Structural and impedance spectroscopy study of Samarium modified Barium Zirconium Titanate ceramic prepared by mechanochemical route

Samarium modified Barium Zirconium Titanate (BZT) ceramics with general formula [Ba_1−xSm_2x/3](Zr_0.05Ti_0.95O₃) [x = 0, 0.01, 0.02, 0.03 and 0.04] have been prepared by high energy ball milling. The Rietveld refinement of BaZr_0.05Ti_0.95O₃ (BZT) shows a single phase tetragonal symmetry with space group P4mmm and TEM micrograph shows that the crystalline size is in the submicron range. X-ray diffraction (XRD) patterns confirm that no phase change occurs with the addition of Samarium in BZT upto x ≤ 0.03 and a small pyrochlore phase exists at x = 0.04. The complex impedance (Nyquist) plots display a single semicircle highlighting the influence of grain resistance on the electrical behavior. Negative temperature coefficient of resistance behavior is observed in all compositions. The activation energy calculated from Z″ and DC conductivity confirms that the oxygen vacancies play an important role in the conduction mechanism.     

Defect studies of doped and undoped barium titanate using computer simulation techniques

We present the results of a computer simulation study of defect energies in BaTiO₃. Our calculations are used to propose a defect model consistent with experimental studies. Intrinsic disorder is masked by oxygen vacancies which compensate accidental acceptor impurities. Holes are trapped by acceptors at high po₂ values whereas doubly ionised oxygen vacancies lead to n-type behavior at lower po₂. Whilst holes and oxygen vacancies are weakly bound by barium vacancies, titanium vacancies act as deep traps for holes, although negligible concentrations will be present in lightly doped BaTiO₃ samples. Three types of competing compensation mechanism are proposed for trivalent dopants, enabling donor ‘effectiveness’ to be predicted in satisfactory agreement with experiment. Binding energies for electron and cation vacancy trapping by donor dopants are generally calculated to be lower than those proposed previously to interpret conductivity measurements.     

Effect of La substitution on conductivity and dielectric properties of Bi1−xLaxFeO3 ceramics: An impedance spectroscopy analysis

Bismuth ferrite (BFO) and La-substituted BFO with composition Bi_1−xLa_xFeO₃ (x=0.05, 0.1 and 0.15) (BLFO_x=0.05-0.15) ceramics were prepared using the solid state reaction route. A structural phase transition from rhombohedral phase to triclinic phase was observed for BLFO_x=0.05-0.15 ceramics. Modulus spectroscopy reveals the deviation of dielectric behavior from ideal Debye characteristics and the dependence of conductivity on ion hopping in BFO and BLFO_x=0.05-0.15 ceramics. The conductivity of the BFO ceramics decreases for La content of 5 mol%, followed by a subsequent increase with 10 and 15 mol% of lanthanum doping. The typical values of the activation energies at high temperature reveal the contribution of short range movement of doubly ionized oxygen vacancies to the conduction process in BFO and BLFO_x=0.05 ceramics. Both short range and long range motion of oxygen vacancies are responsible for large conductivity in BLFO_{x=0.1 and 0.15} ceramics.     

Study of density of localized states in Cd4Se96−xSx (x=0, 4, 8, 12) chalcogenide semiconductor

Thin films of Cd₄Se_96−xS_x (x=0, 4, 8, 12) chalcogenide semiconductor were deposited by the thermal evaporation technique on glass substrates. XRD pattern of CdSeS alloys show that the grain size decreases with the concentration of Sulfur (S). The surface morphology changes due to the addition of sulfur content. The effect of sulfur on the DC conductivity has been investigated, which show that the DC conductivity is a thermally activated process. The Mott parameter shows that dominate conduction is in the localized states, also the addition of sulfur in Cd–Se results an increase in electrical conductivity, which may be due to shift of Fermi level. Current–voltage (I–V) measurements at different fixed temperatures show two regions; Ohmic conduction at low bias having a unit slope, and non ohmic conduction at high bias. Observation of the data shows that conduction is dominated by trap limited space charge limited conduction (SCLC), from where the density of state has been calculated using SCLC measurement data. The increase in the density of states with sulfur concentration may be due to the increase in the defect states.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Investigation of the electronic structure and chemical bonding of lead hexacyanoferrate(III)

