W. Bogusz

Structural and electrical characterisation of BINIVOX

Electrical conductivity behaviour and unit cell parameter variation have been investigated in the double substituted BIMEVOX system Bi2V0.9CoyCu0.1-yO5.35, 0<y<0.1 by a.c. impedance spectroscopy and X-ray powder diffraction. Variation of the unit cell volume indicates a microdomain structure for the material with Co rich and Cu rich regions. Both low and high temperature conductivities increase with increasing Co content. However, the overall conductivities appear to be lower than the two end members, BICOVOX and BICUVOX, which suggests that some type of defect trapping may be present in these double substituted systems.

Electrical and structural study of BICOVOX

Abstract The relationship of structure and conductivity has been investigated in the Co substituted BIMEVOX system Bi 2 V 1 − x Co x O 5.5 − 3x 2 , 0.07 ≤ x ≤ 0.25 (BICOVOX). This system has been characterised using a.c. impedance spectroscopy, X-ray and neutron diffraction. The Arrhenius plots all show two straight line segments, one at low temperatures, below 450 °C, and one at high temperatures, above 550 °C. In the intermediate region, 3 types of behaviour are observed which appears to be composition dependent. All compositions showed differences in conductivity after the first heating cycle. However, this thermal hysteresis persisted in subsequent heating-cooling cycles only in compositions below x = 0.10. All BICOVOX samples in the composition range studied show essentially the tetragonal γ-phase structure, with evidence for a superstructure modulation at low temperatures from the neutron data.

Electrical conductivity dispersion in Co-doped NASICON samples

Impedance spectra of Co-doped NASICON polycrystalline samples (x = 2.0) were measured in the frequency range 0.5 MHz-1.2 GHz and analyzed in terms of the conductivity dispersion. The dispersion of bulk electrical conductivity obeys the power law frequency dependence σ(ω) = σ 0 + Kω n . It is observed from room temperature up to 450 K, i.e. up to the temperature of phase transformation of NASICON. The activation energy of the conductivity of grain interiors and the activation energy of the frequency, at which the dispersion sets on, have the same value of 0.313 eV. The activation energy value of the frequency dependent part of conductivity (0.077 eV) constitutes the proper fraction of the activation energy of the dc conductivity. Similar dispersion is found at lower frequencies for the grain-boundary conductivity. It extends in temperature up to 550 K, well over the temperature of phase transformation of NASICON. The phase transformation, taking place in the crystal grains of the material, does not influence the electrical behavior of grain boundaries. The activation energies for the grain boundary conductivity and for the grain-boundary onset frequency also have equal values (0.440 eV).

Electrical conductivity and structure correlation in BIZNVOX

The structure and electrical conductivity of the fast oxide ion conducting system BIZNVOX, Bi 2 Zn x V 1- O 5.5- (0.05≤x≤0.27), were investigated using X-ray powder diffraction and ac impedance spectroscopy. There is a compositional dependence of the conductivity behaviour as a function of temperature. This behaviour was correlated with the stabilisation of various polymorphs within the system. The x = 0.10 composition was found to have one of the highest low temperature conductivities (σ 300 = 4.09× 10 -3 S cm -1 ) for any BIMEVOX compound.

Relaxation dispersion of ionic conductivity of BICOVOX

Abstract The dispersion of complex dielectric permittivity of BICOVOX (Bi2Co0.1V0.9O5.35) has been investigated between 300 and 800 K in the frequency range from 106 to 109 Hz. The observed relaxation process is attributed to diffusional polarization associated with short range jumping of oxygen vacancies in the crystal lattice. Determination of the conductivity and the relaxation frequency enables estimation of the diffusion coefficient of oxygen vacancies as well as their concentration. A good agreement was found between the activation energy of ionic conductivity and the activation energy of relaxation frequency. It was concluded from the results that the estimated concentration of mobile oxygen vacancies does not change with temperature and is approximately equal to the concentration of oxygen vacancies determined by the refinement of the crystal structure for the high-temperature disordered γ′ phase of the compound. Raman scattering of the samples was also investigated. The maxima of scattering found at around 155 and 240 cm−1 could correspond to the attempt frequency of the mobile oxygen ions.

