AC electrical properties of the mixed crystal (NH4)3H(SO4)1.42(SeO4)0.58

The frequency dependence of the AC conductivity of (NH₄)₃H(SO₄)_1.42(SeO₄)_0.58 (NHSSe) has been presented in the temperature range (299-393 K). The conductivity data has been analysed in terms of two theoretical models: hopping over a potential barrier model and quantum-tunnelling model. Values of the exponent s, decrease from 1.08 to 0.91 with increasing temperature and the experimental data revel that the hopping model is the rate determining mechanism.     

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.     

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.     

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.     

H nuclear magnetic resonance study of the ferroelastic and superionic phase transitions of (NH4)4LiH3(SO4)4 single crystals

We investigated the temperature dependences of the line shape, spin-lattice relaxation time, T₁, and spin-spin relaxation time, T₂, of the ¹H nuclei in (NH₄)₄LiH₃(SO₄)₄ single crystals. On the basis of the data obtained, we were able to distinguish the “ammonium” and “hydrogen-bond” protons in the crystals. For both the ammonium and hydrogen-bond protons in (NH₄)₄LiH₃(SO₄)₄, the curves of T₁ and T₂ versus temperature changed significantly near the ferroelastic and superionic phase transitions at T_C (=232 K) and T_S (=405 K), respectively. In particular, near T_S, the ¹H signal due to the hydrogen-bond protons abruptly narrowed and the T₂ value for these protons abruptly increased, indicating that these protons play an important role in this superionic phase transition. The marked increase in the T₂ of the hydrogen-bond protons above T_S indicates that the breaking of O-H?O bonds and the formation of new H-bonds with HSO₄^- contribute significantly to the high-temperature conductivity of (NH₄)₄LiH₃(SO₄)₄ crystals.     

Structural and electrical properties of high-energy ball-milled NASICON type Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 ceramics

Nano-crystallites of Li_1.3Ti_1.7Al_0.3(PO₄)_2.9(VO₄)_0.1 NASICON type material are prepared by means of solid-state reaction of a stoichiometric mixture after milling it for 22 and 55 h. The milling reduces the average crystallite size of the ceramic to 80 and 60 nm, respectively. Mechanical milling changes structural parameters and the strain induced at the grain-boundaries plays a major role in improving electrical conductivity. An order of magnitude increase in electrical conductivity is observed in the material milled for 55 h compared to the unmilled material, which is also reflected in permittivity loss. Modulus and permittivity representations substantiate the constriction effect of grain-boundaries observed in the complex impedance representation.     

Local order and structural transitions in the molecule-based layer ferrimagnet (n-C4H9)4N FeFe(C2O4)3

Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (n-C₄H₉)₄N Fe^IIFe^III(C₂O₄)₃ by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Néel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction.     

Hopping type of conduction in (Na0.5Bi0.5)ZrO3 ceramic

Polycrystalline sample with (Na_0.5Bi_0.5)ZrO₃ (NBZ) stoichiometry was prepared using a high-temperature solid-state reaction technique. X-ray diffraction (XRD) analyses indicate the formation of a single-phase perovskite-type orthorhombic structure. AC impedance plot is used as tool to analyse the electrical behaviour of the sample as a function of temperature at different frequency. The AC impedance studies revealed the presence of grain boundary effect and evidence of a negative temperature coefficient of resistance (NTCR) character. Pseudo Cole-Cole and complex electric modulus analyses indicated non-Debye-type dielectric relaxation. The AC conductivity obeys the universal power law. The pair approximation type correlated barrier hopping (CBH) model explains the universal behaviour of the s exponent. The apparent activation energy to the conduction process and minimum hopping distance are discussed.     

Phase transition and analysis of AC conductivity data of the (Cs)0.26(Rb)0.74H(SO4)0.89(SeO4)0.11 compound

The X-ray diffraction, vibrational and impedance spectroscopy studies of (Cs)_0.26(Rb)_0.74H(SO₄)_0.89(SeO₄)_0.11 (CsRbHSSe) new solid solution are presented. The title compound undergo a superionic phase transition (SPT) at This transition was confirmed by an abrupt increase of conductivity. The bulk impedance parameters of CsRbHSSe, RbH(SO₄)_0.81(SeO₄)_0.19 (RbHSSe) and CsH(SO₄)_0.76(SeO₄)_0.24 (CsHSSe) were determined from an analysis of AC conductivity data measured in a wide temperature range. The charge carriers concentration in the samples investigated has been evaluated using the Almond-West formalism and shown to be independent of temperature.     

