Mn2+doping-induced-effects on commensuration and incommensuration of potassium zinc chloride crystal

Undoped and Mn²⁺-doped with different concentrations of potassium zinc chloride (KZC) crystals were grown from aqueous solutions by slow evaporation. The dielectric constant (ε), dielectric losses (tan?δ) and ac conductivity (σ_ac) of the crystals in the ferroelectric-commensurate, incommensurate and normal phases have been measured as a function of frequency, in the range 1–100?kHz, and temperature, in the range 300–580?K. Virgin samples were subjected to measurements of the frequency dependence at selected temperatures and measurements of the temperature dependence was then followed using the same samples. The increase of ε with T could be due to a combination of conductivity, structural variations and discommensuration (DC) formation and pinning as well. The increase of tan?δ with temperature was attributed to relaxation loss in addition to conduction loss, which increases more rapidly with temperature. The ac conductivity (σ_ac) and tan?δ along the polar axis of KZC increased significantly with increasing Mn²⁺ content while ε decreased. σ_ac changed with frequency according to a power law of the form σ_ac?=?f? ^s where 0.15<s<1.27. A linear decrease of ε and tan?δ with increasing the frequency was also found. The obtained results were treated by considering the effect of Mn²⁺-doping on stripples nucleation, DC evolution/annihilation, DC-lattice formation and DCs pinning by the crystal lattice and/or structural defects for virgin and thermally treated samples.     

Coexistence of the relaxor-like and ferroelectric behavior in K1-xLixTaO3

ABSTRACT

The results of low-temperature linear and nonlinear susceptibilities, polarization measurements and the dc electric field dependence of the dielectric properties of the lithium-doped potassium tantalate K_1-xLi_xTaO₃, x = 0.034 (KLT-3.4%Li) solid solution are presented. The coexistence of the relaxor-like and ferroelectric behavior and different mechanisms leading to either of them are discussed. The observed ferroelectric phase transition is of the first-order type with temperature hysteresis. This transition is due to the off-center motions of Ta ions in the octahedral environment of oxygen ions. Clusters of Li⁺ ions produce a relaxor-like behavior and random electric field. This field reduces the depolarization field and allows off-center motions of Ta ions and an appearance of spontaneous polarization.     

AC conductivity and dielectric behavior of bulk Furfurylidenemalononitrile

AC conductivity and dielectric behavior for bulk Furfurylidenemalononitrile have been studied over a temperature range (293–333 K) and frequency range (50–5×10⁶ Hz). The frequency dependence of ac conductivity, σ_ac, has been investigated by the universal power law, σ_ac(ω)=Aω^s. The variation of the frequency exponent (s) with temperature was analyzed in terms of different conduction mechanisms, and it was found that the correlated barrier hopping (CBH) model is the predominant conduction mechanism. The temperature dependence of σ_ac(ω) showed a linear increase with the increase in temperature at different frequencies. The ac activation energy was determined at different frequencies. Dielectric data were analyzed using complex permittivity and complex electric modulus for bulk Furfurylidenemalononitrile at various temperatures.     

A.c. conductivity and relaxation in LiCoVO4 ceramics

The LiCoVO₄ compound is synthesized by solution-based chemical method. X-ray diffraction analysis exhibits a single phase nature of the compound with cubic structure. The dielectric constant (ε_r), tangent loss (tanδ) and a.c. conductivity (σ_ac) have been studied as a function of frequency and temperature using complex impedance spectroscopy (CIS) technique. The variation of (ε_r and tanδ) with frequency at studied temperatures shows a dispersive behavior at low frequencies. Frequency dependence of σ_ac at different temperatures obeys Jonscher’s universal power law governed by the relation: σ_ac = σ_dc + Aωⁿ, where n is the frequency exponent in the range of 0 ⩽ n ⩽ 1 and A is a constant that depends upon temperature.     

