Vibrational Properties of LaNb0.8 M0.2 O4-δ (M=As, Sb, V, and Ta).

LaNb0.8 M0.2 O4-δ (where M=As, Sb, V, and Ta) oxides with pentavalent elements of different ionic sizes were synthesized by a solid-state reaction method. The vibrational properties of these oxides have been investigated. These studies revealed that the substituent element influences both Debye temperature value as well as the Raman active vibrational modes. Additionally, the low-temperature vibrational properties of LaNb0.8 Sb0.2 O4-δ have been determined to show the phase transition occurrence at 260 K which is lower than previously reported.

Proton conductivity in multi-component ABO4-type oxides.

This work investigates how configurational entropy in oxides could affect proton conductivity. For this purpose, three samples of different elemental compositions are synthesized. Five, six and seven elements were introduced into the A-site of ANbO4, forming La1/5 Nd1/5 Sm1/5Gd1/5 Eu1/5NbO4, La1/6Nd1/6Sm1/6Gd1/6Eu1/6Ho1/6NbO4 and La1/7Nd1/7Sm1/7Gd1/7Eu1/7Ho1/7Er1/7NbO4, respectively. The high configuration disorder changes the local environment, which can have a notable effect on many properties, including proton transport, which is the focus of this work. The conductivity was measured in different atmospheres; dry and wet and in a different temperature range (600-800 °C) to compare the proton transport as well as study the effect of temperature. A homogenous single-phase monoclinic fergusonite was obtained for the three samples. Proton conductivity, measured by means of comparing the conductivity in dry and wet atmospheres, was observed in all samples. La1/5 Nd1/5 Sm1/5Gd1/5 Eu1/5NbO4 exhibited the highest conductivity, about 3.0 × 10-6 S cm-1 at 800 °C in the wet atmosphere, while in the dry atmosphere it was about 2.2 × 10-6 S cm-1 at the same temperature, which implies a modest proton conductivity in this class of materials.

High-Temperature Proton Conduction in LaSbO4.

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.

Energetics of formation and stability in high pressure steam of barium lanthanide cobaltite double perovskites.

This study concerns energetics of formation and the stability in high water partial pressure of BaLnCo2O6-δ, (Ln = La, Pr, Nd, and Gd) (BLnC) and BaGd1-xLaxCo2O6-δ, where x = 0.2, 0.5, and 0.7 (BGLC) double perovskite cobaltites. Those materials are extensively studied due to their potential applications as a positrode in electrochemical devices. Therefore, their stability under such conditions is a key issue. All investigated materials are thermodynamically stable relative to binary oxides and exhibit strongly exothermic enthalpies of formation. Moreover, BaGd0.3La0.7Co2O6-δ and BaGd0.8La0.2Co2O6-δ remain the main perovskite structure up to 3 bars of water vapor at 400 °C. At higher steam pressure, reaching 10 bar at 300 °C, the partial decomposition to constituent oxides and hydroxides was observed. The BGLC compounds exhibit higher negative formation enthalpies in comparison to single-Ln compositions, which does not translate into higher chemical stability under high steam pressures since the BLnC series retained the main perovskite structure at higher temperatures as well as in higher water vapor pressures.

Praseodymium Orthoniobate and Praseodymium Substituted Lanthanum Orthoniobate: Electrical and Structural Properties.

In this paper, the structural properties and the electrical conductivity of La1-xPrxNbO4+δ (x = 0.00, 0.05, 0.1, 0.15, 0.2, 0.3) and PrNbO4+δ are presented and discussed. All synthesized samples crystallized in a monoclinic structure with similar thermal expansion coefficients. The phase transition temperature between the monoclinic and tetragonal structure increases with increasing praseodymium content from 500 °C for undoped LaNbO4+δ to 700 °C for PrNbO4+δ. Thermogravimetry, along with X-ray photoelectron spectroscopy, confirmed a mixed 3+/4+ oxidation state of praseodymium. All studied materials, in humid air, exhibited mixed protonic, oxygen ionic and hole conductivity. The highest total conductivity was measured in dry air at 700 °C for PrNbO4+δ, and its value was 1.4 × 10-3 S/cm.

Structural properties of mixed conductor Ba1-xGd1-yLax+yCo2O6-δ.

