An insight into the structure,conductivity and ion dynamics of Sr-Sm co-doped ceria oxygen ion conductors: Effect of defect interaction

In this work, we investigate the structure, conductivity and ion dynamics of mixed di and tri-valent doped Ce_0.8Sm_0.2-xSr_xO_2-δ (x = 0–0.2) oxygen ion conductors. The lattice parameter and root mean square strain are significantly affected by the ionic radius of dopants and their solubility into ceria lattice. Due to the solubility limit of Sr²⁺ ions, SrCeO₃ phase increases with the doping concentration of Sr²⁺. The increase of Sr²⁺ ions into ceria lattice promotes the formation of large defect clusters by expense of formed oxygen vacancies. The coulombic interaction between oxygen vacancies with substituted dopant cations enhances with Sr²⁺ ions due to decrease of the value of dielectric constant of the compositions. The defect interaction significantly affects the conductivity values by means of increase of SrCeO₃ phase and defect clusters. The conductivity values are found to be consistent with the migration and association energy. The scaled spectra of dielectric tangent loss and real part of complex conductivity confirm the temperature and defect interaction independent nature of hoping mechanism in the compositions.     

Dielectric and ac impedance studies of nanostructured CaCu3Ti3.90Ce0.10O12 electro-ceramic synthesized by citrate-gel route

Effect of doping at Ti⁴⁺ site by Ce³⁺ has been examined in CaCu₃Ti_3.90Ce_0.10O₁₂ synthesized by citrate-gel route. DTA/TG analysis of dry powder gives pre-information about formation of final product around 850 °C. X ray diffraction analysis confirmed the formation of CaCu₃Ti_3.90Ce_0.10O₁₂ phase of the ceramic sintered at 950 °C for 12 h. Microstructure has been studied using scanning electron microscopy and confirmed the average grain size found in nano range 200–400 nm in system CaCu₃Ti_3.90Ce_0.10O₁₂.The nature of relaxation behavior of ceramic was also rationalized by using the impedance and modulus spectroscopy. The bulk conductivity indicates an Arrhenius-type thermally activated process. The ac conductivity spectrum obeyed the Jonscher power law. The complex impedance diagrams of the ceramic exhibited a significant contribution from the grains, grain boundaries and electrode. The activation energies calculated from the grain-boundary relaxation time constant was found to be 0.49 eV which confirmed the Maxwell–Wagner type of relaxation present in the ceramic.     

Apparent giant dielectric constants, dielectric relaxation, and ac-conductivity of hexagonal perovskites La1.2Sr2.7BO7.33 (B=Ru, Ir)

We present a thorough dielectric investigation of the hexagonal perovskites La_1.2Sr_2.7IrO_7.33 and La_1.2Sr_2.7RuO_7.33 in a broad frequency and temperature range, supplemented by additional infrared measurements. The occurrence of giant dielectric constants up to 10⁵ is revealed to be due to electrode polarization. Aside of dc and ac conductivity contributions, we detect two intrinsic relaxation processes that can be ascribed to ionic hopping between different off-center positions. In both materials we find evidence for charge transport via hopping of localized charge carriers. In the infrared region, three phonon bands are detected, followed by several electronic excitations. In addition, these materials provide further examples for the occurrence of a superlinear power law in the broadband ac conductivity, which recently was proposed to be a universal feature of all disordered matter.     

Behavior of Mn3+ ions in four-layered hexagonal (Sr1−x−yLaxBay)MnO3

Stoichiometric four-layered hexagonal (4H) (Sr_1?x?yBa_xLa_y)MnO₃ was synthesized using a standard ceramic technique. Rietveld analysis at room temperature indicated that the Mn–O(1) distance increased and the Mn–O(2) distance decreased with the increase in x. The samples were n-type semiconductors and exhibited hopping conductivity in a small-polaron model below 533 K. The Mn³⁺ ion acted as a donor and the electron transfer became active through the Mn³⁺–O–Mn⁴⁺ path. The samples were antiferromagnetic and the Néel temperature (T_N) was constant regardless of y when x was fixed to 0.3, whereas T_N shifted to a high temperature when y was fixed to 0.02. The face-sharing Mn³⁺–O(2)–Mn⁴⁺ interaction strengthened as the Mn–O(2) distance decreased, and T_N shifted to a high temperature as a result.     