Experimental and theoretical studies on dynamic properties of Li ions in LixMn2O4

In order to study the dynamic properties of Li_xMn₂O₄, potential relaxation techniques (PRT) is used to measure the chemical diffusion coefficient of Li_xMn₂O₄. Results are presented for x ranges from x=0.1 to 0.9. They show that the chemical diffusion coefficient at the two-phase coexistent stage near x=0.3 and 0.7 is higher than at the single-phase stage during the insertion and extraction process. Monte Carlo (MC) simulations are also used to simulate the ionic conductivity σ of Li ions in Li_xMn₂O₄ and its dependence as a function of lithium concentration x. The results show an M shaped curve in the plot of ionic conductivity σ versus x when the simulation temperature is 293 K, which confirms the experimental PRT results. The voltage profiles of Li_xMn₂O₄/Li cells were also simulated with different boundary conditions.     

Impact of MoO3 concentration,frequency and temperature on the dielectric properties of zinc phosphate glasses

Phosphate glasses with the chemical composition of 47P₂O₅–24ZnO-(29-x)Na₂O-xMoO₃, x = 0, 2, 4, 6, 8 and 10, have been prepared using the melt quenching technique. Dielectric properties of these phosphate glasses are carried out in the frequency range from 1 to 100 kHz at different temperatures. Dielectric parameters such as dielectric constant ε′, dielectric loss ε′′ and ac conductivity of the investigated glasses have been evaluated. The dependences of these dielectric parameters on frequency, composition and temperature have been discussed. It is found that dielectric constant decreases with increasing frequency due to the reduction of space-charge polarization and dipole polarization. The dependence of ac conductivity on the MoO₃ content indicates a competition between electronic and ionic conduction. The temperature dependence of the dielectric parameters reveals a rising trend of the dielectric parameters with temperature. This rising trend is indicated due to the increase of the amplitude of the thermal vibration of the charge carriers which facilitates the electron hopping and drifting of the mobile ions. The linear trend of the ln(σ_ac)-1000/T plot indicates that ac conductivity of the investigated glasses is thermally-activated transport process and follows the Arrhenius equation. The activation energy and its composition dependence have been reported.     

Role of manganese ions on the stability of ZnF2-P2O5-TeO2 glass system by the study of dielectric dispersion and some other physical properties

ZnF₂-P₂O₅-TeO₂ glasses containing different concentrations of MnO (ranging from 0 to 0.6%) were prepared. A number of studies, viz. differential thermal analysis, optical absorption, thermo luminescence, infrared spectra, magnetic susceptibility, elastic properties (Young's modulus Y, shear modulus n and micro hardness H) and dielectric properties (constant ε, loss tan δ, a.c. conductivity σ_ac over a range of frequency and temperature and dielectric breakdown strength), of these glasses were carried out as a function of manganese ion concentration. The analysis of the results indicate manganese ions mostly exist in Mn²⁺ state in these glasses when the concentration of MnO≤0.4% and above this concentration manganese ions predominantly exist in Mn³⁺ state; from this analysis an attempt is made to identify the role of these ions on the stability of glass network.     

Effect of magnesium substitution on dielectric and magnetic properties of Ni-Zn ferrite

The dielectric and magnetic properties of Mg incorporated Ni-Zn spinel ferrites have been investigated. Ni_0.5−xZn_0.5Mg_xFe₂O₄ ferrites have been prepared by sol-gel auto-combustion technique. The as prepared ferrites were annealed at 673, 873 and 1073 K. The X-ray diffraction studies reveal the spinel structure of annealed ferrites. The TEM results are in agreement with XRD results. FTIR study has also been carried out to get insight into the structure of these ferrites. The dielectric measurements show that the dielectric constant (ε′), dielectric loss (tan δ) and conductivity (σ_ac) increase on incorporation of Mg in the Ni-Zn ferrite. ε′, tan δ and σ_ac also show dependence on temperature, frequency of external applied electric field and microstructure of the samples. The magnetic moment measurements reveal that the saturation magnetization (M_s) increases and coercivity (H_c) decreases with the increase in concentration of Mg²⁺ ions. M_s and H_c also show dependence on the annealing temperature.     