Structural and electrical consequences of high dopant levels in the BIMGVOX system

Abstract The influence of high Mg2+ dopant levels in BIMGVOX, Bi2MgxV1−xO5.5−3x/2 (0.05≤x≤0.40) on structure and conductivity has been investigated using X-ray powder diffraction and a.c. impedance spectroscopy. Four, compositionally dependent, structural ranges are observed at room temperature, with emergence of a new orthorhombic phase at high dopant levels. Generally the Arrhenius plots of conductivity show two linear regions the limits of which are compositionally dependent. The results have been correlated to the stability ranges of various polymorphs within the system.

Impedance study of BICUVOX ceramics

Abstract Impedance spectra of BICUVOX (Bi 2 Cu 0.1 V 0.9 O 5.35 with platinum or gold electrodes, measured over broad frequency and temperature range, were simulated by the impedance of an equivalent circuit composed of resistors, capacitors and constant phase elements. The assignment of the circuit elements to the bulk of electrolyteand to the electrode process was proved by their scaling with the geometric factor and by comparison with the ac four-probe measurement. Separation of contributions of grain interiors and grain boundaries to the resistance of BICUVOX was possible below 670 K. The Arrhenius plot of the total conductivity consisted of two straight sections, with a change of slope at about 740 K. The charge transfer reaction at the electrode was evident above 770 K.

Studies on preparation and electrical conductivity of BICOVOX

Abstract A member of the BIMEVOX family with vanadium partially substituted by cobalt (Bi 2 Co 0.1 V 0.9 O 5.35 -BICOVOX.1) was synthesized. The synthesis process was investigated by means of DTA. Dense ceramic samples were obtained after several minutes of sintering at 850°C, which is a higher temperature than that required for BICUVOX.1 (800°C). The Arrhenius plots of the total electrical conductivity for the obtained samples consist of two straight line sections with distinctly different values of activation energy. Between 400 and 500°C a rapid increase of conductivity with increasing temperature is observed. At temperatures above 500°C the conductivity of BICOVOX.1 is slightly higher than that of BICUVOX.1. Except for the first heating, the conductivity values during heating and cooling cycles are approximately the same.

Phase stability, structure and electrical conductivity in the system Bi2ZrxV1−xO5.5−(x/2)−δ

Abstract The BIMEVOX system, Bi 2 Zr x V 1− x O 5.5− x /2 , has been investigated using ac impedance spectroscopy and X-ray powder diffraction, in order to examine the effects of vacancy and dopant cation concentration on γ-phase stability and ionic conductivity. Four crystallographically distinct phases are observed at ambient temperature over the composition range 0.05≤ x ≤0.50. Below x =0.10, the orthorhombic α-phase is seen. Between x =0.10 and 0.16, the β-phase is stabilised. The stabilisation of orthorhombic α and β phases at lower compositions is typical for BIMEVOXes and results from an ordering of oxide ion vacancies. At x =0.19, the data were modelled on a tetragonal γ-phase cell, while at compositions where x ≥0.22 a mixture of the tetragonal γ-phase and Bi 8 V 2 O 17 is observed. The electrical conductivity is correlated with the stabilisation of the various polymorphs within the system. The observed γ-phase stabilisation region and the solid solution limit are discussed with respect to the defect structure.

Influence of preparation procedure on some physical properties of BICUVOX

Abstract The influence of preparation conditions on some physical properties of BICUVOX.10 (Bi 2 V 0.90 Cu 0.10 O 5.35 ) is reported. Differential thermal and X-ray analyses indicate that synthesis of the compound can be accomplished at 610 °C. The densification characteristics show that high density BICUVOX ceramic can be obtained after several minutes of sintering at 800 °C. The electrical conductivity was studied by a.c. impedance methods. Separation of the contributions of grain interiors and grain boundaries to the total resistance was possible at temperatures below around 350 °C. The Arrhenius plots of the total conductivity consist of two straight line sections with a change in slope at around 500 °C. The activation energy was about 0.48 eV at high temperatures for all the samples. Below around 500 °C the activation energy was in the range from 0.62 to 0.72 eV depending on the preparation procedure. Lower values of activation energy and higher values of conductivity were obtained for well-sintered samples, which is related to the smaller contribution of the grain boundaries to the total electrical resistance of a sample.