Nikolai P. Vyshatko

Oxygen Ionic and Electronic Transport in Apatite-Type Solid Electrolytes

The oxygen ionic conductivity of apatite-type La 9.83 Si 4.5 Al 1.5-y Fe y O 26±δ (y = 0-1.5), La 10-x Si 6-y Fe y O 26±δ (x = 0-0.77; y = 1-2), and La 7-x Sr 3 Si 6 O 26-δ (x = 0-1) increases with increasing oxygen content. The ion transference numbers, determined by faradaic efficiency measurements at 973-1223 K in air, are close to unity for La 9.83 Si 4.5 Al 1.5-y Fe v O 26+δ and La 10 Si 5 FeO 26.5 , and vary in the range 0.96-0.99 for other compositions. Doping of La 9.83 (Si, Al) 6 O 26 with iron results in an increasing Fe 4+ fraction, which was evaluated by Mossbauer spectroscopy and correlates with partial ionic and p-type electronic conductivities, whereas La-stoichiometric La 10 (Si, Fe)O 26+δ apatites stabilize the Fe 3+ state. Among the studied materials, the highest ionic and electronic transport is observed for La 10 Si 5 FeO 26.5 , where oxygen interstitials are close neighbors of Si-site cations. Data on transference numbers, total conductivity, and Seebeck coefficient as a function of the oxygen partial pressure confirm that the ionic conduction in Fe-substituted apatites remains dominant under solid oxide fuel cell operation conditions. However, reducing p (O 2 ) leads to a drastic decrease in the ionic transport, presumably due to a transition from the prevailing interstitial to a vacancy diffusion mechanism, which is similar to the effect of acceptor doping. Iron additions improve the sinterability of silicate ceramics, increase the n-type electronic conductivity at low p(O 2 ), and probably partly suppress the ionic conductivity drop. The thermal expansion coefficients of apatite solid electrolytes in air are (8.8-9.9) X 10 -6 K -1 at 300-1250 K.

Electron-hole transport in (La0.9Sr0.1)0.98Ga0.8Mg0.2O3-δ electrolyte: effects of ceramic microstructure

The oxygen ion transference numbers of a series of (La0.9Sr0.1)0.98Ga0.8Mg0.2O3−δ (LSGM) ceramics with different microstructures, prepared by sintering at 1673 K for 0.5–120 h, were determined at 973–1223 K by a modified Faradaic efficiency technique, taking electrode polarization into account. In air, the transference numbers vary in the range 0.984–0.998, decreasing when temperature or oxygen partial pressure increases. Longer sintering times lead to grain growth and to the dissolution of Sr-rich secondary phases and magnesium oxide, present in trace amounts at the grain boundaries, into the major perovskite phase. This is accompanied with a slight decrease of the total grain-interior resistivity and thermal expansion, while the boundary resistance evaluated from impedance spectroscopy data decreases 3–7 times. The electron-hole transport in LSGM ceramics was found to decrease when the sintering time increases from 0.5 to 40 h, probably indicating a considerable contribution of acceptor-enriched boundaries in the hole conduction. Due to reducing boundary area in single-phase materials, further sintering leads to higher p-type conductivity. The results show that, as for ionic conductivity, electronic transport in solid electrolytes significantly depends on ceramic microstructure.

Ionic and p-type electronic transport in zircon-type Ce1 − xAxVO4 ± δ(A = Ca, Sr)

Incorporation of alkaline-earth cations into the zircon-type lattice of Ce1 − xAxVO4 (A = Ca, Sr; x = 0–0.2) leads to higher p-type electronic conductivity, while the tetragonal unit cell volume and Seebeck coefficient decrease due to increasing concentration of electron holes localised on cerium cations. The oxygen ion transference numbers of Ce1 − xCaxVO4 in air, determined by faradaic efficiency measurements, vary in the range from 2 × 10−4 to 6 × 10−3 at 973–1223 K, increasing with temperature. The ionic conductivity is essentially independent of calcium content and decreases with reducing oxygen partial pressure. The activation energy for ionic transport in Ce(Ca)VO4 is 90–125 kJ mol−1. Doping with calcium enhances the stability of cerium orthovanadate at reduced oxygen pressures, shifting the phase decomposition limits down to oxygen activity values of 10−16–10−14 atm at 1023 K. The results on structure, Seebeck coefficient, and the partial p-type electronic and oxygen ionic conductivities suggest the presence of hyperstoichiometric oxygen in the Ce1 − xAxVO4 + δ lattice. The hyperstoichiometry, estimated from Seebeck coefficient data in the p(O2) range from 10−19 to 0.75 atm at 923–1223 K, may achieve 2–3% of the total oxygen content and weakly depends on the temperature and oxygen pressure variations within the zircon phase existence domain. Thermal expansion coefficients of Ce1 − xAxVO4 + δ ceramics in air, calculated from dilatometric data, are in the narrow range (5.6–5.9) × 10−6 K−1 at 400–800 K.