Structural characterization,thermal, vibrational properties and molecular motions in perovskite-type diaminopropanetetrachlorocadmate NH3(CH2)3NH3CdCl4 crystal

The work presents a detailed analysis of the sequencing of the structural phase transitions in NH₃(CH₂)₃NH₃CdCl₄ crystal by differential scanning calorimetry (DSC), X-ray, infrared, far infrared and Raman spectroscopy. DSC studies have shown that in analyzed crystal occurring one reversible continuous phase transition at 375/374 K (on heating/cooling). Observed in Nujol and Fluorolube mulls in the wide temperature range between 296 K and 413 K spectral changes through the structural phase transition can be attributed to an onset of motion of cations. An assignment of some bands due to internal modes has been also proposed.     

Proton conduction in (NH4)3 H(SO4)2 single crystals

D.C. electrical conductivity, DTA and coulometric studies on (NH₄)₃ H(SO₄)₂ single crystals are made. Conductivity is markedly anisotropic with maximum along c¹ direction. A sudden jump in the conductivity plot along c¹ direction at 413 K is supported by a large endothermic peak in DTA, confirming the presence of transition at this temperature. The values of activation energy calculated from conductivity measurements indicated that the charge carriers are protons. This was further confirmed by coulometric experiment where the gas evolved was hydrogen, as established by a gas chromatograph and the volume of H₂ released agreed with that expected from electrolysis. The mechanism of protonic conduction in this crystal is discussed.     

High field ESR measurements of spin gap system MCu2(PO4)2

Submillimeter and millimeter wave ESR measurements of spin gap systems SrCu₂(PO₄)₂ and PbCu₂(PO₄)₂, which have four kinds of dimers, have been performed to investigate the magnetic properties of spin gap systems using the pulsed magnetic field up to 35T. The observed ESR spectra of powder sample SrCu₂(PO₄)₂ show sharp and single peak in the temperature range from 4.2 to 80 K. The anisotropy of the g-values turned out to be very small compared to the usual anisotropic powder spectra of copper compounds. The dynamical properties will be discussed from the temperature dependence measurements.     

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.     

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.     

Brillouin scattering study of phase transitions in LiK0.80(NH4)0.20SO4 mixed crystals

Brillouin spectroscopy was used to study the phase transitions of LiK_0.80(NH₄)_0.20SO₄ mixed crystals in the temperature range 10-300 K. The relevant elastic stiffness coefficients were evaluated at room temperature. The quasi-longitudinal γ₁₆ and the quasi-transverse γ₁₇ mode frequencies were measured in the above temperature range. From their frequency vs. temperature curve, three different phase transitions were determined. Two of the four phases presented by the crystal were found to be ferroelastic. The observed phases are tentatively assigned through a comparison with the phase transitions undergone by LiKSO₄ and LiK_0.96(NH₄)_0.04SO₄ crystals. An anomalous behavior of the Brillouin linewidth near the 260 K phase transition was observed.     

Optical studies of (NH4)2SO4 single crystal in the paraelectric phase

Transmittance and absorbance spectra of (NH₄)₂SO₄ single crystals along [010] direction were measured at different temperatures (296, 308, 318, 328 and 348 K) in the paraelectric phase. The absorption coefficient was computed and the analysis of the data revealed the existence of two optical transitions in (NH₄)₂SO₄ single crystals. The direct and indirect band gaps were shifted towards the longer wavelength with increasing temperature. The data on the allowed indirect transition was analyzed and interpreted in terms of two valence bands originated by spin orbit interaction and crystal field splitting. The momenta E_p were calculated as the difference between E_g1, the first valence to conduction band, and E_g2 for the second valence band at different temperatures. The results of extinction coefficient (k), the refractive index (n), and dielectric constants (ε) were also discussed and calculated as a function of wave length (λ). The heat treatment of the crystals proved that the variations of these optical parameters can be consequence of the internal microstructure changes caused by annealing.     