The electronic structure of lead hexacyanoferrate(III) is calculated by the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the LSDA + U approximation. The influence of vacancies in the lead sublattice on the electronic spectrum, chemical bonding, and magnetic properties of the Pb_1.5Fe(CN)₆ phase is investigated. Analysis of the electronic spectrum shows that this compound is characterized by semiconductor conductivity. It is demonstrated that the semiconductor gap is associated with the charge ordering of iron(III) ions.     

Synthesis,refinement of the structure,and AC conductivity behavior of sodium lithium orthonavadates

The Na _x Li_1-x CdVO₄ (x = 0.5, 1) orthovanadates were prepared using a solid-state reaction method. The x-ray diffraction patterns (XRDP) of both materials reveal the formation of the Na₂CrO₄ structure. Vibrational study confirms the existence of [VO₄]^3? group. Electrical measurements of our compounds have been investigated using complex impedance spectroscopy (CIS) in the frequency and temperature range 209 Hz–1 MHz and 589–703 K, respectively. Nyquist plots reveal the presence of tow contributions, an equivalent circuit was proposed. DC conductivity shows electrical conduction in the material as a thermally activated process. The AC conductivity is explained using the non-overlapping small polaron tunneling (NSPT) conduction mechanism. A relationship between crystal structure and ionic conductivity was established and discussed.     

Infrared spectra of solid germane

An IR spectroscopic study has shown that GeH₄ molecules are situated on one set of sites of C, symmetry in phase IV. The III–IV transition at 63 K is apparently a second-order phase transition. In phases II and III the molecules are on sites of C_3v. or C₃ symmetry. The II–III phase transition was observed at 67 K. In solid GeD₄, phase transitions were observed at 68.5 and 77 K. In phases II and III the site symmetry is C_I. The II–III phase transition in GeD₄ is apparently second order. There is evidence that the ν₁, vibration of GeD₄ is IR active in the solid state.     

AC impedance spectroscopic studies of transport properties in metal oxide doped α-NPD

The potential application of molecular organic semiconductor needs the control adjustment of conductivity. The hole transport materials (HTL) α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenylbenzidin) has been doped with metal oxide (molybdenum oxide) for the study of transport phenomena. Impedance spectroscopic measurements were performed with the applied external electric filed. The activation energy calculated (0.55, 0.48 and 0.50 eV) decreases with doping of metal oxide. There is a clear indication of space charge limited (SCL) conduction in the doped and undoped thin films. The cole–cole plot indicates the device can be represented by a parallel resistance and capacitance network in series with a series resistance (of 70 Ω). At high frequency; the conduction follows the universal power law for both doped–undoped α-NPD with MoO₃ and the onset frequency increases with increasing bias voltages. The conduction mechanism can be explained by classical hopping barriers (CHB) mechanism for the system.     

EPR investigations of phase transitions in lithium potassium sulfate: LiKSO4

Single crystal EPR studies of NH⁺₃ doped in the K⁺ sites of lithium potassium sulfate (LKS) have been carried out from room temperature down to −150°C. The spin Hamiltonian parameters have been evaluated for the three phases which occur in this temperature range, P I (room temperature) (A_N)_XX = (A_N)_YY = 23.2 G and (A_N)_ZZ = 10.4 G; P II (below −69°C) (A_N)_XX = (A_N)_XX = 22.3 G and (A_N)_ZZ = 11.7 G; P III (below −83°C) (A_N)_XX = 8.9 G, (A_N)_YY = 20.8 G and (A_N)_ZZ = 25.9 G. The P I–P II transition at −69° C is rather sluggish and shows a mixed phase for 2.5–3 h. In P III the principal component (A_N)_ZZ moves out by an angle of 1.4° from the b-axis. The space group of this phase is most probably not Cmc2₁.     

