K. Hariharan

The superionic Agl-Ag2O-V2O5 system: electrical conductivity studies on glass and polycrystalline forms

Superionic conducting glasses in the Agl-Ag2O-V2O5 system were prepared by heating the appropriate amounts of raw materials at 723 K and quenching in liquid nitrogen. The polycrystalline materials were prepared by slowly cooling the melt to room temperature. X-ray diffraction was used for material characterization. The electrical conductivity of the pulverized samples, pressed together with an electrode mixture of silver and electrolyte (1:2 by weight) under 5000 kg cm−2 pressure to form pellets 10 mm in diameter and 2 to 3 mm in thickness, was measured in the temperature range 300 to 365 K at 1 kHz. The ionic conductivities of the glasses were always higher than those of their polycrystalline counterparts, while their activation energies were also slightly higher. Conductivity measurements on annealed glassy samples indicated that the conductivity decreases with the time of annealing, and reaches a constant value which is nearly the same as that of the polycrystalline sample. Electronic conductivities of both types of sample were obtained by using Wagners polarization cell technique, which showed that the electronic conductivity for both types was five orders of magnitude less than the total conductivity. Typical galvanic cells having the configuration Ag,electrolyte/electrolyte/C,electrolyte,I2 were constructed and the silver ion transport number was calculated by the e.m.f. method.

Silver phosphovanadte vitreous electrolytes

Abstract Glass formation and electrical conductivity studies on the AgI-Ag 2 O-[ x V 2 O 5 + (1- x )P 2 O 5 ], O ⩽ x ⩽ 1, have been carried out. Glass forming region is found to be within 40 and 66.7 m/o AgI. For similar mole fraction of AgI, quaternary glasses show higher conductivities than the individual ternary systems. Glass transition temperature is found to be almost independent of the V 2 O 5 /P 2 O 5 ratio.

Raman spectroscopic study of (Pb1−xBax)(Yb1∕2Ta1∕2)O3 ceramics

The ordered complex perovskite lead ytterbium tantalate undergoes a change in structural and dielectric properties as the concentration of Ba increases. The x-ray diffraction patterns show a decrease in the intensity of superlattice reflections due to Pb-antiparallel displacement as well as to B-site ordering. The structure approaches cubic symmetry with increase in Ba content. The dielectric response shows a gradual change from antiferroelectric to relaxor ferroelectric and then the relaxor behavior starts decreasing as Ba content increases. Raman-scattering study of the samples shows that some of the modes seen for Pb(Yb0.5Ta0.5)O3 disappear and an additional mode appears at 420cm−1 with increase in Ba concentration. A qualitative analysis of various modes suggests that the substitution of Ba for Pb leads to a decrease in the degree of ordering and occurrence of local distortions in the lattice as evidenced by the broadening of certain peaks in the Raman spectra.

Dielectric Study of the Phase Transitions in Pb 1 − x Ba x (Yb 1/2 Ta 1/2 )O 3 Ceramics

The structure and low frequency dielectric properties of Pb 1 − x Ba x (Yb 1/2 Ta 1/2 )O 3 solid solutions with x = 0.00, 0.10, 0.15, 0.20 and 0.30 have been investigated using X-ray diffraction and dielectric constant measurements. As Ba 2+ concentration increases beyond x = 0.15, the crystal structure changes from monoclinic to pseudocubic at room temperature. The sharp paraelectric-antiferroelectric phase occuring in pure Pb(Yb 1/2 Ta 1/2 )O 3 , remarkably decreases and simultaneously the weak ferroelectric phase gets enhanced with increase in x. The maximum value of the dielectric constant (K max ) varies with x, showing a peak value at x = 0.20 whereas dielectric maximum temperature (T max ) continuously decreases with increase in x. The diffuseness exponent and degree of diffuseness are calculated for x = 0.15, 0.20 and 0.30 by fitting the dielectric data to the modified Curie-Weiss law and the values indicate that the compounds exhibit diffuse phase transition. The paper also discusses the influence of calcination and sintering temperature on the dielectric properties of pure Pb(Yb 1/2 Ta 1/2 )O 3 .

Electronic and vibrational spectra of CrO2-4 in cesium halide lattices

Abstract The electronic spectra of CrO2−4 ions embedded in cesium halide lattices show two well defined bands due to dipole allowed transitions along with some weak bands due to forbidden transitions. All the above bands are interpreted in terms of transitions within molecu ar orbital levels using the Ballhausen and Liehr scheme. It is observed that superposed on the electronic transitions there is a vibrational fine structure which arises due to the coupling between the electronic transitions and the totally symmetric stretching frequency of the ion. IR absorption spectrum of Analar grade CsI :CrO2−4 shows seven lines, whereas Specpure CsI :CrO2−4 shows five lines. The above spectra are interpreted on the basis of different local symmetries of CrO2−4 ions in the lattice.