Sensitized red luminescence from Ce, Mn-doped glaserite-type alkaline-earth silicates

Bright red luminescence is observed from Ce, Mn-doped glaserite-type alkaline-earth silicates with M₂BaMgSi₂O₈ (M: Ba, Sr, Ca) chemical composition. Under UV excitation, Ce-doped M₂BaMgSi₂O₈ exhibits strong near-UV emission with asymmetric peak shape. UV-excited Mn-doped M₂BaMgSi₂O₈ compounds show visible red emission only when Ce³⁺ ions are doped together. These results indicate that Mn²⁺-derived red emission is caused by an efficient energy transfer from Ce³⁺ to Mn²⁺. The red emission becomes intense with an increase in Ba-amount. This trend originates from the relaxation of the selection rule for 3d-3d transition in Mn²⁺ ions, which is caused by the structural deformation due to Ba²⁺ occupation for layer-pockets.     

Characterization and physical properties of Li2O-CaF2-P2O5 glass ceramics with Cr2O3 as a nucleating agent—Physical properties

In this paper, studies on various physical properties, viz., dielectric properties (dielectric constant, loss tan δ, a.c. conductivity σ) over a wide range of frequency and temperature, optical absorption, ESR at liquid nitrogen temperature and magnetic susceptibility at room temperature of Li₂O-CaF₂-P₂O₅: Cr₂O₃ glass ceramics, have been reported. The optical absorption, ESR and magnetic susceptibility studies indicate that the chromium ions exist in Cr⁵⁺, Cr⁴⁺ and Cr⁶⁺ states in addition to Cr³⁺ state in these samples. The dielectric constant and loss variation with the concentration of Cr₂O₃ have been explained on the basis of space charge polarization mechanism. The dielectric relaxation effects exhibited by these samples have been analysed by a graphical method and the spreading of dielectric relaxation has been established. The a.c. conductivity in the high-temperature region seems to be connected both with electronic and ionic movements.     

Energy Transfer from Ce3+ to Tb3+/Dy3+/Mn2+ in Ca9Ga(PO4)7 Phosphors: Synthesis,Structure and Tunable Multicolor Luminescent Properties

A series of Ca₉Ga(PO₄)₇:Ce³⁺/Tb³⁺/Dy³⁺/Mn²⁺ phosphors with tunable color, in which Ce³⁺ acts as the sensitizer, was synthesized. Energy transfer (ET) from Ce³⁺ to Tb³⁺/Dy³⁺/Mn²⁺ was investigated in detail. Tb³⁺/Dy³⁺/Mn²⁺ single-doped Ca₉Ga(PO₄)₇ can exhibit green, yellow, and red emission, respectively. Incorporating Ce³⁺ into a Tb³⁺/Dy³⁺/Mn²⁺ single-doped Ca₉Ga(PO₄)₇ phosphor can remarkably promote the luminous efficiency of the Tb³⁺/Dy³⁺/Mn²⁺ ions. This enhancement originates from an efficient ET from Ce³⁺ to Tb³⁺/Dy³⁺/Mn²⁺. The ET was validated by luminescence spectra, decay dynamics, and schematic energy levels. Moreover, the intensity ratio of red emission of Mn²⁺ to violet emission of Ce³⁺ was analyzed based on energy-transfer and lifetime measurements. In Ce³⁺-Tb³⁺, Ce³⁺-Dy³⁺, and Ce³⁺-Mn²⁺ doped Ca₉Ga(PO₄)₇, the emitting color changed from violet to green, yellow, and red, respectively, which indicates the potential use of this new tunable phosphor in UV light-emitting diodes.     

The origin of DC electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) films

We present here the evidence for the origin of dc electrical conduction and dielectric relaxation in pristine and doped poly(3‐hexylthiophene) (P3HT) films. P3HT has been synthesized and purified to obtain pristine P3HT polymer films. P3HT films are chemically doped to make conducting P3HT films with different conductivity level. Temperature (77–350 K) dependent dc conductivity (σ_dc) and dielectric constant (ε′(ω)) measurements on pristine and doped P3HT films have been conducted to evaluate dc and ac electrical conduction parameters. The relaxation frequency (f_R) and static dielectric constant (ε₀) have been estimated from dielectric constant measurements. A correlation between dc electrical conduction and dielectric relaxation data indicates that both dc and ac electrical conductions originate from the same hopping process in this system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1047–1053, 2010     

Valence Distributions of Iron and Manganese in Pure,Manganese‐doped and Calcium‐doped Yttrium Orthoferrite

