Electrical conductivity of (Er,U)O2+x and (Ce,U)O2+x

The electrical conductivity (σ) of (Er_yU_1−y)O_2+x (y=0.06, 0.20) and (Ce_yU_1−y)O_2+x (y=0.05, 0.15, 0.25) has been measured as a function of oxygen partial pressure in the temperature range of 1100≤T/°C≤1300 by a d.c. 4-probe method. Both of the oxides exhibited Po₂-regions where the electrical conductivity is independent of oxygen partial pressure, which indicates that doped Er and Ce exist as trivalent cations on uranium sites and fix the hole concentration by acting as electron acceptors, i.e. [h]=[Er′_U] and [h]=[Ce′_U], respectively. It is considered that strong oxidization tendency of uranium and reduction tendency of cerium simultaneously render the cerium ions exist exclusively as Ce³⁺ in the uranium dioxide. The electron-hole mobility of (Er_yU_1−y)O_2+x and (Ce_yU_1−y)O_2+x in the Po₂ region where σ is constant has been calculated by the combination of the electrical conductivity and charge carrier concentration; the activation energy (E_H) of each oxide has been obtained from the temperature dependence of the mobility. Small polaron hopping conduction mechanism was confirmed by small magnitude of the mobility (0.018-0.052 cm² V⁻¹ s⁻¹) and the activation energy (0.12-0.22 eV).     

Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.     

Effect of ZnO/CdO on the structure and electrical conductivity in Li2O·MO·Bi2O3·B2O3 glasses (M=Zn, Cd)

Studies of structural and electrical properties have been carried out on a number of glasses with wide ranging compositions in the glass systems Li₂O·MO·Bi₂O₃·B₂O₃ (where M=Zn or Cd), in order to understand the effect of transition metal (TM) ions on the structure of these glasses. The density and molar volume measurements have also been made to understand the structural changes occurring in these glasses. The dc conductivity measured in the temperature range 423-623 K obeys Arrhenius law. It increases with increase in Li₂O/MO ratio. The results of infrared spectra indicate that TM ions (Zn²⁺ or Cd²⁺) behave as network former in the present system. Boron exists in both tri- and tetra-hedral units in these glasses and no boroxol ring formation takes place in the glass structure. Values of theoretical optical basicity have also been reported.     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.     

Linear and nonlinear optical susceptibilities of orthorhombic rare earth molybdates RE2(MoO4)3

The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu.     

Luminescent properties of R2(MoO4)3:Eu (R=La, Y, Gd) phosphors prepared by sol-gel process

A series of red phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) have been synthesized by sol-gel method. The crystallization processes of the phosphor precursors were characterized by X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA), and the properties of these resulting phosphors have also been characterized by photoluminescence (PL) spectra and reflectance spectra. Field emission scanning electron microscopy (FE-SEM) was also used to characterize the shape and the size of the samples. The results of TG-DTA and XRD indicated that all of the R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors crystallized completely at 650 °C. Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ have two structures, monoclinic and orthorhombic, while La_0.8Eu_1.2(MoO₄)₃ only adopts monoclinic structure. The luminescent properties of phosphors R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) are dependent on their structures to some extent. The orthorhombic Y_0.8Eu_1.2(MoO₄)₃ and Gd_0.8Eu_1.2(MoO₄)₃ phosphors show very similar luminescent properties, which differ from those of phosphors with monoclinic structure. For all of R_0.8Eu_1.2(MoO₄)₃ (R=La, Y, and Gd) phosphors, intense red emission is obtained by exciting at ∼394 and ∼465 nm which are owing to the sharp ⁷F₀→⁵L₆ and ⁷F₀→⁵D₂ lines of Eu³⁺. Two strongest lines at 394 and 465 nm in excitation spectra of these phosphors match well with the two popular emissions from near-UV and blue GaN-based LEDs, so they could be used as red components for white light-emitting diodes.     

A novel blue-emitting long-lasting proyphosphate phosphor Sr2P2O7:Eu,Y

By introducing the Y³⁺ into Sr₂P₂O₇:Eu²⁺, we successfully prepared a kind of new phosphor with blue long-lasting phosphorescence by the high-temperature solid-state reaction method. In this paper, the properties of Sr₂P₂O₇:Eu²⁺,Y³⁺ were investigated utilizing XRD, photoluminescence, luminescence decay, long-lasting phosphorescence and thermoluminescence (TL) spectra. The phosphor emitted blue light that was related to the 4f⁶5d¹-⁸S_7/2 transition of Eu²⁺. The bright blue phosphorescence could be observed by naked eyes even 8 h after the excitation source was removed. Two TL peaks at 317 and 378 K related to two types of defects appeared in the TL spectrum. By analyzing the TL curve the depths of traps were calculated to be 0.61 and 0.66 eV. Also, the mechanism of LLP was discussed in this report.     

