Crystal structure,NMR study,dielectric relaxation and AC conductivity of a new compound [Cd3(SCN)2Br6(C2H9N2)2]n

The crystal structure, the ¹³C NMR spectroscopy and the complex impedance have been carried out on [Cd₃(SCN)₂Br₆(C₂H₉N₂)₂]_n. Crystal structure shows a 2D polymeric network built up of two crystallographically independent cadmium atoms with two different octahedral coordinations. This compound exhibits a phase transition at (T=355±2 K) which has been characterized by differential scanning calorimetry (DSC), X-rays powder diffraction, AC conductivity and dielectric measurements. Examination of ¹³C CP/MAS line shapes shows indirect spin–spin coupling (¹⁴N and ¹³C) with a dipolar coupling constant of 1339 Hz. The AC conductivity of this compound has been carried out in the temperature range 325–376 K and the frequency range from 10⁻² Hz to 10 MHz. The impedance data were well fitted to two equivalent electrical circuits. The results of the modulus study reveal the presence of two distinct relaxation processes. One, at low frequency side, is thermally activated due to the ionic conduction of the crystal and the other, at higher frequency side, gradually disappears when temperature reaches 355 K which is attributed to the localized dipoles in the crystal. Moreover, the temperature dependence of DC-conductivity in both phases follows the Arrhenius law and the frequency dependence of σ(ω,T) follows Jonscher's universal law. The near values of activation energies obtained from the conductivity data and impedance confirm that the transport is through the ion hopping mechanism.     

Galvanomagnetic properties of air-stable and highly conductive potassium-intercalated graphite sheet

Magnetoresistance and Hall coefficient of air-stable potassium-intercalated graphite sheets (hereafter abbreviated as K-PGS) were determined at room temperature. The magnitude of the magnetoresistance and the absolute value of Hall coefficient of K-PGS decreased with increasing potassium content of K-PGS, n_K/n_C. Two-carrier model was used for calculating carrier density and mobility. The electron density increased with increasing n_K/n_C: 3.07×10²⁰ cm⁻³ (n_K/n_C=0.005), 5.67×10²⁰ cm⁻³ (n_K/n_C=0.008) and 6.40×10²⁰ cm⁻³ (n_K/n_C=0.011). The value of the electron density of K-PGS with n_K/n_C=0.011 (nominal composition KC₉₁) was about 80% of the reported value, 7.8×10²⁰ cm⁻³, for KC₄₈ (n_K/n_C=0.021) prepared from HOPG (highly oriented pyrolytic graphite). The mobility decreased with increasing n_K/n_C: 2.11×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.005), 1.42×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.008) and 1.34×10³ cm² V⁻¹ s⁻¹ (n_K/n_C=0.011). The value of the mobility of K-PGS with n_K/n_C=0.011 was about 60% of the reported value (2300 cm² V⁻¹ s⁻¹) for KC₄₈ prepared from HOPG.     

Perpendicular anisotropy and magneto-optical Kerr effect of (Ni/Pd) multilayers

Nickel film, with total thickness t_Ni in the range 1000-2000 Å, is known to exhibit perpendicular magnetic anisotropy (PMA), if the film has been deposited at room temperature. This phenomenon is due to the magneto-elastic (ME) effect. The same is also true for the (Ni/Pd)_n multilayers, where n is the period (n≥3). In this paper, we have made two kinds of multilayers: one, which does not have a Pd cap layer, belongs to the A-group, and the other, which has, belongs to the B-group. The polar Kerr rotation θ_k, the polar Kerr ellipticity ε_k, and the figure of merit (θ_k)²R, where R is the reflectance, were measured for the two wavelengths, i.e. λ=633 and 442 nm, respectively. The effective PMA energy K_⊥ was measured from the vibrating sample magnetometer. It was found that the most favorable multilayer for the magneto-optical (MO) application exists among the A-group samples: i.e. the t_Ni=1300 Å, t_Pd=50 Å (seed layer), and n=1 sample. We obtained θ_k=−9.76 min, ε_k=−9.13 min, (θ_k)²R=1.51 (rad)² at λ=442 nm, and K_⊥=3.21×10⁶ erg/cc for this optimal multilayer. Finally, the effects of the Pd seed layer on PMA and MO are also studied.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).     

