0.08, 0.17, 0.25, 0.33) prepared by the sol–gel method was measured between 293 and 473 K at a relative humidity of 12%. Partial substitution of tungsten for vanadium or V⁵⁺ and H⁺ for tungsten was found to reduce the conductivity of the phases studied. The only exception is the x = 0.33 phase. Below 373 K, the activation energy of conduction is 0.22–0.29 eV in the amorphous phases (0 y 3; x= 0.25, 0.33) and 0.06–0.11 eV in the crystalline phases (x = 0, 0.08, 0.17). The possible mechanisms of electrical transport are discussed.     

Mixtures of (N2)1−x : (O2) x at High Pressures and Low Temperatures

We performed Raman measurements at 18 K and pressures up to 25 GPa in order to construct a tentative phase diagram of the (N ₂)_1–x :(O ₂)_x –system at low temperatures. We varied the composition of the mixed system over the whole concentration range. Here we focused on the systems with high nitrogen concentration and pressures above 2 GPa. It is known that at room temperature oxygen is highly solvable in the –phase of N ₂. The experimental results show that oxygen suppresses the disorder–order transition –N ₂.     

Electrical and Magnetoresistance Properties of Mg x Mn1−x Fe2O4 Compounds

A systematic study of the electrical properties and magnetoresistivity of Mg _x Mn_1?x Fe₂O₄ (x ranging from 0 to 1.0 in steps of 0.1) compounds is presented. All samples were produced by glycolthermal reaction under a low reaction temperature of 200 ^°C. The X-ray diffraction data indicate formation of a pure cubic spinel phase in all the samples. The effects of Mg concentration and particle size on the properties have been investigated. A decreasing trend in the resistivity of all the compounds with increasing magnetic field has been observed.     

High-temperature electrical conductivity and thermal expansion of SmBa2(Cu1-xFex 3O6+δ (x = 0–0.2)

The electrical conductivity and thermal expansion of SmBa₂(Cu_1-xFe_{x 3}O_6+δ (x = 0-0.2) were measured in air in the temperature range from 20 to 900°C. The linear thermal expansion coefficient was determined to be (12.8–13.5) × 10^-6 K^-1 in the temperature range from 20 to 350°C and (16.2–17.8) × 10^-6 K^-1 in the range from 350 to 800°C. Between 400 and 900°C, the conductivity of SmBa₂(Cu_1-xFe_x)₃O_6+δ was found to decrease with increasingx, mainly in the rangex = 0–0.1. This finding was interpreted in terms of the Fe occupancies on the Cu(1) and Cu(2) sites.     

Electronic conductivity of Sr3-3xLa2x█x(VO4)2 solid solutions

The electronic conductivity of Sr_3-3xLa_2x█_x(VO₄)₂ solid solutions was measured at oxygen pressures from 10^-13 to 10⁵ Pa and temperatures from 1070 to 1270 K, and their band gap was determined as a function of composition. The activation energy (≏2.85 eV) and enthalpies of electron generation (≏4.2 eV) and migration (≏0.75 eV) were determined. A correlation between the band gap and electronic conductivity of the solid solutions was revealed     

Electrical conductivity of Cux(As0.4Se0.3Te0.3)100−x glasses

Results of the dc electrical conductivity () measurements (from 90 to 420 K) on 15 compositions of the Cux(As0.4 Se0.3 Te0.3)100–x glasses (x from 0 to 30) are presented. Similar to that observed [1] in the composition dependencies of the mean atomic volume (V) and the glass transition temperature (Tg) of these glasses, it is possible to delineate three regions in the composition dependence of the of these glasses. (i) For addition of Cu up to 1 at %, the register a decrease compared to that of the parent As0.4Se0.3Te0.3 glass. (ii) For Cu > 1 at %, the conduction activation energy (E) and the pre-exponential factor (C) decrease, with a concomitant increase in (at 250 K) by about six orders of magnitude. (iii) Both E and C show saturation for Cu > 23 at %. The -composition data are examined using the model developed earlier [1] to understand, the V (and Tg)-composition dependencies of these glasses.     