Structural and electrical properties of Cu doped NiFe2O4 nanoparticles prepared through modified citrate gel method

Nanoparticles of polycrystalline NiFe_2−xCu_xO₄ (0.0≤x≤0.05) ferrites were prepared through the modified citrate-gel method. The samples were obtained as dried gel after the successful chemical reaction of their respective metal nitrate solutions in the midst of citric acid as catalyst. X-ray diffraction (XRD) and selective area electron diffraction (SAED) confirmed the single phase nature of all the samples with an average particle size of 19.8 (±1). Fourier transformation infrared spectroscopy (FTIR) shows the presence of two broad vibrational bands between 400 and 1000 cm⁻¹ corresponding to the tetrahedral and the octahedral sites. The variation of dielectric properties (ε′, ε″, tan δ) and ac conductivity (σ_ac), with frequency reveals that the dispersion is due to the Maxwell–Wagner type of interfacial polarization in general and due to hopping of charges between Fe⁺² and Fe⁺³ as well as between Ni⁺² and Ni⁺³ ions at B-sites. The complex impedance spectroscopy has been used to study the effect of grain and grain boundary on the electrical properties of all the ferrite nanoparticles.     

Influence of iron ions on dielectric properties of the PbO–Bi2O3–B2O3 glass system

PbO–Bi₂O₃–B₂O₃ glasses containing small concentrations of Fe₂O₃ (0–1 mol%) were subject to dielectric studies (dielectric constant ε′; loss tan δ; and ac conductivity σ _ac) over a wide range of frequency and temperature. From spectroscopic (infrared, optical absorption and ESR spectra) and magnetic susceptibility studies, variations in these properties with dopant ion concentration were analyzed in terms of different oxidation states and iron ion environment in the glass network.     

Electrical conduction and dielectric properties of vanadium phosphate glasses doped with lithium

AC conductivity and dielectric studies on vanadium phosphate glasses doped with lithium have been carried out in the frequency range 0.2-100 kHz and temperature range 290-493 K. The frequency dependence of the conductivity at higher frequencies in glasses obeys a power relationship, σ_ac=Aω^s. The obtained values of the power s lie in the range 0.5≤s≤1 for both undoped and doped with low lithium content which confirms the electron hopping between V⁴⁺ and V⁵⁺ ions. For doped glasses with high lithium content, the values of s≤0.5 which confirm the domination of ionic conductivity. The study of frequency dependence of both dielectric constant and dielectric loss showed a decrease with increasing frequency while they increase with increasing temperature. The results have been explained on the basis of frequency assistance of electron hopping besides the ionic polarization of the glasses. The bulk conductivity increases with increasing temperature whereas decreases with increasing lithium content which means a reduction of the V⁵⁺.     

Partial reduction of graphene oxide upon intercalation into exfoliated manganese thiophosphate

Abstract

We have prepared novel nanohybrid Li_2xMn_1?xPS₃/GO films consisting of graphene oxide (GO) and exfoliated MnPS₃ nanosheets via an easy mixing protocol at room temperature. These films were investigated by SEM and AFM, as well as XRD, Raman and XPS spectroscopies, obtaining information about the nature of the interaction between MnPS₃ and GO. In particular, the dark colour of the nanocomposite film and, more to the point, the analysis of (i) the C1s core level XPS spectra, (ii) the interlayer spacing obtained by XRD measurements and (iii) the increment in the intensity ratio I_D/I_G of the two bands typical of the carbon region of GO Raman spectra, suggest that a partial reduction of GO can be obtained right upon intercalation into Li_2xMn_1?xPS₃.     

Influence of the composition of TlGa1 − x Er x Se2 crystals on their dielectric characteristics and parameters of localized states

Frequency dependences of the real (?′) and imaginary (?″) parts of the complex permittivity, the dielectric loss tangent (tanδ), and the ac conductivity (σ_ac) in frequency range f = 5 × 10⁴?3.5 × 10⁷ Hz have been investigated for TlGa_{1 ? x} Er _x Se₂ crystals of various compositions. It has been established that the relaxation dispersion of ?′ and ?″ takes place for the studied crystals. The influence of the erbium content in the crystals on their dielectric coefficients has been studied. The ac conductivity of the TlGa_{1 ? x} Er _x Se₂ single crystals in the high-frequency range obeys the law σ_ac ～ f ^0.8, which is characteristic of the hopping mechanism of charge transfer over the states localized in the vicinity of the Fermi level. Parameters of the states localized in the band gap of TlGa_{1 ? x} Er _x Se₂ and the influence of the composition of the crystals on these parameters have been evaluated.     