Influence of the shape of inclusions on the percolation thresholds in 2D models of composites

The influence of the shape of inclusions on the conductivity of composites, including critical concentration N _c (percolation threshold), is considered using 2D models as an example. A relationship between constant N _I , which characterizes effective conductivity ?? _e for low concentrations N of inclusions and percolation threshold N _c is established.     

Electrical conductivity behaviour of pure and polyblends samples of polyvinyl chloride (PVC) and polymethyl methacrylate (PMMA)

Electrical conductivity of pure PVC, PMMA and their polyblends samples has been studied in detail as a function of polarizing fields at constant temperatures. Different plots were drawn to investigate the nature of mechanism responsible for conduction. The nature of all the thermograms is nonlinear but similar for all temperatures. The plots have two slopes, i.e., ohmic conduction with slope of curve ≈1 at lower voltage region and a non-ohmic conduction with slope ≈1.9 at higher voltage region are observed. The increase in the conductivity and decrease in the activation energy, suggest that plasticization effect is taking place between the polymers when they are blended. Fowler–Nordheim plots are not consistent and showing negative and positive slopes simultaneously for lower as well as higher values of applied voltage. Theoretical and experimental values of β_SR and β_PF were calculated and the β_exp of pure and polyblend samples is in agreement with theoretically β_PF. The β_exp value lies close to β_PF, this shows that Poole–Frenkel mechanism is also effective. The calculated metal electrode potential barrier at a constant voltage suggests that the dominant charge carrier mechanism is Schottky–Richardson type. Hence in the present case both SR and PF mechanisms are seem to be operative.     

Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

Electrical and thermal studies in the commensurate incommensurate phase region of (NH4)2ZnCl4 single crystal

DC electrical conductivity for a virgin and poled annealed (NH₄)₂ZnCl₄b-axis single crystal shows a defect controlled property. A Schottky mechanism is a probable mechanism of conduction in regions of strong structural transitions. The rise of conductivity in the incommensurate and paraelectric phases is linked to an increase in discommensurations density. The activation energies (ΔE) in the three phases region were calculated. DTA measurements shows that the crystal is stable up to 200 °C and the phase transition temperatures were observed at 42, 94.8 and 137 °C. The effective activation energy (E_e) was obtained using Kissinger and Mahadevan equations. It was found to be equal to 0.49 eV. This correlates with the value obtained through DC conductivity.     

Mixed conducting perovskite-like ceramics on the base of lanthanum gallate

Ceramic perovskite solid solutions (La_0.9Sr_0.1)[(Ga_1−xM_x)_0.8Mg_0.2]O_3−y, 0 ≤ x ≤ 0.5, M = Fe, Ni, Cr (systems I–III) and brownmillerite solid solutions (La_0.2Sr_1.8)[Ga(Fe_1−xMg_x)]O_5−z, 0 ≤ x ≤ 0.5, (system IV) have been prepared. The samples have been studied by X-ray diffraction and electron microscopy methods, dielectric spectroscopy and permeability measurements. The correlation between the composition, unit cell parameter changes, electrical transport and oxygen permeation properties has been revealed. Introduction of transition metals (Fe, Ni, or Cr), substituting for gallium, ensures the enhancement of the electronic constituent of the conductivity in the perovskite systems I–III. Stabilization of the transition metal high valence states 4+ or 5+ has been suggested for compositions I and III. This leads to a unit cell volume contraction and provides a decrease in the concentration of oxygen vacancies. The oxygen permeability reaches its maximum values in compositions I–III with x ∼ 0.3. On the contrary, increasing concentration of the doping element with lower valence state (magnesium), substituting for iron, determines the expansion of the brownmillerite unit cell volume and provides an increase of the oxygen vacancy concentration, which in turn, favors the enhancement of oxygen permeability of composition IV.     

Rubidium-cation conductivity of Rb<Subscript>3–2<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> solid solutions

New Rb₃PO₄-based ceramic materials with high rubidium-cation conductivity in the Rb_3–2x Pb _x PO₄ system have been synthesized and studied. Introduction of Pb²⁺ cations leads to a sharp increase in the conductivity of rubidium orthophosphate due to formation of cation vacancies and, at temperatures 350–550°C, also due to the stabilization of high-temperature cubic modification Rb₃PO₄. At high temperatures, the electrolytes prepared have very high ion conductivity higher than 10^–1 S cm^–1 at 700°C, which is higher than the values previously obtained in similar systems with additions of tin and cadmium ions. The factors influencing the transport properties of the materials under study are discussed.     