Fe4+ formation in brownmillerite CaAl0.5Fe0.5O2.5+δ

A Mossbauer spectroscopy study of brownmillerite Ca2FeAlO5+δ, in combination with the structure refinement from X-ray powder diffraction data, showed that hyperstoichiometric oxygen incorporation into the lattice is accompanied with Fe4+ formation in the perovskite-like layers, which are built of oxygen–metal octahedra and preferably occupied by iron cations. This behavior differs from that of another layered ferrite, Sr4Fe6O13+δ, where the point defects formed due to oxidation are primarily accumulated in non-perovskite layers. The incorporation of even minor amounts of extra oxygen in Ca2FeAlO5+δ leads to decreasing water absorption by the lattice, probably due to blocking of oxygen vacancies located in the tetrahedral layers of brownmillerite phase.

Structural characterization of mixed conducting perovskites La(Ga,M)O3−δ (M=Mn, Fe, Co, Ni)

Abstract Comparative analysis of the structure refinement results of perovskite-like LaGa0.5M0.5O3−δ (M=Mn, Fe, Co, Ni) and data on other LaGaO3-based phases, heavily doped with transition metal cations, shows that on doping the structural changes in these oxides follow common trends for the perovskite-type systems. The maximum ionic conductivity, observed in various perovskites when the tolerance factor values are approximately 0.96–0.97, was found to correlate with the transition from orthorhombic to rhombohedral structure and maximum lattice distortion. The perovskite unit cell distortion near the orthorhombic–rhombohedral phase boundary may hence play a positive role in the ionic transport processes.

Fiber-optic based method for the measurements of electric-field induced displacements in ferroelectric materials

A novel technique for the measurements of electric field-induced displacements in ferroelectric materials is presented. The method relies on a high sensitivity of the fiber-optic probe (Fotonic Sensor™, MTI Inc.) that measures the displacement of a specially designed cantilever beam having both electrical and mechanical contact with deforming sample. In this way, the major disadvantages of the standard Fotonic Sensor technique can be avoided. The method provides relatively high sensitivity (down to ∼4A), high stability (7% over 8h), and sufficiently broad frequency range. The capabilities of the proposed measurement setup are validated by the strain measurements in bulk Pb(Zr,Ti)O3(PZT) ceramics and thin films.

Temperature impedance spectroscopy of (1 − x)Na1/2Bi1/2TiO3-xLaMg1/2Ti1/2O3 solid solutions

The dielectric properties of (1 − x)Na1/2Bi1/2TiO3-xLaMg1/2Ti1/2O3 solid solutions (0 ≤ x ≤ 0.4) were studied over the temperature range 650–1030 K by measuring the impedance spectra over the frequency range 25–106 Hz. The Curie temperature TC was determined as a function of the composition of the solid solutions. It is shown that TC decreases linearly with increasing x. The temperature dependences of the dc component (σdc) and the polarization component (σ′ac) of the real part of the conductivity are found. The activation energy for the conductivity σdc is shown to increase abruptly as the temperature increases above approximately 770 K for all solid solution compositions. Over the temperature range corresponding approximately to the region of existence of the tetragonal phase of Na1/2Bi1/2TiO3, the polarization component σ′ac of the solid solutions, as well as that of the pure compound, is anomalously high at low frequencies, which can be due to structural inhomogeneities.

Processing and Characterization of (1-x)(Na1/2Bi1/2)TiO3 - xLa(Mg1/2Ti1/2)O3 Ceramics

Lead-free relaxor ceramics based on sodium-bismuth titanate, (1-x)(Na1/2Bi1/2)TiO3 - xLa(Mg1/2Ti1/2)O3 [(1-x)NBT-xLMT] (0≤x≤0.25), were prepared by both the conventional mixed oxide method and by the Pechini route, and their crystal structure and dielectric properties were investigated. All the compositions studied were found to possess a rhombohedrally distorted crystal lattice at room temperature. A distortion degree of the unit cell decreases and its volume increases with LMT content. The temperature of the dielectric permittivity maximum does not change with increasing substitution rate. As x is increased, the frequency-dependent dielectric peaks are flattened. The compositional evolution of structure and dielectric characteristics of the ceramics obtained are analysed and discussed in respect to size, charge and polarizability of the cations involved.

Powder X-ray diffraction study of LaCo0.5Ni0.5O3−δ and LaCo0.5Fe0.5O3−δ

The crystal structures of LaCo 0.5 Ni 0.5 O 3−δ and LaCo 0.5 Fe 0.5 O 3−δ solid solutions, studied by powder X-ray diffraction, were found to be rhombohedral perovskite. The unit cell parameters in the hexagonal setting are a =5.491(6) A and c =13.231(3) A for LaCo 0.5 Fe 0.5 O 3−δ , and a =5.464(4) A and c =13.125(3) A for LaCo 0.5 Ni 0.5 O 3−δ . The space group is R 3 c (No. 167).

Structure transformations and dielectric properties of PbY1/2Nb1/2O3 and PbHo1/2Nb1/2O3 compounds

Abstract The metastable pseudomonoclinic perovskite phases, PbY 1/2 Nb 1/2 O 3 and PbHo 1/2 Nb 1/2 O 3 were prepared under conditions of high pressures and temperatures. Investigations of structural parameters, thermal stability and some dielectric properties were carried out. The data were analyzed by comparison with corresponding data of other representatives of the PbB 1/2 3+ Nb 1/2 O 3 series. The dependence of unit cell parameters on the B 3+ rare earth cation radius for the PbB 1/2 3+ Nb 1/2 O 3 perovskites was correlated with a change of electronic structure of the rare earth elements, in particular, with the 4f-shell filling.