Magnetic, Electrical and Thermodynamic Properties of UCuT x Al11−x Alloys Where T = Mn, Fe and x=4 and 5

The structure, magnetic, electrical, and thermodynamic properties of UCuT _x Al_11?x alloys, where T = Mn or Fe and x=4 or 5 are presented. The behavior of the Fe alloys is ferromagnetic-like with the Curie points amounting to 180 and 230 K, and the saturation magnetic moments under magnetic field of 5 T equal to 4.75 and 6.02 μ_B/f.u., respectively, whereas under a magnetic field of about 34 T the magnetic moments amount to 6.9 and 9.0 μ_B/f.u. for the alloys with x=4 and 5, respectively. The Curie points are reflected in the temperature dependence of the specific heat in which the anomalies are found at 180–200 and 230 K for alloys with x=4 and 5, respectively, however, it shows no reflection in the temperature dependence of the electrical resistivity. The field dependence of the magnetization at T=1.9 K for both compounds exhibits considerable hysteresis. There is a pronounced difference between ZFC and FC magnetization in its temperature dependence below the Curie point for materials with x=4 and 5. The Mn alloys exhibit ferrimagnetic-like character for which, supposedly, the interplay of the uranium and manganize sublattices is responsible. Magnetic transitions are determined at T _N =300 (x=4) and 380 K (x=5). However, those anomalies do not find confirmation in the temperature dependence of the specific heat and the electrical resistivity. Magnetic moments determined at T=1.9 K and in a magnetic field of 5 T are very low and in both cases amount to about 0.35 μ_B/f.u. and these values are slightly higher in a magnetic field of 34 T reaching a value of about 1.5 μ_B/f.u. Also for the Mn alloys the clear difference between ZFC and FC magnetization in its temperature dependence below the Curie point is observed.     

Electrical properties of n-Cd1 ? x Co x GeAs2 (x = 0.05–0.15) at high pressures

The resistivity (??) and Hall coefficient (R _H) of n-Cd_{1 ? x} Co _x GeAs₂ with x = 0.05?C0.15 were measured as functions of temperature (at atmospheric pressure) and pressure (up to p = 7 GPa). The temperature-dependent ?? data were used to determine the ionization energy of a cobalt-related impurity center. In the ??(p) and R _H(p) curves, we identified structural semiconductor-semiconductor phase transitions under both increasing and decreasing pressure. Using a mixed-phase structure-effective medium model, we assessed the dynamics of the variation of phase composition with pressure.     

Electrical conductivity of acid crystallohydrate MgHPO4· 3H2O and its structural transformations

Electrical and thermal characteristics of solid MgHPO₄· 3H₂O have been investigated in a temperature interval from 20–400°C, and correlated with data of X-ray diffraction analysis. It is noted that in the temperature interval from 20–140°C, despite the presence of considerable hydrogen concentrations, the sample shows very small electrical conductivity. This is a consequence of crystal structure, where the strongest hydrogen bond exists between acid proton and oxygen of P0₄³⁻ group. So a strong hydrogen bond decreases the number of potential electric carriers and prevents rotation of the P0₄³⁻acceptor group, which causes crystallohydrate to show poor conductivity. With the breakage of crystal structure, caused by dehydration, amorphous MgHPO₄, salt is formed and conductivity abruptly increases. At about 550°C crystallization starts and the newly formed phase Mg₂P₂O₇ again shows poorer conductivity.     

Nanocrystalline NiFe2O4 and nanocomposites of (NiFe2O4)(1-x)(Al2O3)x (x = 0.25, 0.40): superparamagnetic behavior and Mössbauer spectroscopy

