Synthesis of polymorphic modifications of samarium tungstate from a mechanically activated oxide mixture.

The phase formation of samarium tungstate (Sm2WO6) from a mechanically activated oxide mixture of Sm2O3 : WO3 = 1 : 1 was studied using thermal analysis methods. The microstructure of the powder samples and high-temperature ceramics was examined by SEM. The influence of activation in mills with different specific power intensities was considered: a high-energy mill designed by Aronov and a SPEX 8000M mill. The conditions for obtaining polycrystalline samples of new modifications, beta-Sm2WO6, alpha-Sm2WO6 and delta-Sm2WO6, whose existence is reported here for the first time, have been determined. The structures of the new polymorphs were characterized by XRD with Rietveld refinement and Raman spectroscopy.

In the present work, extensive studies of ionic conductors with pyrochlore and fluorite structures in the Gd2O3-HfO2 system are carried out. Gd2Hf2O7 and Gd2.1Hf1.9O6.95, Gd2.16Hf1.84O6.92, and Gd2HfO5 solid solutions known as "stuffed" pyrochlores were synthesized by mechanical activation of oxides followed by high temperature annealing at 1500-1600 degrees C. The main investigation methods were X-ray diffraction with Rietveld refinement, SEM microscopy and impedance spectroscopy in dry and wet air and N2. Proton conductivity was first found in Gd2HfO5 (1 x 10-6 at 600 degrees C), and solid solutions based on it, doped with lanthanum and calcium with pure fluorite structure. Proton conduction of the best proton-conducting ceramics Gd1.9La0.1HfO5 (3 x 10-6 at 600 degrees C) was confirmed by measurements in O2/D2O, Ar/D2O atmospheres. At all temperatures, the conductivity sigma D2O was lower than that of sigma H2O. Gd1.9Ca0.1HfO4.95 has a higher oxygen ion component of conductivity in dry air than Gd2HfO5, but the proton conductivity is the same. An attempt was made to synthesize the HEO composition (Lu0.2Y0.2Ho0.2Nd0.2La0.2)2HfO5, which turned out to be two-phase, containing similar to 70% fluorite and similar to 30% pyrochlore. The two-phase HEO (Lu0.2Y0.2Ho0.2Nd0.2La0.2)2HfO5 shows a total conductivity of similar to 3 x 10-6 at 600 degrees C and similar to 3 x 10-5 at 750 degrees C, respectively.

Materials of the system Pr2O3-ZrO2, namely the Pr2Zr2O7-based pyrochlores, have received considerable attention in the last decade, being a very interesting structure for defect chemistry because of its high solubility for various dopants, anti-site behaviour between A and B, and the multitude of possible combinations of A and B that are compatible in this type of structure. The compositions (Pr2-xZrx)Zr2O7+x/2 (x = 0.15), Pr2Zr2O7, and Pr2(Zr2-xPrx)O7-x/2 (x = 0.1), were prepared in previous works through the coprecipitation method and were characterised by impedance spectroscopy as a function of the oxygen partial pressure. In the present work, a defect chemistry model is proposed, and, based on the previously obtained experimental conductivity data, the relevant thermodynamic parameters were obtained by fitting, using a non-linear optimisation numerical method. The mobility of oxygen vacancies and interstitials oxygen were accurately determined, as well as the equilibrium constant of the formation of anti-Frenkel defects. It was observed that deviations from the stoichiometry promote an increase in ionic conductivity, respectively, 1.3x10-4, 1.4x10-3 and 1.7x10-2 S/cm, for the stoichiometric, excess of Pr and excess of Zr composition. The higher value obtained for the composition with an excess of Zr4+, suggests a higher interstitial oxygen mobility when compared with the oxygen vacancy mobility. It is also demonstrated that the novel applied methodology of fitting conductivity experimental data with an optimisation numerical method is suitable for determining the thermodynamically relevant parameters of defect chemistry models, allowing the prediction of material properties.