Effect of Bi2O3 on EPR, optical transmission and DC conductivity of vanadyl doped alkali bismuth borate glasses

Glasses with composition xBi₂O₃·(30−x)M₂O·70B₂O₃ (M=Li, Na) containing 2 mol% V₂O₅ have been prepared over the range 0≤x≤15 (x is in mol%). The electron paramagnetic resonance spectra of VO²⁺ of these glasses have been recorded in the X-band (≈9.3 GHz) at room temperature (RT≈300 K). Spin Hamiltonian parameters, g_∥, g_⊥, A_∥, A_⊥, dipolar hyperfine coupling parameter, P, and Fermi contact interaction parameter, K, have been calculated. The molecular orbital coefficients, α² and γ², have been calculated by recording the optical transmission spectra. In xBi₂O₃·(30−x)Li₂O·70B₂O₃ glasses there is decrease in the tetragonality of the V⁴⁺O₆ complex for x up to 6 mol% whereas for x≥6 mol%, tetragonality increases. In xBi₂O₃·(30−x)Na₂O·70B₂O₃ glasses there is increase in the tetragonality of the V⁴⁺O₆ complex with increasing x. The 3d_xy orbit expands with increase in Bi₂O₃:M₂O ratio. Values of the theoretical optical basicity, Λ_th, have also been reported. The DC conductivity increases with increase in temperature. The order of conductivity is 10⁻⁵ ohm⁻¹ m⁻¹ at low temperature and 10⁻³ ohm⁻¹ m⁻¹ at high temperature. The DC conductivity decreases and the activation energy increases with increase in Bi₂O₃:M₂O ratio.     

Relaxation and conductivity behaviour in the compounds: FeRGe2O7 (R=Pr,Tb)

AC measurements were performed on the thortveitite-like layered compounds, FeRGe₂O (R=Pr,Tb) in order to study their dielectric features, e.g. as a function of temperature. The main electrical response lies on impedance plots composed of two successive arcs with depressed centers. Bulk conductivity behavior is mostly Arrhenius for the measured temperatures. The associated bulk activation energies are close to 1 eV. Raw data were used to follow the temperature dependence of the dynamic parameters, ε′(ω) and σ′(ω). From logarithmic σ′(ω) vs. ω curves the dc component was obtained. ε′(ω) vs. log ω curves exhibit a dispersive behavior at low frequencies, reflecting blocking effects. E_dc and E_ac activation energies were also calculated, the last one obtained from σ′(ω) vs. 1000/T plots. Conductivity results suggest the occurrence of an extrinsic conducting mechanism. A structural instability was detected via the temperature dependence of permittivity, which has been ascribed to the presence of Ga-O-Ga bonds having associated angles different of 180°. Analyses of the results show that the interchange of Tb and Pr in the general formula FeRGe₂O₇ (R=Pr, Tb) involves only small differences in their global ac and dc behavior.     

On the power-law behavior of the AC conductivity of the mixed crystal (NH4)3H(SO4)1.42(SeO4)0.58

A power law used to describe the AC conductivity from 299 to 393 K of the mixed crystal (NH₄)₃H(SO₄)_1.42(SeO₄)_0.58 led to fractional exponent values ranging from 1.08 to 0.91, depending on structural changes induced on temperature variation [B. Louati, M. Gargouri, K. Guidara and T. Mhiri, J. Phys. Chem. Solids 66 (2005) 762]. In the present note, we suggest that the fractional law exhibits features of lattice relaxation. Despite the structural changes, the parameters of the power law are mutually interconnected to yield a temperature independent phenomenon. Such behavior is probably of general validity and characterizes the universal fractional dispersion of the AC conductivity, as it was also observed in glasses of different composition.     

Preparation, structure and oxide ion conductivity in Ce6−xYxMoO15−δ (x=0.1-1.4) solid solutions

The Ce_6−xY_xMoO_15−δ solid solution with fluorite-related structure have been characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), IR, Raman, scanning electric microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) methods. The electric conductivity of samples is investigated by Ac impedance spectroscopy. An essentially pure oxide-ion conductivity of the oxygen-deficiency was observed in pure argon, oxygen and air. The highest oxygen-ion conductivity was found in Ce_5.5Y_0.5MoO_15−δ ranging from 5.9×10⁻⁵ (S cm⁻¹) at 300 °C to 1.3×10⁻² (S cm⁻¹) at 650 °C, respectively. The oxide-ion conductivities remained stable over 80 h-long test at 800 °C. These properties suggested that significant oxide-ionic conductivity exists in these materials at moderately elevated temperatures.     

Effects of protons and acceptor substitution on the electrical conductivity of La6WO12

Transport properties and non-stoichiometry of La_1−xCa_xW_1/6O₂ and La_1−yW_1/6O₂ (x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H⁺/D⁺ isotope exchange, and thermogravimetry in the temperature range 300-1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW_1/6O₂ with values reaching 3×10⁻³ S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.     

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10 mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900 °C for 500 h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.     