The valence distributions of Fe and Mn in yttrium orthoferrite (YFeO,), YFe_0.6Mn0.4O3 and Y_0.9Ca_0.1Fe_0.6Mn_0.4O_3.8 were studied by the measurement of thermal power. Pure YFeO₃ shows the n‐type electrical conductivity associated with small polaron hopping between Fe²⁺ and Fe³⁺. Both YFe_0.4Mn_0.4‐O₃ and Y_0–9Ca_0–1.Fe_0–6Mn_0–4O_3–8 show n‐type and p‐type conductivities at high and low temperatures respectively associated with small polaron hopping between Mn³⁺ and Mn⁴⁺, iron has only oxidation state of Fe³⁺ and does not have contribution to the electrical conductivity.     

The effect of Ce3+ cations on polyaniline morphology and electric properties

Polyaniline (PANI) is synthesized in the potentiostatic pulse mode from an electrolyte containing Ce₂(SO₄)₃. Cations Ce³⁺ are incorporated into the polymer composition during PANI redox transformations. It is shown that PANI in its conducting and dielectric forms contains different amounts of Ce³⁺ cations. Starting with the beginning of polymerization, the Ce³⁺ cations actively form the special polymer morphology as demonstrated by SEM images. The chief consequence of the formation of so well-developed uniform nanostructure is that the latter allows the dopant anions, cations, and protons to easily enter and leave it. This, in turn, results in the high electrochemical activity of this polymer and enhances the conductivity of PANI samples doped with Ce³⁺ cations as compared with those doped with only protons.     

Luminescence Tuning of Sr8MgCe(PO4)7:Eu2+,Mn2+ Phosphors: Structure Refinement,Site Occupancy,and Energy Transfer

Sr₈MgCe(PO₄)₇:Eu²⁺,Mn²⁺ phosphor with whitlockite‐type structure was prepared by a combustion‐assisted solid‐state reaction. The crystal structure and luminescence properties were investigated. Under UV radiation, Sr₈MgCe(PO₄)₇ host exhibits a violet‐blue emission band from Ce³⁺ ions. When Eu²⁺/Mn²⁺ are doped into the host, the samples excited with 270 nm UV radiation present multicolor emissions due to the energy transfer (ET) from Ce³⁺ to Eu²⁺/Mn²⁺. The emitting color of Sr₈MgCe(PO₄)₇:Eu²⁺ can be tuned from violet‐blue to yellow‐green, whereas Sr₈MgCe(PO₄)₇:Mn²⁺ can emit red light. Under excitation with long wavelength at 360 nm, Sr₈MgCe(PO₄)₇:Eu²⁺ phosphor shows a broadband emission from 390 to 700 nm, which is attributed to the 4f⁶5d¹→4f⁷ transition of Eu²⁺ without the contribution from Ce³⁺ emission. Tunable full‐color emitting light can be achieved in the Eu²⁺ and Mn²⁺‐codoped Sr₈MgCe(PO₄)₇ phosphor by ET_Eu–Mn through control of the levels of doped Eu²⁺ and Mn²⁺ ions. These results suggest that Sr₈MgCe(PO₄)₇:Eu²⁺,Mn²⁺ phosphor has potential applications in NUV chip pumped white LEDs.     

Grafting and initiation reactions

Grafting of acrylic monomers onto cellulose substrates in acid aqueous medium using complex of Mn³⁺ ions as a redox initiator is described. Assuming that the /Mn³⁺/ ions react with aldehydes and vicinal diols, the rate of these reactions has been measured spectroscopically. (ESR and visible light) using model compounds. It is concluded that the /Mn³⁺/ grafting onto cellulose is a radical reaction initiated mainly by oxidation of aldehydes at reducing end groups and C-C bond scission of vicinal diols at the end groups and along the cellulose chains. The initiation rates of the Ce⁴⁺, /Mn³⁺/ and VO₂²⁺ redox ions are related to their oxidation potential.     