Dielectric and AC conductivity behavior of BaBi2Nb2O9 ceramics

The complex dielectric and AC conductivity response of BaBi₂Nb₂O₉ relaxor ferroelectric ceramics were studied as a function of frequency (100 Hz-10 MHz) at various temperatures. The observed dielectric behavior was characterized by two types of relaxation processes which were described by the ‘universal relaxation law’. The frequency dependence of conductivity which showed a classical relaxor behavior followed the Jonscher's universal law σ(ω)=σ₀+Aωⁿ. The exponent n exhibited a minimum in the vicinity of temperatures of dielectric anomaly while the pre-factor A showed a maximum. The temperature dependence of n followed the Vogel-Fulcher relation with activation energy of about 0.14 eV.     

Study of dielectric and impedance properties of Mn ferrites

The paper reports on the effect of Al substitution on the structural and electrical properties of bulk ferrite series of basic composition MnFe_2−2xAl_2xO₄ (0.0≤x≤0.5) synthesized using solid state reaction method. XRD analysis confirms that all the samples exhibit single phase cubic spinel structure excluding presence of any secondary phase. The dielectric constant shows a normal behaviour with frequency, whereas the loss tangent exhibits an anomalous behaviour with frequency for all compositions. Variation of dielectric properties and ac conductivity with frequency reveals that the dispersion is due to Maxwell-Wagner type of interfacial polarization in general and hopping of charge between Fe⁺² and Fe⁺³ as well as between Mn⁺² and Mn⁺³ ions at octahedral sites. The complex impedance plane spectra shows the presence of two semicircles up to x=0.2, and only one semicircle for the higher values of x. The analysis of the data shows that the resistive and capacitive properties of the Mn ferrite are mainly due to processes associated with grain and grain boundaries.     

Dielectric spectroscopy characteristics of ferroelectric Pb0.77K0.26Li0.2Ti0.25Nb1.8O6 ceramics

Temperature and frequency dependence of dielectric constant and conductivity properties of Pb_0.77K_0.26Li_0.2Ti_0.25Nb_1.8O₆ (PKLTN) ceramics are modelled through the universal dielectric response (UDR). Partial substitution of Ti⁴⁺ for Nb⁵⁺ was compensated by charge and the creation of oxygen vacancies according to the Kroger-Vink notation. The electrons released by this reaction are captured by Nb⁵⁺ and Ti⁴⁺ to generate Nb³⁺ and/or Ti³⁺. The hopping of electrons between Nb⁵⁺–Nb³⁺ and Ti⁴⁺–Ti³⁺ are believed to participate in conductivity. Characterization of the dielectric constant has been performed from room temperature to 590°C in the frequency range from 45 Hz to 5 MHz. The measured dielectric constant obeyed Jonscher's dielectric dispersion relations: ε ^l = ε _∞ + sin(n(T)π/2)(a(T)/ε ₀)(ω^{n ( T )?1}) and ε ^ll = σ/iε ₀ ω + cos(n(T)π/2)(a(T)/ε ₀)(ω^{n ( T )?1}). Cole-Cole plots inclined at an angle (1 ? n(T))π/2 and followed the trend of universal material behavior ε _∞ + A(T)(ω^{n ( T )?1}). The exponent n(T) and coefficient A(T) = (a(T)S/L) exhibited a minimum and maximum at T _c = 425°C, respectively. The conductivity studies show the contribution of the hopping of bound charge carriers to conduction in PKLTN.     

