J.E. Garbarczyk

Studies of silver–vanadate–phosphate glasses by Raman, EPR and impedance spectroscopy methods

Abstract Four different series of AgI–Ag 2 O–V 2 O 5 –P 2 O 5 glasses have been prepared, in a wide range of compositions defined by the ratio of r =[Ag 2 O]/[V 2 O 5 +P 2 O 5 ] and a parameter x (mol.% of AgI). DTA and Raman scattering experiments reveal that the glass-networks of the investigated glasses are determined mainly by r values, and the x (up to 40%) parameter plays a secondary role. EPR spectra show that the interactions between vanadium centers (which are essential for electronic hopping) strongly depend on crosslinking of the glass network. They dramatically weaken if the glass-network is broken by addition of a large amount of a glass-modifier (Ag 2 O). Depending on values of r and x the character of electric conduction of the studied glasses changes from purely electronic through mixed to predominantly ionic (via Ag + cations).

Studies of silver-vanadate superionic glasses using Raman spectroscopy

Abstract Series of superionic glasses of compositions: 50AgI·(50− x )Ag 2 O· x V 2 O 5 , 14≤ x ≤25, and y AgI·(80− y )Ag 2 O·20V 2 O 5 , 35≤ y ≤50 have been prepared. The Raman spectra in the wave number range 700–1000 cm −1 were recorded for all compositions. The observed spectra were interpreted in terms of various vibrational modes of the vanadate glass-network. A correlation between the spectral features and glass compositions (especially V 2 O 5 and Ag 2 O contents) was found. An influence of the laser beam on the Raman spectra of some compositions was observed.

EPR studies of mixed-conductive glasses in the AgI–Ag2O–V2O5–P2O5 system

Abstract Three families of mixed conductive silver-vanadate–phosphate glasses have been studied by electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra are sensitive to glass modifier (Ag 2 O) to network former (V 2 O 5 –P 2 O 5 ) ratio r . For r values close to 1, the hyperfine structure (hfs) of the EPR bands is observed. The presence of a hfs is an indicator of the glass network disruption. Therefore, its absence may indirectly point on enhanced electron hopping between aliovalent vanadium centers along V 4+ OV 5+ bonds. A method was proposed to extract hfs from apparently unresolved EPR spectra. Using a simulation procedure the parameters of a Hamiltonian responsible for interaction between spins of 3d 1 electron and 51 V nucleus, were determined and a local order around vanadium ions was proposed.

Transition from ionic to electronic conduction in silver–vanadate–phosphate glasses

A series of silver-vanadate-phosphate glasses of the AgI-Ag 2 O-V 2 O 5 -P 2 O 5 system has been prepared and their electrical properties have been studied. Compositions of high AgI content and low V 2 O 5 content exhibited pure ionic conduction. On the other hand, vanadium-rich and silver-poor glasses were almost pure electronic conductors, for which, depending on temperature, the models of phonon-assisted hopping of small polarons or variable range hopping can be applied. Samples of intermediate compositions exhibited mixed ionic-electronic conductivity. The observed transition from ionic- to electronic-type conduction in the frame of the same system could be of interest for possible practical applications, e.g. in integrated microbatteries.

Studies of Li2O–V2O5–P2O5 glasses by DSC, EPR and impedance spectroscopy

Abstract Glasses of the Li 2 O–V 2 O 5 –P 2 O 5 system of different extent of glass-network disruption, characterized by a parameter r =[Li 2 O]/[V 2 O 5 +P 2 O 5 ], were synthesized. They were subsequently characterized by differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR) and impedance spectroscopy. As in the case of glasses of the Ag 2 O–V 2 O 5 –P 2 O 5 system, studied earlier by us, one observes a transition from predominantly electronic to predominantly ionic conduction. EPR spectra of predominantly ionically conducting glasses under study exhibit hyperfine structure (hfs). In the case of electronically conducting glasses, the hyperfine structure of EPR lines is absent.

Electrical properties of AgI–Ag2O–V2O5–P2O5 glasses

Abstract Mixed conductive silver vanadate–phosphate glasses were prepared and their electrical properties were studied. Compositions of the glasses were characterized by parameters: x (AgI content) and r =[Ag 2 O]/[V 2 O 5 +P 2 O 5 ]. Total conductivity σ t ( x , r =constant) of a series of samples with fixed r values exhibited minima when plotted vs. x . The conductivity minima were accompanied by maxima of apparent activation energy E t ( x , r =constant). The observed dependencies were attributed to the ion–polaron effect (IPE), i.e. to electrostatic attraction between mobile species: electrons (polarons) and Ag + ions.

Some properties of mixed electronic-ionic glassy conductors of the AgIAg2OV2O5P2O5 system

Abstract The mixed electronic-ionic conducting glasses of the AgIAg2OV2O5P2O5 system have been prepared. XRD and DTA analyses have confirmed the amorphous nature of these glasses. DTA analysis was used to determine glass transition and recrystallization temperatures. EPR measurements showed that at the V2O5-rich end of the compositions, the number of V4+ ions, essential for electronic conduction, is considerable. Electrical properties have been studied using complex immittance spectroscopy. Numerical analysis of the admittance spectra showed that the ionic transference number at the AgI-rich end of the compositions is close to one and for the compositions with high V2O5 content it is considerably lower.

PREPARATION OF TRIPHYLITE-LIKE GLASSES AND NANOMATERIALS IN THE LiFePO4-V2O5 SYSTEM AND STUDY ON THEIR ELECTRICAL CONDUCTIVITY

Research on lithium iron phosphates is stimulated by their application as cathodes in Li-ion rechargeable batteries. The aim of this study was to enhance its initially poor electronic conductivity. A thermal nanocrystallization is applied to lithium-iron-phosphate and lithium-vanadium-iron-phosphates materials resulting in a significant increase of the electronic conductivity of the latter (almost 10-6 S/cm). The obtained nanomaterial exhibits very good thermal stability (up to 625°C), the activation energy 0.51 eV and moderate electronic conductivity at the room temperature, which is, however, a good starting point for further enhancement.

ac conductivity of Na-Ag β” -alumina polycrystalline samples

Abstract The results of ac conductivity measurements carried out on ceramic samples of Li 2 O and NiO-Li 2 O-doped β7rdquo;-alumina with Na + , Ag + and Na + -Ag + mobile ions are presented. The modifications of doping only slightly influenced either bulk or grain boundary conductivity in Na β” -alumina. The activation energies of conduction in Na β”-alumina equal to 0.28 eV at low temperatures decreased to 0.14–0.15 eV at temperatures above 200°C. The Arrhenius plots for Ag β” -alumina were found to be linear in whole 20–450°C temperature range with the activation energies of 0.19 eV (Li 2 O) and 0.24 eV (NiO-Li 2 O). The conductivity measurements done on partially exchanged samples revealed the presence of the mixed alkali effect.

Effect of selected mobile ions on moisture uptake by beta″ alumina

Abstract The effect of mobile Na+, Ag+ and Li+ ions on water uptake of lithia stabilized beta″ alumina ceramics was investigated. The extent of water uptake was deduced from ionic a.c. conductivity, TGA and DTA measurements done on “moist” samples. It was found that contact with humid air influences electrical properties of bulk and grain boundary regions of Na+ beta″ alumina. The most influenced was Li+ Na+ beta″ alumina, the least Ag+ beta″. The TGA/DTA data are in good agreement with the electrical results.