X-ray diffraction and differential scanning calorimetry of a novel CuI Ag oxysalt solid electrolyte

Abstract Solid electrolytes of the system : xCuI(1−x) (Ag 2 O + MoO 3 ) have been prepared for various values of x, the molar fraction. X-ray Diffraction data has revealed the presence of AgI and Differential Scanning Calorimetry has confirmed this by the presence of an endothermic peak characteristic of the superionic transition of AgI at 150°C. The transport number for silver ions was nearly unity, and preliminary conductivity results showed a value of 10 −3 (Ωcm) −1 at room temperature. The formation of AgI by an exchange reaction between CuI and the silver oxysalt facilitates superionic behaviour similar to AgIag Oxysalts.

Lithium iron(II) pyrophosphate as a cathode material: structure and transport studies

A single phase cathode material Li2FeP2O7 was synthesized by a solid state reaction using inexpensive precursors and optimization of the calcination conditions. Reitveld refinement of the XRD data reveals the value of Rwp and reduced-χ2 as 0.97% and 1.006, respectively. Impedance spectroscopy indicates an electronically dominated conduction mechanism with a conductivity of 3.4 × 10−9 S cm−1 at 303 K.

Gel growth and characterization of pure and vanadyl-doped strontium tartrate tetrahydrate single crystals

Pure and vanadyl-doped single crystals of SrC4H4O6·4H2O (SrTr·4H2O) were grown by controlled diffusion in silica gel. The crystals were characterized by EPR and electronic absorption techniques. The angular variation EPR spectra showed that the VO2+ ion has a fixed orientation in the lattice. Theg andA tensors and their direction cosines were evaluated from the EPR analysis. The above parameters for the observed sites were analysed assuming a spin Hamiltonian corresponding to a tetragonal symmetry. Electronic absorption studies carried out at 300 K as well as at 80 K showed three characteristic absorption bands at 12 500, 13 201 and 18 348 cm−1 which were assigned to crystal field (d-d) transitions and the site symmetry of the ion has been estimated to be tetragonal. To confirm the ascribed transitions, the band positions were calculated by solving the energy expressions corresponding to the tetragonal symmetry and compared with the observed band positions. The crystal field parameters (Dq) and tetragonal distortion parameters (Ds andDt) were obtained from the best fit of the observed and calculated band positions. The EPR and electronic spectral results showed that the VO2+ ion has occupied the interstitial site having C4V symmetry.

Structural aspects and ac conductivity of the solid electrolyte AgxCu2−xHgI4

Abstract Lattice parameters of the solid electrolyte Ag x Cu 2− x HgI 4 have been determined for the different values of x between 0 and 2. All compositions studied showed a tetragonal phase, except for a small range from x = 1.14 to x = 1.30. Compositions in this range had a cubic phase. The lattice parameters a and c of compositions outside this range increased steadily from 6.07 and 12.15A˚, respectively, for x = 0 (Cu 2 HgI 4 ) to 6.29 and 12.55A˚for x = 2 (Ag 2 HgI 4 ). Compositions in the region1.14 ≤ x ≦ 1.3 that had cubic phases were found to have lattice parameter a between 6.22 and 6.26A˚. The frequency-dependent conductivity of all compositions showed two distinct regions at low temperatures: a frequency-independent plateau region at lower frequencies (1 Hz to 10 kHz) and a power law variation,Aω n , at higher frequencies (10 to 65 kHz). At higher temperatures (>328 K) electrode polarization effects were seen at low frequencies and a frequency-independent plateau region at higher frequencies. The frequency-independent (i.e., dc) conductivity was on the order of 10 −5 (Ω cm −1 ) for compositions1.14 ≤ x ≤ 1.3.

Rapid Quenching Setup: Design, Development and Applications in Solid State Ionics

Quenching rate of the order of 105 K/s has been achieved using a home-built twin-roller rapid quenching setup which is two orders of magnitude higher as compared to the rate possible through conventional quenching methods. The glass forming limit for the two ternary glassy electrolytes Li2O–B2O3–P2O5 and Li2O–Nb2O5–P2O5 has been extended using the twin roller setup by incorporating mol% 66.7 and 50 of the glass modifier (Li2O) in the above glass systems. Further, the effect of mixed glass formers B2O3–P2O5 and Nb2O5–P2O5, investigated in the above extended glassy systems, reveal an initial increase in the glass transition temperature (Tg) and ionic conductivity (σdc), as one glass former replaces the other. The Tg and σdc show maximum values for the compositions mol% 66.7Li2O–33.3[0.6B2O3–0.4P2O5] and mol% 50Li2O–50[0.6Nb2O5–0.4 P2O5], indicating the “mixed glass former effect”. The composition mol% 66.7Li2O–33.3[0.6B2O3–0.4P2O5], showing good thermal stability as well as conductivity, is a potential electrolyte material for device applications.