Metal-insulator transition in the spinel-type Cu(Ir1−xCrx)2S4 system

A normal thiospinel CuIr₂S₄ exhibits a temperature-induced metal-insulator (M-I) transition around 230 K with structural transformation, showing hysteresis on heating and cooling. On the other hand, CuCr₂S₄ has the same normal spinel structure without the structural transformation. CuCr₂S₄ has been found to be metallic and ferromagnetic with the Curie temperature T_c~377 K. In order to see the effect of substituting Cr for Ir on the M-I transition, we have carried out a systematic experimental study of electrical and magnetic properties of Cu(Ir_1−xCr_x)₂S₄. The M-I transition temperature shifts to lower temperature with increasing Cr-concentration x and this transition is not detected above x~0.05. The ferromagnetic transition temperature decreases as x is decreased and the transition does not occur below x~0.20.     

Conduction behaviour and thermoelectric power of Agx (As0.4Se0.6) 100−x chalcogenide system

Chalcogenide bulk alloys of Ag_x (As_0.4Se_0.6) _100−x (x=5, 7.5, 10, 12.5, 15 and 17.5) system were prepared by the conventional melt-quench technique. The d.c. electrical conductivity (σ) and thermoelectric power (TEP) measurements were carried out in the temperature range from 83 to 373 K and from 253 to 373 K, respectively. Variations of both σ and TEP with ambient temperature proved the p-type semiconducting behaviour of these materials. The current density-electric field characteristics were found to be linear. The activation energies, calculated from both the electrical conductivity E_σ and thermoelectric power E_s, were found to be dependent on composition.     

Electron paramagnetic resonance of gamma irradiated [(CH3)4N]InCl4 and [(CH3)4N]2CdCl4 single crystals

Gamma irradiated [(CH₃)₄N]InCl₄ and [(CH₃)₄N]₂CdCl₄ single crystals were investigated by electron paramagnetic resonance at ambient temperature, and it has been found that both compounds indicate the existence of (CH₃)₃N⁺ radicals. The g factors were found to be isotropic, and the hyperfine constant for H atoms was measured as 2.86 mT and is isotropic for this radical in these substances. The hyperfine coupling constant of the N nucleus with the hole in (CH₃)₃N⁺ in [(CH₃)₄N]InCl₄ was found to be anisotropic with the A_zz=2.92, A_yy=1.62 and A_xx=1.40 mT. From these, it has been revealed that the C_3v-axis of (CH₃)₃N⁺ radical performs rotational or jumping reorientational motions around a fixed axis, in addition to the rotations of protons in CH₃ groups and the rotational motions of CH₃ groups around the C_3v-axis of the radical. The g, and the hyperfine coupling factors of the N nucleus were isotropic in (CH₃)₃N⁺ in [(CH₃)₄N]₂CdCl₄. This indicates the motional behaviour of the radical in this compound is as in a liquid. This isotropic behaviour of the hyperfine coupling constants was found to be same until the attainable lowest temperature of 113 K in our laboratory.     

Influence of lithium phosphate concentration and temperature on the electrical conduction of (76V2O5-24P2O5)1−X (Li3PO4)X glasses

Characterization of the (76V₂O₅-24P₂O₅)_1−X (Li₃PO₅)_X, where X=0.0,0.01,0.02,0.10 and 0.15, glass has been done using X-ray diffraction and differential thermal analysis (DTA). The dc conductivity of the glass samples was studied over a temperature range from 300 to 593 K. The temperature dependence of dc conductivity shows two regions. One at relatively high temperature range, above θ_D/2, and the other at relatively low temperature range, below θ_D/2. The I-V characteristics of the glasses have been studied as a function of both temperature and Li₃PO₄ content. The I-V characteristics exhibits threshold switching with differential negative resistance. It's found that both the threshold voltage (V_th) and threshold current (I_th) are dependent on the temperature and lithium phosphate concentration.