Nanocrystalline NiFe2O4 with different particle sizes and nanocomposites of (NiFe2O4)(1-x)(Al2O3)x (x = 0.25, 0.40) were prepared by using co-precipitation method. In this method two techniques viz., 'ultrasonication' and 'magnetic stirring' during co-precipitation were used. The as prepared samples were annealed at different temperatures to obtain samples with different particle sizes. The formation of the nanocrystalline spinel phases of all the samples were confirmed by X-ray diffraction (XRD) patterns. The sizes of the nanoparticles of all the samples were calculated from the broadening of the (311) line in the XRD pattern. The distribution of sizes are remarkably less in samples obtained from 'ultrasonication' technique compared to those obtained in 'magnetic stirring' technique. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses etc. were extracted from the ac hysteresis loops observed at different frequencies. The variations of coercive field and hysteresis loss as functions of frequency and particle sizes have been studied. M?ssbauer spectra of the samples along with the hysteresis loops recorded at room temperature indicate the presence of superparamagnetic (SPM) relaxations of the nanoparticles. Also, SPM relaxations have been introduced in the samples annealed at higher temperature by encapsulating the nanoparticles in non-magnetic matrix of Al2O3 with 40% coating.     

Synthesis and properties of dielectric (HfO2)1 − x (Sc2O3) x films

(HfO₂)_{1 ? x} (Sc₂O₃) _x films have been grown by chemical vapor deposition (CVD) using the volatile complexes hafnium 2,2,6,6-tetramethyl-3,5-heptanedionate (Hf(thd)₄) and scandium 2,2,6,6-tetramethyl-3,5-heptanedionate (Sc(thd)₃) as precursors. The composition and crystal structure of the films containing 1 to 36 at % Sc have been determined. The results demonstrate that, in the composition range 9 to 14 at % scandium, the films are nanocrystalline and consist of an orthorhombic three-component phase, which has not been reported previously. Using Al/(HfO₂)_{1 ? x} (Sc₂O₃) _x /Si test structures, we have determined the dielectric permittivity of the films and the leakage current through the insulator as functions of scandium concentration. The permittivity of the films with the orthorhombic structure reaches k = 42–44, with a leakage current density no higher than ～10^?8 A/cm².     

Diamagnetic and Electrical Properties of Si x Mo2S3−x (x=0.1, 0.2, 0.33, 0.5) and A0.1Mo2S3−x (A=C, B, and Ru)

Mo₂S₃ doped with Si, C, B, and Ru, is identified to bear the same crystalline structure P2₁/m as that of Mo₂S₃ through XRD analysis. Diamagnetic transitions with χ _m ～10^?4 emu/g?Oe at temperature ranging from 2 K to 6 K were observed in the doped samples of Si _x Mo₂S_3?x (x=0.1, 0.2, 0.33, 0.5). And both of the x=0.2 and 0.5 samples were found to have double diamagnetic transitions with higher T _c at the same temperature of 6.01 K, while Si _x Mo₂S_3?x of x=0.33 displayed an extra ferromagnetic-like response at 63 K. The corresponding transition in resistivity of Si _x Mo₂S_3?x with x=0.1 was noticed to show a mild drop with less than 10 % of its original transition values as measured down to 2 K. But a superconducting-like magnetic field dependence on the phase transition of resistivity was also noted. Its diamagnetic signals were greatly reduced when the applied magnetic fields were raised to 10³ Oes. In the doped samples of A_0.1Mo₂S_2.9 (A=C, B, and Ru), the phase transition in resistivity at 4.08 K, 4.62 K, and 4.35 K, respectively, exhibited similar fashion as that in the case of Si_0.1Mo₂S_2.9.     

D.c. conductivity of V2O5–MnO–TeO2 glasses

Semiconductive oxide glasses in the system V2O5–MnO–TeO2 were prepared, and the mechanism of d.c. conduction was studied. The Seebeck coefficient measurements at temperatures from 375–475 K indicated the glasses to be n-type semiconducting. The d.c. conductivity ranged from 5×10–5 to 1.9×10–6 S cm–1 at 405 K for V2O5=60 mol% and MnO=0–20 mol%, and decreased with increasing MnO content. The conduction was confirmed to obey the adiabatic small polaron hopping model, and was due to mainly hopping between V-ions in the glasses. The polaron band width J was estimated to be J=0.10–0.20 eV. The electron–phonon interaction coefficient p was very large (21–26). The hopping mobility evaluated as 2.3×10–7–2.7×10–6 cm2 V–1 s–1 increased with increasing V2O5 content. The estimated carrier concentration was the order of 1019 cm–3. The principal factor determining conductivity was the polaron hopping mobility in these glasses. © 1998 Chapman & Hall