Low-temperature property investigation of the lead indium-niobate-lead nickel-niobate solid solution

The complex perovskite solid solution (1−x) Pb(In_1/2Nb_1/2)O₃-(x) Pb(Ni_1/3Nb_2/3)O₃ has been successfully prepared by the Columbite precursor method. The temperature dependencies of the dielectric constant and pyroelectric coefficient were measured between −261 and 200 °C. Relaxor ferroelectric behavior has been noticed in all compositions across the solid solution. The room-temperature electrostrictive coefficient, Q₃₃, was 1.83×10⁻² C²/m⁴ for x=0.10. No room-temperature piezoelectric activity was detected; however, upon cooling to −261 °C the maximum coupling coefficients k_p=29%, k_t=11%, and k₃₃=31% were observed for the composition x=1.00.     

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

A new compound, K₄(SO₄)(HSO₄)₂(H₃AsO₄) was synthesized from water solution of KHSO₄/K₃H(SO₄)₂/H₃AsO₄. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a=8.9076(2) Å, b=10.1258(2) Å, c=10.6785(3) Å; α=72.5250(14)°, β=66.3990(13)°, γ=65.5159(13)°, V=792.74(3) Å³, Z=2 and ρ_cal=2.466 g cm⁻³. The refinement of 3760 observed reflections (I>2σ(I)) leads to R₁=0.0394 and wR₂=0.0755. The structure is characterized by SO₄²⁻, HSO₄⁻ and H₃AsO₄ tetrahedra connected by hydrogen bridge to form two types of dimer (H(16)S(3)O₄⁻?S(1)O₄²⁻ and H(12)S(2)O₄⁻?H₃AsO₄). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)-H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H₃AsO₄ group to build the chain S(1)O₄?H₃AsO₄ which are parallel to the “a” direction. The potassium cations are coordinated by eight oxygen atoms with K-O distance ranging from 2.678(2) to 3.354(2) Å.Crystals of K₄(SO₄)(HSO₄)₂(H₃AsO₄) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Low-temperature thermoelectric properties of Bi85Sb15−xNbx alloys prepared by high-press sintering

The nanocrystalline materials with the general formula Bi₈₅Sb_15−xNb_x (x=0, 0.5, 1, 2, 3) were prepared by mechanical alloying and subsequent high-pressure sintering. Their transport properties involving electrical conductivity, Seebeck coefficient and thermal conductivity have been investigated in the temperature range of 80-300 K. The absolute value of Seebeck coefficient of Bi₈₅Sb₁₃Nb₂ reaches a maximum of 161 μV/K at 105 K, which is 69% larger than that of Bi₈₅Sb₁₅ at the same temperature. The power factor and figure-of-merit are 4.45×10⁻³ WK⁻²m⁻¹ at 220 K and 1.79×10⁻³ K⁻¹ at 196 K, respectively. These results suggest that thermoelectric properties of Bi₈₅Sb₁₅ based material can be improved by Nb doping.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4(TCNQ) showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH₄-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH₄-TCNQ. The enthalpy and entropy of transition are Δ_trsH=(667±7) J mol⁻¹ and Δ_trsS=(2.19±0.02) J K⁻¹ mol⁻¹, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ_trsS_mag=0.61 J K⁻¹ mol⁻¹ and the lattice contribution as Δ_trsS_lat=1.58 J K⁻¹ mol⁻¹. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K⁻¹ mol⁻¹), a majority of the magnetic entropy (∼4.6 J K⁻¹ mol⁻¹) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.     

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.     

Kondo behaviour of the solid solution (Ce1−xLax)PtGa

The solid solution (Ce_1−xLa_x)PtGa has been studied through X-ray diffraction, magnetization (σ(B)), magnetic susceptibility (χ(T)), electrical resistivity (ρ(T)), magnetoresistivity (MR) and heat capacity (C_P(T)) measurements. The Néel temperature (T_N=3.3 K) for CePtGa is lowered upon La substitution as observed from χ(T) and ρ(T) measurements. The Kondo temperature T_K as calculated from MR measurements is comparable to T_N and also decreases with La substitution. The volume dependence of T_K is in accordance with the compressible Kondo lattice model and a Doniach diagram of the results is presented. C_P(T) measurements are presented for CePtGa, Ce_0.2La_0.8PtGa and LaPtGa and the results are discussed in terms of the electronic and magnetic properties. Other features of interest are anomalies in ρ(T) and C_P(T) due to crystalline electric field effects and metamagnetism as observed in σ(B) studies for samples with 0≤x≤ 0.3.     