Ba1-xGd1-yLax+yCo2O6-δ (BGLC) compositions with large compositional ranges of Ba, Gd, and La have been characterised with respect to phase compositions, structure, and thermal and chemical expansion. The results show a system with large compositional flexibility, enabling tuning of functional properties and thermal and chemical expansion. We show anisotropic chemical expansion and detailed refinements of emerging phases as La is substituted for Ba and Gd. The dominating phase is the double perovskite structure Pmmm, which is A-site ordered along the c-axes and with O vacancy ordering along the b-axis in the Ln-layer. Phases emerging when substituting La for Ba are orthorhombic Ba-deficient Pbnm and cubic LaCoO3-based R3̄c. When La is almost completely substituted for Gd, the material can be stabilised in Pmmm, or cubic Pm3̄m, depending on thermal and atmospheric history. We list thermal expansion coefficients for x = 0-0.3, y = 0.2.

Microstructural Design of Ba0.5La0.5Co0.5Fe0.5O3 Perovskite Ceramics.

Ba0.5La0.5Co0.5Fe0.5O3-δ was synthesized in the solid-state reaction route. The influence of ball milling parameters (such as milling media size, angular velocity, and time), pelletizing pressure, and annealing parameters on the microstructure was studied. The grain size distribution and density or specific surface area changes were investigated in each approach while the individual parameters were changed. The evaluation of BLCF synthesis parameters enables tailoring the microstructure to various applications. It was observed that with lowering the size of milling balls and increasing the angular velocity the material will be porous and thus more appropriate as electrode material in proton ceramic fuel cell or electrolyzer. An increase of time, balls diameter, and/or angular velocity of milling enables one to densify the material in case of membrane application in, e.g., as a gas sensor. The significant influence on densification has also annealing temperature increase. Applying 1200 °C during annealing leads to dense material, while at 1100 °C shows visible porosity of the product. In this work, we present the results of the BLCF synthesis parameters change allowing the selection of appropriate parameter values depending on the further application as PCCs.

Structural Properties and Water Uptake of SrTi1-xFexO3-x/2-δ.

In this work, Fe-doped strontium titanate SrTi1-xFexO3-x/2-δ, for x = 0-1 (STFx), has been fabricated and studied. The structure and microstructure analysis showed that the Fe amount in SrTi1-xFexO3-x/2-δ 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 > 0.3) exhibit higher oxygen vacancies concentration of the order of magnitude 1022-1023 cm-3. The proton uptake investigations have been done using thermogravimetry in wet conditions, and the results showed that the compositions with x < 0.5 exhibit hydrogenation redox reactions. Proton concentration at 400 °C depends on the Fe content and was estimated to be 1.0 × 10-2 mol/mol for SrTi0.9Fe0.1O2.95 and 1.8 × 10-5 mol/mol for SrTi0.5Fe0.5O2.75. Above 20 mol% of iron content, a significant drop of proton molar concentrations at 400 °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-δ. 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.

Conductivity, structure, and thermodynamics of Y2Ti2O7-Y3NbO7 solid solutions.

The defect fluorite yttrium niobate Y3NbO7 and pyrochlore yttrium titanate Y2Ti2O7 solid solutions have been synthesized via a solid state synthesis route. The resulting stoichiometry of the oxides is Y2+xTi2-2xNbxO7, where x = 0 to x = 1. All of the samples were single-phase; however, for those with a predominant fluorite phase, a small amount of additional pyrochlore phase was detected. The volume of the solid solution unit cells linearly increases with increase in yttrium niobate content. The water uptake increases with (x) and the protonic defect concentration reaches almost 4.5 × 10-3 mol mol-1 at 300 °C. The calculated enthalpy of formation from oxides suggests strong stability for all of the compositions, with the values of enthalpy ranging from -84.6 to -114.3 kJ mol-1. The total conductivity does not have a visible dependence on Y3NbO7 content. For each compound, the total conductivity is higher in wet air. Interestingly, for samples where x < 0.5, the ratio of conductivity in hydrogen to air increases with increasing temperature, while for x > 0.5, the trend is the opposite.

High-Temperature Structural and Electrical Properties of BaLnCo2O6 Positrodes.

The application of double perovskite cobaltites BaLnCo2O6-δ (Ln = lanthanide element) in electrochemical devices for energy conversion requires control of their properties at operating conditions. This work presents a study of a series of BaLnCo2O6-δ (Ln = La, Pr, Nd) with a focus on the evolution of structural and electrical properties with temperature. Symmetry, oxygen non-stoichiometry, and cobalt valence state have been examined by means of Synchrotron Radiation Powder X-ray Diffraction (SR-PXD), thermogravimetry (TG), and X-ray Absorption Spectroscopy (XAS). The results indicate that all three compositions maintain mainly orthorhombic structure from RT to 1000 °C. Chemical expansion from Co reduction and formation of oxygen vacancies is observed and characterized above 350 °C. Following XAS experiments, the high spin of Co was ascertained in the whole range of temperatures for BLC, BPC, and BNC.