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Lanthanum orthoantimonate was synthesized using a solid-state synthesis method. To enhance the possible protonic conductivity, samples with the addition of 1 mol % Ca in La-site were also prepared. The structure was studied by the means of X-ray diffraction, which showed that both specimens were single phase. The materials crystallized in the space group P21 /n. Dilatometry revealed that the material expanded non-linearly with the temperature. The nature of this deviation is unknown; however, the calculated linear fraction thermal expansion coefficient was 9.56×10-6 K-1 . Electrical properties studies showed that the material is a proton conductor in oxidizing conditions, which was confirmed both by temperature studies in wet in dry air, but also by the H/D isotope exchange experiment. The conductivity was rather modest, peaking at the order of 10-6 S cm-1 at 800 °C, but this could be further improved by microstructure and doping optimization. This is the first time protonic conductivity in lanthanum orthoantimonates is reported.

The single phase lanthanum orthoniobate with tetragonal structure has been synthesized by the means of mechanosynthesis method. The studies have shown the crystal structure of La0.98Ca0.02NbO4 depends on the synthesis stage. The samples were predominantly in the tetragonal phase with a trace amount of the monoclinic phase. The SEM studies of morphology and microstructure have shown nanocrystallinity of the materials. The Raman studies showed the significant difference between the Raman spectra of nano- and microcrystalline La0.98Ca0.02NbO4. A decrease of Debye temperature, shown a change of thermal properties due to the presence of nano-sized particles in La0.98Ca0.02NbO4. Electrical characterization results suggest the protons are predominant charge carries in the material for the whole measured range of temperature and shows the total conductivity value of 4.0 × 10−5 S cm−1 at 600 °C in wet air. The calculated activation energies were equal to (0.81 ± 0.04) eV under dry and (0.73 ± 0.06) eV under humidified conditions. That indicates that nanostructuring was successfully stabilized the high-temperature tetragonal phase.

The results of ionic transport and structural measurements for the lanthanum orthoniobate doped by praseodymium are presented and discussed. The influence of calcium co-doping on these properties has also been analyzed. The results suggest the predominant protonic conductivity for the investigated system in the whole range of investigated temperatures. The influence of calcium co-doping on phase purity is analyzed.

A tetragonal polymorph of lanthanum orthoniobate can be stabilized to room temperature by the substitution of Nb with an isovalent element. LaNb1−xAsxO4 (0 < x ≤ 0.3), where As is an element stabilizing the tetragonal structure, were successfully synthesized using a combined co-precipitation and solid-state reaction method. The phase transition temperature, above which the material has a tetragonal structure, decreases linearly with increasing As content, and LaNb0.7As0.3O4 is tetragonal at room temperature. The analysis of the influence of different isovalent substituents, namely V, Sb and Ta, has shown that there is a relation between the properties of the chemical element and its effect on the structure. It was proposed that the electronegativity of the substituent determines the type of stabilization – the tetragonal/monoclinic structure is stabilized by chemical elements with electronegativity higher/lower than that of niobium. The slope of the phase transition temperature dependence on the substituent content has been proposed as a parameter determining the “quality” of the stabilization, since a steeper function leads to a larger decrease of transition temperature for the same content of different substituents. The analysis has shown that apart from the electronegativity, the stabilization of the tetragonal structure depends also on the ionic radius of a substituent. Arsenic has been found to be a better stabilizer of the tetragonal polymorph of lanthanum orthoniobate than Sb, but worse than V.

In this work, Fe-doped strontium titanate SrTi1-xFexO3-x/2-&delta, for x = 0&ndash, 1 (STFx), has been fabricated and studied. The structure and microstructure analysis showed that the Fe amount in SrTi1-xFexO3-x/2-&delta, has a great influence on the lattice parameter and microstructure, including the porosity and grain size. Oxygen nonstoichiometry studies performed by thermogravimetry at different atmospheres showed that the Fe-rich compositions (x &gt, 0.3) exhibit higher oxygen vacancies concentration of the order of magnitude 1022&ndash, 1023 cm&minus, 3. The proton uptake investigations have been done using thermogravimetry in wet conditions, and the results showed that the compositions with x &lt, 0.5 exhibit hydrogenation redox reactions. Proton concentration at 400 &deg, C depends on the Fe content and was estimated to be 1.0 &times, 10&minus, 2 mol/mol for SrTi0.9Fe0.1O2.95 and 1.8 &times, 5 mol/mol for SrTi0.5Fe0.5O2.75. Above 20 mol% of iron content, a significant drop of proton molar concentrations at 400 &deg, C was observed. This is related to the stronger overlapping of Fe and O orbitals after reaching the percolation level of approximately 30 mol% of the iron in SrTi1-xFexO3-x/2-&delta, The relation between the proton concentration and Fe dopant content has been discussed in relation to the B-site average electronegativity, oxygen nonstoichiometry, and electronic structure.

In this work, lanthanum orthoniobates doped with either antimony, calcium, or both have been synthesized and studied. The water uptake of the investigated materials has been analyzed by means of thermogravimetric studies. The results show the difference between the thermodynamics of hydration between the lanthanum orthoniobate system and other proton conducting ceramics. The relation between the water uptake and effective acceptor doping for the investigated system has been found, and the energetics of the water uptake relation are discussed.

Composite materials consisting of acceptor doped lanthanum orthoniobate electrolyte phase (La0.98Ca0.02NbO4) and Li2O:NiO:ZnO semiconducting phase were synthesized. The precursor powder of La0.98Ca0.02NbO4 was prepared in nanocrystalline (mechanosynthesis) and microcrystalline (solid-state synthesis) form. The composite can be applied in a single-layer fuel cell, because of the presence of two phases acting as an anode and a cathode simultaneously. X-ray diffraction data show that the materials consist of two expected phases. Scanning Electron Microscope images, with Energy Dispersive X-Ray analysis show that La0.98Ca0.02NbO4 as well as Li2O:NiO:ZnO are mixed together in the volume of the material. Open circuit voltage both for nano- and microcrystalline composite do not exceed 0.8 V. The single-layer fuel cell is degrading upon time and the voltage drop is observed. The processes of ZnO reduction and Zn diffusion and evaporation as responsible for cell degradation are discussed.

The results of electrical conductivity studies, structural measurements and thermogravimetric analysis of La1&minus, xTbxNbO4+&delta, (x = 0.00, 0.05, 0.1, 0.15, 0.2, 0.3) are presented and discussed. The phase transition temperatures, measured by high-temperature x-ray diffraction, were 480 &deg, C, 500 &deg, C, and 530 &deg, C for La0.9Tb0.1NbO4+&delta, La0.8Tb0.2NbO4+&delta, and La0.7Tb0.3NbO4+&delta, respectively. The impedance spectroscopy results suggest mixed conductivity of oxygen ions and electron holes in dry conditions and protons in wet. The water uptake has been analyzed by the means of thermogravimetry revealing a small mass increase in the order of 0.002% upon hydration, which is similar to the one achieved for undoped lanthanum orthoniobate.