The influence of imperfection of the crystal lattice on the electrokinetic and magnetic properties of disordered titanium monoxide

The conductivity and magnetic susceptibility of disordered titanium monoxide TiO_y (0.920≤y≤1.262) containing vacancies in titanium and oxygen sublattices are investigated. For TiO_y monoxides with an oxygen content y≤1.069, the temperature dependences of the conductivity are described by the Bloch-Grüneisen function at a Debye temperature ranging from 400 to 480 K and the temperature dependences of the magnetic susceptibility are characterized by the contribution from the Pauli paramagnetism due to conduction electrons. The behavior of the conductivity and magnetic susceptibility of TiO_y monoxides with an oxygen content y≥1.087 is characteristic of narrow-gap semiconductors with nondegenerate charge carriers governed by the Boltzmann statistics. The band gap ΔE between the valence and conduction bands of TiO_y monoxides with y≥1.087 falls in the range 0.06–0.17 eV.     

Characteristics of the thermodynamic and kinetic properties of NbH0.83 in the phase transition region

Measurements were made of the specific heat (in the temperature range 80–400 K), electrical resistivity (4.2–300 K), thermo-emf (4.2–300 K), thermal conductivity (7–300 K), magnetic susceptibility (4.2–400 K), and lattice parameters (30–300 K) of the alloy NbH_0.83. The λ′→λβ phase transitions were studied. It was established that the structure of the niobium matrix of the hydride remains unchanged as a result of these transitions. It is shown that the β→λ phase transition (one-dimensional hydrogen ordering) is a three-stage one and is accompanied by a substantial change in the shear modulus of the hydride. The λ′ phase existing at T<110 K was determined. The electron thermal conductivity κ_el calculated for the λ′ phase in the range T<23 K is 25% higher than the measured thermal conductivity. In order to explain this fact and also the discontinuity in the concentration dependence of the coefficient of thermal expansion of NbHx for x?0.83–0.84 and the approximately 1.5% compression of the NbH_0.84 volume it is assumed that in the region of x _c in the λ′ phase the topology of the Fermi surface of the NbH_x interstitial alloy changes substantially.     

Abelian lattice gauge theories in 3 + 1 dimensions: The linked cluster approach

We use the linked cluster expansion methods of Nickel to derive strong couping series for Z_N abelian gauge theories. These new results together with corresponding estimates using the exact linked cluster expansion algorithm are analysed and compared with previously obtained results for U(1) lattice gauge theory in 3 + 1 dimensions. We confirm the phase structure of these theories as found by other techniques. The critical value of N at which the phase structure of Z_N alters is estimated to be N_C = 4.5 ± 0.2. In each case the string tension estimates using the ELCE algorithm are found to be stable in the presence of a roughening transition.     

Dielectric-spectroscopic and a.c. conductivity studies on layered Na2-XKXTi3O7 (X=0.2, 0.3, 0.4) ceramics

The dependence of loss tangent (tanδ) and relative permittivity (ε_r) on temperature and frequency has been reported for Na_2-XK_XTi₃O₇ (with X=0.2, 0.3, 0.4) ceramics. The losses are characteristic of dipole mechanism and electrical conduction. The peaks of ε_r at high temperature indicate a possible ferroelectric phase transition for all three compositions. The results of a.c. conductivity studies on the same samples have also been reported. The corresponding ln(σT) versus 1000/T plots have been divided into five regions namely I, II, III, IV and V. The various conduction mechanisms in the different regions have been stressed. Furthermore, the log(σ) versus frequency plots for all the above samples reveal that the electronic hopping (polaron) conduction, which diminishes with the rise in temperature, is dominant in the lower temperature region. The interlayer ionic conduction seems to play a major role in conduction towards higher temperature.