Enhanced luminescence from spontaneously ordered Gd2O3:Eu based nanostructures

Nanostructured Gd₂O₃:Eu³⁺ and Li⁺ doped Gd₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation technique. The effects of annealing and Li⁺ doping on the structural, morphological, optical and luminescent properties are discussed. X-ray diffraction and Micro-Raman investigations indicate a phase transformation from amorphous to nanocrystalline phase and an early crystallization was observed in Li⁺ doped Gd₂O₃:Eu³⁺ thin films on annealing. AFM images of Li⁺ doped Gd₂O₃:Eu³⁺ films annealed at different temperatures especially at 973 K show a spontaneous ordering of the nanocrystals distributed uniformly all over the surface, with a hillocks (or tips) like self-assembly of nanoparticles driven by thermodynamic and kinetic considerations. Enhanced photoemission from locations corresponding to the tips suggest their use in high resolution display devices. An investigation on the photoluminescence of Gd_2−xEu_xO₃ (x=0.10) and Gd_2−x−yEu_xLi_yO₃ (x=0.10, y=0.08) thin films annealed at 973 K reveals that the enhancement in luminescence intensity of about 3.04 times on Li⁺ doping is solely due to the increase in oxygen vacancies and the flux effect of Li⁺ ions. The observed decrease in the values of asymmetric ratio from the luminescence spectra of Li⁺ doped Gd₂O₃:Eu³⁺ films at high temperature region is discussed in terms of increased EuO bond length as a result of Li⁺ doping.     

Structure, electric and dielectric studies of indium-substituted magnesium copper manganese ferrites

The structure, electric and dielectric properties of In-substituted Mg-Cu-Mn ferrites having the general formula of Mg_0.9Cu_0.1Mn_0.1In_xFe_1.9−xO₄ with 0.0≤x≤0.4 have been studied. X-ray diffraction (XRD) patterns of the samples indicated the formation of single-phase cubic spinel structure up to 0.2 and mixed phase (cubic and tetragonal phase) for samples x≥0.3. The relation of conductivity with temperature revealed a semiconductor to semimetal behavior as In⁺³ concentration increases. Variation in the universal exponent s with temperature indicates the presence of two hopping conduction mechanisms: the correlated barrier hopping (CHB) at low In⁺³ content x≤0.1 and small-polaron (SP) hopping at In⁺³ content x≥0.2. The variation in dielectric permittivity (ε′, ε″) with temperature at different frequencies shows a normal behavior for the studied compounds, while the variation in dielectric loss tangent with frequency at different temperatures shows abnormal behavior with more than relaxation peak. The conduction mechanism used in the present study has been discussed in the light of electron exchange between Fe³⁺ and Fe²⁺ ions and hole hopping between Mn²⁺ and Mn³⁺ ions at the octahedral B-sites.     

Structural and electrochemical effects of cobalt doping on lithium manganese oxide spinel

Pure LiMn₂O₄ and lithium manganese oxide spinels with partial replacement of manganese by cobalt up to 20 mole%, LiCo_xMn_2−xO₄, were prepared. The effect of extended cycling on the crystal structure was investigated. A capacity decrease with increasing cobalt content was observed in the potential range about 4100 mV vs. Li/Li⁺. Cycling behavior is significantly improved, compared to LiMn₂O₄. LiCo_xMn_2−xO₄ is discharged in a single phase reaction in the upper potential range around 4100 mV vs. Li/Li⁺, whereas pure LiMn₂O₄ shows a two phase behavior. LiMn₂O₄ shows a significant broadening of peaks in plots of differential capacity and change in shape of the voltage profile upon extended cycling. LiCo_xMn_2−xO₄ shows neither broadening nor change. Voltage profiles and plots of the differential capacity differ significantly compared to spinels with lithium substitution, Li_1+xMn_2−xO₄. In contrast to Li_1+xMn_2-xO₄, LiCo_xMn_2-xO₄ is discharged in a two step process in the range of 0 ≤ × ≤ 0,5. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrical properties of novel Li4.08Zn0.04Si0.96O4 ceramic electrolyte at high temperatures

Novel Li_4.08Zn_0.04Si_0.96O₄ electrolyte was synthesized by citric acid-assisted sol–gel method. The compound was studied by X-ray diffraction and complex impedance spectroscopy in the frequency range from 10 Hz to 10 MHz and temperature range from 573 to 773 K. The conductivity–frequency spectra exhibited two regions of conductivity dispersion related to Li⁺ ion transport in the bulk and grain boundaries. The activation energy of the bulk conductivity was found to be equal to the activation energy of relaxation frequency in the bulk. This indicated that the increase in conductivity with temperature was due to the increase in ion mobility while the number of charge carrier concentration was found to be constant with selected temperature range. The observation was in agreement with the calculated charge carrier concentration and ion mobility derived from conductance spectra, σ _ac(ω)?=?σ _o ?+?Aω ^α .