Frequency dependence of ionic conductivity and dielectric relaxation studies in Na3H(SO4)2 single crystal

Electrical impedance measurements of Na₃H(SO₄)₂ were performed as a function of both temperature and frequency. The electrical conductivity and dielectric relaxation have been evaluated. The temperature dependence of electrical conductivity reveals that the sample crystals transformed to the fast ionic state in the high temperature phase. The dynamical disordering of hydrogen and sodium atoms and the orientation of SO₄ tetrahedra results in fast ionic conductivity. In addition to the proton conduction, the possibility of a Na⁺ contribution to the conductivity in the high temperature phase is proposed. The frequency dependence of AC conductivity is proportional to ω^s. The value of the exponent, s, lies between 0.85 and 0.46 in the room temperature phase, whereas it remains almost constant, 0.6, in the high-temperature phase. The dielectric dispersion is examined using the modulus formalism. An Arrhenius-type behavior is observed when the crystal undergoes the structural phase transition.     

Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Electrical conductivity and amorphization of Sc2(WO4)3 at high pressures and temperatures

Impedance spectroscopy measurements and synchrotron X-ray diffraction studies of Sc₂(WO₄)₃ at 400°C have been carried out as a function of pressure up to 4.4 GPa. Ionic conductivity shows normal decrease with increase in pressure up to 2.9 GPa, but then increases at higher pressures. The XRD results show that Sc₂(WO₄)₃ undergoes pressure-induced amorphization at pressures coincident with the reversal in conductivity behavior. The loss of crystal structure at high pressure is consistent with growing evidence of pressure-induced amorphization in negative thermal expansion materials, such as Sc₂(WO₄)₃. The increase in conductivity in the amorphized state is interpreted as the result of an increase in structural entropy and a concomitant reduction of energy barriers for ionic transport.     

Effects of uniaxial pressure and annealing on the resistivity of Ba(Fe1−xCox)2As2

Single crystals of underdoped Ba(Fe_1−xCo_x)₂As₂ were detwinned by applying uniaxial pressure. The anisotropic in-plane resistivity was measured using the Montgomery method without releasing pressure. The resistivity along the a-axis shows metallic behavior down to 5 K, while the resistivity along the b-axis shows an insulator-like behavior in some temperature range. Annealing the sample radically reduces the residual resistivity for x=0, and at the same time the anisotropy becomes much smaller at low temperatures.     

Temperature dependence of electrical resistivity for Ca-doped perovskite-type Y1−xCaxCoO3 prepared by sol-gel process

The temperature dependences of DC electrical resistivity for perovskite-type oxides Y_1−xCa_xCoO₃ (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y_1−xCa_xCoO₃ decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced E_a (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρ∝T^−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T^−1/4 law, we estimated the density of localized states N(E_F) at the Fermi level, which was found to increase with increasing doping content.     

Thermoluminescence studies of LiBa2B5O10:RE (RE=Dy, Tb and Tm)

LiBa₂B₅O₁₀:RE³⁺ (RE=Dy, Tb and Tm) was synthesized by the method of high-temperature solid-state reaction and the thermoluminescence (TL) properties of the samples under the irradiation of the γ-ray were studied. The result showed that Dy³⁺ ion was the most efficient activator. When the concentration of Dy³⁺ was 2 mol%, LiBa₂B₅O₁₀:Dy³⁺ exhibited a maximum TL output. The kinetic parameter of LiBa₂B₅O₁₀:0.02Dy was estimated by the peak shape method, for which the average activation energy was 0.757 eV and the frequency factor was 1.50×10⁷ s⁻¹. By the three-dimensional (3D) TL spectrum, the TL of the sample was contributed to the characteristic f-f transition of Dy³⁺. The dose-response of LiBa₂B₅O₁₀:0.02Dy to γ-ray was linear in the range from 1 to 1000 mGy. In addition, the decay of the TL intensity of LiBa₂B₅O₁₀:0.02Dy was also investigated.     

Synthesis and luminescence properties of YVO4:Eu,Bi phosphor with enhanced photoluminescence by Bi doping

YVO₄:Eu³⁺,Bi³⁺ phosphors have been prepared by the high-temperature solid-state (HT) method and the Pechini-type sol-gel (SG) method. Spherical SiO₂ particles have been further coated with YVO₄:Eu³⁺,Bi³⁺ phosphor layers by the Pechini-type SG process, and it leads to the formation of core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ phosphors. Therefore, the phase formations, structures, morphologies, and photoluminescence properties of the three types of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors were studied in detail. The average diameters for the phosphor particles are 2-4 μm for HT method, 0.1-0.4 μm for SG method, and 0.5 μm for core-shell structured SiO₂/YVO₄:Eu³⁺,Bi³⁺ particles, respectively. Photoluminescence spectra show that effective energy transfer takes place between Bi³⁺ and Eu³⁺ ions in each type of as-prepared YVO₄:Eu³⁺,Bi³⁺ phosphors. Introduction of Bi³⁺ into YVO₄:Eu³⁺ leads to the shift of excitation band to the long-wavelength region, thus the emission intensities of ⁵D₀-⁷F₂ electric dipole transition of Eu³⁺ at 615 nm upon 365 nm excitation increases sharply, which makes this phosphor a suitable red-emitting materials that can be pumped with near-UV light emitting diodes (LEDs).