Enhancing Photoactivity of TiO2(B)/Anatase Core–Shell Nanofibers by Selectively Doping Cerium Ions into the TiO2(B) Core

Cerium ions (Ce³⁺⁾ can be selectively doped into the TiO₂(B) core of TiO₂(B)/anatase core–shell nanofibers by means of a simple one‐pot hydrothermal treatment of a starting material of hydrogen trititanate (H₂Ti₃O₇) nanofibers. These Ce³⁺ ions (≈0.202 nm) are located on the (110) lattice planes of the TiO₂(B) core in tunnels (width≈0.297 nm). The introduction of Ce³⁺ ions reduces the defects of the TiO₂(B) core by inhibiting the faster growth of (110) lattice planes. More importantly, the redox potential of the Ce³⁺/Ce⁴⁺ couple (E°(Ce³⁺/Ce⁴⁺)=1.715 V versus the normal hydrogen electrode) is more negative than the valence band of TiO₂(B). Therefore, once the Ce³⁺‐doped nanofibers are irradiated by UV light, the doped Ce³⁺ ions—in close vicinity to the interface between the TiO₂(B) core and anatase nanoshell—can efficiently trap the photogenerated holes. This facilitates the migration of holes from the anatase shell and leaves more photogenerated electrons in the anatase nanoshell, which results in a highly efficient separation of photogenerated charges in the anatase nanoshell. Hence, this enhanced charge‐separation mechanism accelerates dye degradation and alcohol oxidation processes. The one‐pot treatment doping strategy is also used to selectively dope other metal ions with variable oxidation states such as Co^2+/3+ and Cu^+/2+ ions. The doping substantially improves the photocatalytic activity of the mixed‐phase nanofibers. In contrast, the doping of ions with an invariable oxidation state, such as Zn²⁺, Ca²⁺, or Mg²⁺, does not enhance the photoactivity of the mixed‐phase nanofibers as the ions could not trap the photogenerated holes.     

ESR and fluorescence studies of Mn-doped Na2ZnP2O7 single crystal and glasses

X-band electron spin resonance (ESR) spectra and fluorescence measurements were performed on Mn-doped Na₂ZnP₂O₇ (NZPO) single crystal synthesized by the Czochralski pulling method and glasses synthesized by the quenching process. For the single crystal, ESR angular dependences were measured in both the zx and xy plans of the NZPO lattice. The fine and hyperfine structure parameters and g-factor values were determined by modelling the experimental spectra. Using the Newman superposition model, the resonating centres in the single crystal and powder (crushed from crystals) samples are assigned to Mn²⁺ ions substituting for both zinc and sodium. For the glass sample the analysis of the ESR data shows that Mn²⁺ ions substitute for the Na⁺ ions. These interpretations are confirmed by the fluorescence measurements with a unique broad red band for the glassy compound and the presence of two emission bands (green and red) in the case of the crystal sample.     

Synthesis,Structure, and Photoluminescence Properties of Novel KBaSc2(PO4)3:Ce3+/Eu2+/Tb3+ Phosphors for White‐Light‐Emitting Diodes

A series of novel KBaSc₂(PO₄)₃:Ce³⁺/Eu²⁺/Tb³⁺phosphors are prepared using a solid‐state reaction. X‐ray diffraction analysis and Rietveld structure refinement are used to check the phase purity and crystal structure of the prepared samples. Ce³⁺‐ and Eu²⁺‐doped phosphors both have broad excitation and emission bands, owing to the spin‐ and orbital‐allowed electron transition between the 4f and 5d energy levels. By co‐doping the KBaSc₂(PO₄)₃:Eu²⁺ and KBaSc₂(PO₄)₃:Ce³⁺ phosphors with Tb³⁺ ions, tunable colors from blue to green can be obtained. The critical distance between the Eu²⁺ and Tb³⁺ ions is calculated by a concentration quenching method and the energy‐transfer mechanism for Eu²⁺→Tb³⁺ is studied by utilizing the Inokuti–Hirayama model. In addition, the quantum efficiencies of the prepared samples are measured. The results indicate that KBaSc₂(PO₄)₃:Eu²⁺,Tb³⁺ and KBaSc₂(PO₄)₃:Ce³⁺,Tb³⁺ phosphors might have potential applications in UV‐excited white‐light‐emitting diodes.     

Influence of the structure on electric and magnetic properties of La0.8Na0.2Mn1−xCoxO3 perovskites

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La_0.8Na_0.2Mn_1−xCo_xO₃ (0?x?0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral () structure and the insulator-metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I-M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn³⁺→Mn⁴⁺ leading to the formation of stable couples Mn⁴⁺-Co²⁺. Therefore the double-exchange interactions Mn³⁺-O²⁻-Mn⁴⁺ partly vanish and they are replaced by positive superexchange interactions Mn⁴⁺-O²⁻-Co²⁺, but of a semiconducting character.     