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.     

Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

Fluorophosphate glasses containing manganese

New fluorophosphate glasses based on MnF₂, NaPO₃ and MF_n (M=Zn²⁺, Sr²⁺, Mg²⁺, Ba²⁺, Li⁺, Na⁺ and K⁺) have been synthesised and characterized. Large vitreous areas were observed. Samples of 4 mm in thickness have been obtained. These glasses are easy to prepare and stable in ambient air. Depending on the composition and the nature of the M cation, glass transition temperature, T_g, lies between 230 and 314 °C, crystallisation temperature, T_x is between 320 and 475 °C. These glasses are pink coloured, and infrared transmission extends up to 4.5 μm with extrinsic OH⁻ absorption band at 3200 cm⁻¹ and other bands around 2200 and 1600 cm⁻¹ that relate to PO₄ tetrahedron vibration. Other physical properties including density, microhardness, Young modulus, thermal expansion and refractive index were investigated and correlated to composition.     

A novel high-k ‘Y5V’ barium titanate ceramics co-doped with lanthanum and cerium

Structural, dielectric, and ferroelectric properties of a novel high-k ‘Y5V’ (Ba_1−xLa_x)(Ti_1−x/4−yCe_y)O₃ ceramics (where x=0.03 and y=0.05, denoted by BL3TC5) with the highest ‘Y5V’ dielectric response (ε′>10 000) among rare-earth-doped BaTiO₃ ceramics to date are investigated in detail using SEM, TEM, XRD, DSC, EPR, Raman spectroscopy (RS), temperature and frequency, electric field dependences of dielectric permittivity (ε′), and temperature and electric field dependences of ferroelectric hysteresis loops. The BL3TC5 diffusion of ferroelectric phase transition occurs around dielectric peak temperatures (T_m) near a room temperature characteristic of dielectric thermal relaxation. Powder XRD data and defect complex model were given. “Relaxor” behavior associated with an order/disorder model and formation of a solid solution were discussed. The EPR results provided the evidence of Ti vacancies as compensating for lattice defects. High-k relaxor nature of BL3TC5 is characterized by an average cubic structure with long-range lattice disordering and local polar ordering; a slow change of the ε′ (T) and P_r(T) curves around T_m; no phase transition observed by DSC; and a broad, red-shifted A₁ (TO₂) Raman phonon mode at 251 cm⁻¹ accompanying the disappearance of the “silent” mode at 305 cm⁻¹ and a clear anti-resonance effect at 126 cm⁻¹ at room temperature.     

Impedance and dielectric studies of ferroelectric SrBi2Nb2O9 ceramics

This paper reports the dielectric and impedance characteristics of ferroelectric SrBi₂Nb₂O₉ (SBN) ceramics in the 100 Hz-1 MHz frequency range at various temperatures (300-823 K). A strong low frequency dielectric dispersion (LFDD) associated with an impedance relaxation has been found to exist in these ceramics in the temperature range 573-823 K. The Z″ of the AC complex impedance showed two distinct slopes in the frequency range 100 Hz-1 MHz suggesting the existence of two dispersion mechanisms. This non-ideal behavior has been explained on the basis of the expression, Z*=R₀/(1+(iω/ω₁)^m+(iω/ω₂)ⁿ) [J. Phys. Chem. Solids 53 (1992) 1] where ω₁ and ω₂ characterize the lattice response and the charge carrier behavior, respectively. The exponents m and n were obtained from the curve fitting. The exponent n was found to exhibit a minimum at the Curie temperature, T_c (723 K) whereas the m was temperature independent.     

Dielectric properties and relaxation behavior of [TMA]2Zn0.5Cu0.5Cl4 compound

The [TMA]₂Zn_0.5Cu_0.5Cl₄ hybrid material was prepared and its dielectric spectra were measured in the 10⁻¹ Hz-10⁶ Hz frequency range and 200-305 K temperature interval. The dielectric permittivity showed a ferroelectric-paraelectric phase transition at 293 K. Double relaxation peaks are observed in the imaginary part of the electrical modulus, suggesting the presence of grain and grain boundary in the sample. The frequency dependent conductivity was interpreted in term of Jonscher's law: σ(ω)=σ_dc+Aωⁿ. The temperature dependent of the dc conductivity (σ_dc) was well described by the Arrhenius equation: σ_dcT=σ_o×exp(−E_a/kT).