Conductivity and dielectric relaxation in various polyvinyl alcohol/ammonium salt composites

We studied electrical conductivity and dielectric relaxation in polyvinyl alcohol/ammonium chloride and polyvinyl alcohol/ammonium acetate composite films. Infrared absorbance showed the presence of H-bonding interaction between the salt and the polymer. X-ray diffraction showed the reduction of the grain size of ordered regions in the polymer matrix after adding salt. Thermo gravimetric analysis (TGA) showed water wt% content between 4.2 and 5.8%. Differential Scanning Calorimetry (DSC) showed the decrease of the glass transition due to retained water indicating its plasticizer effect. The ac conductivity studied in the frequency range from 10^?1 Hz to 1 MHz and the temperature range from 10 to 150°C is described by the universal law of Jonsher characterizing the charge transport in disordered materials. With NH₄Cl inclusion, the dc conductivity showed a higher value in the vicinity of 4% but with NH₄CH₃CO₂ the dc conductivity decreases monotonically by increasing the salt amount. By using the dielectric permittivity and dielectric modulus we detected three relaxation processes which we attributed to electrode/sample polarization, alpha relaxation and conductivity relaxation respectively.     

Studies of layered uranium(VI) compounds. I. High proton conductivity in polycrystalline hydrogen uranyl phosphate tetrahydrate

We have found that hydrogen uranyl phosphate tetrahydrate HUO₂PO₄·4H₂O has a high proton conductivity. The ac conductivity was 0.4 ohm^?1 m^?1 at 290°K measured parallel to the faces of sintered disks of the compound. The activation energy was found to be 31 ± 3 kJ mole^?1. The values of conductivity were between 3 and 10 times lower when measured perpendicular to the disk faces due to preferred orientation of the plate-like crystals. Both the powder and sintered disks are stable in air and insoluble in phosphoric acid solution of pH 2.5. Experiments are described which enable possible grain boundary contributions to the conductivity to be determined in such hydrates. The extrinsic grain boundary contribution to the conductivity was found to be small from experiments in which the pH in a solution cell was varied. The abnormally high bulk H⁺ conductivity thus inferred is attributed primarily to the high concentration of H⁺, which exists as H₃O⁺ in the interlamellar hydrogen-bonded network. A Grotthus-type mechanism of conduction is proposed which involves intermolecular transfer steps (hopping) and intramolecular transfer steps, in comparable numbers, the former facilitated by the high concentration of H₃O⁺ ions in the structure, and the latter most likely facilitated by the high concentration of H-bond vacancies.     

A combined diffraction and dielectric properties investigation of Ba3MnNb2O9 complex perovskites

A combined synthesis, diffraction and dielectric properties investigation of the dependence (and effect) of Mn²⁺/Nb⁵⁺ ordering in Ba₃MnNb₂O₉ (BMN) upon annealing atmosphere and processing conditions has been carried out. Annealing in different atmospheres was not found to significantly alter either nominal stoichiometry or structure type. The obtained structure type (disordered metrically cubic or ordered trigonal) as well as the measured electrical properties (in particular, the dielectric loss) were, however, found to be sensitive to the synthesis route. Samples obtained via solid-state reaction were found to be predominantly of 1:2 Mn²⁺/Nb⁵⁺ ordered, trigonal structure type whereas samples obtained via an aqueous solution route were found to be of a Mn²⁺/Nb⁵⁺ ‘disordered’, metrically cubic structure type. All solid-state synthesized samples showed reasonable dielectric properties. The microwave dielectric constant and dielectric quality factor, Q, at 8 GHz of an as-synthesized BMN sample were 38 and 100, respectively. By contrast, the dielectric loss of the metrically cubic, Mn²⁺/Nb⁵⁺ ‘disordered’ samples obtained via an aqueous solution synthesis process were significantly worse.     

Mechanical and electrical properties of Ni0.65Zn0.35CuxFe2−xO4 ferrite

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2?xO₄ (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. X-ray analysis showed that they were cubic spinel (single phase). Young's modulus, the dielectric loss and the change in capacitance under mechanical stress were measured for the samples. Young's modulus decreased with increasing Cu content. This is due to the fact that Cu²⁺ ions entered the lattice substitutionally for Fe³⁺ ions at the octahedral sites, creating lattice vacancies gave rise to lattice strain. The minimum value of the dielectric loss corresponding tox=0.3 may be due to the formation of lattice vacancies retarding the jump frequency to be far from the frequency of the applied a.c. field. The increase in capacitance of the samples with mechanical stress may be explained via the mechanism of dielectric polarization.