Using mechanical activation of an oxide mixture containing 0.2 wt.% hexagonal boron nitride, followed by high-temperature firing (1400–1500 °C), we prepared single-phase (La0.2Nd0.2Ho0.2Lu0.2Y0.2)2ZrO5 ceramic, a high-entropy oxide (HEO) analog of Gd2ZrO5, whereas we failed to obtain single-phase Gd2ZrO5 under similar conditions: the material consisted of two phases, with the fluorite and bixbyite structures, like in the case of conventional synthesis or coprecipitation in previous work. Thus, the use of the HEOs allowed us to obtain a phase-pure compound with the fluorite structure at a markedly lower synthesis temperature. An important role was played by 0.2 wt.% hyperstoichiometric hexagonal BN additions, which ensured considerable amorphization of the starting oxides. The start powders and resulting ceramics were studied by X-ray diffraction, SEM analyses and conductivity was measured by impedance spectroscopy method in dry and wet air.The highest proton conductivity, ∼2.5×10-5 S/cm at 630 °C, was offered by the (La0.2Nd0.2Ho0.2Lu0.2Y0.2)2ZrO5 HEO ceramic. The use of HEO analogs of various compounds can be helpful for not only assessing thermodynamic properties of ceramics (reduction in synthesis and polymorphic transformation temperatures), but also preparing proton conductors when it is necessary to enhance hydrating properties of the cation sublattice.

Solid solutions of rare earth titanates with high contents of rare earth oxides of up to 50-62% have been synthesized by the co-precipitation method and their structure, microstructure and conductivity in dry and wet air have been studied. Proton conductors have been found for the first time in solid solutions of rare earth titanates with a high content of Ln2O3 (>50%) with a nominal formula composition of (LnxTi1-x)4O8-2x (Ln = Yb, Er, Ho, 0.667 ≤ x ≤ 0.765). Among (LnxTi1-x)4O8-2x (Ln = Yb, Er, Ho, x = 0.684), (HoxTi1-x)4O8-2x (x = 0.684) showed the maximum conductivity in wet air. In this context, four additional compositions (HoxTi1-x)4O8-2x (x = 0.718, 0.734, 0.75, and 0.765) were synthesized in the holmium series. An increase in the holmium content leads to an increase in the proton transfer coefficients; at the same time, a more complex nature of the dependence of the conductivity under dry and wet atmospheres is observed. For the fluorite-like solid solution (HoxTi1-x)4O8-2x (0.701 ≤ x ≤ 0.765), the proton transfer coefficients were found to be ∼0.9 in the range of 200-450 °C. As the temperature continues to rise, the proton conductivity decreases quite sharply and the transfer coefficient becomes as low as 0.3 at 700 °C. The increase in proton conductivity in the Yb-Er-Ho series is associated with an increase in the hydrophilic properties of rare earth cations. In the (HoxTi1-x)4O8-2x (x = 0.667 ≤ x ≤ 0.765) series, the conductivity in wet air was ∼1 × 10-6 S cm-1 at 450 °C for most compositions. The conductivity of ceramics with x = 0.701 and 0.75 is about 2 times higher, which may be due to the optimal size of pyrochlore nanodomains in the fluorite matrix for x = 0.701 and the formation of pure fluorite for x = 0.75, respectively.

The mechanism of phase formation from the initial and the mechanically activated (m/a) mixture of Sm2O3+MoO3 oxides is studied by differential scanning calorimetry (DSC) in an oxygen atmosphere. It is shown that different mechanisms of samarium oxymolybdate synthesis are realized in these two cases. As a result of the mechanochemical action at room temperature, a nanosized mixture of Sm2(MoO4)3 and Sm2O3 is formed. Upon heating, Sm2(MoO4)3 is crystallized at the first stage and its interaction with Sm2O3 in the second stage at 900°C leads to the synthesis of oxymolybdate Sm2MoO6 with a scheelite structure, and this structure type is stable up to 1400°C. The kinetic experiment in a DSC cell shows only an apparent similarity of the phase formation mechanism with a decrease of the main exoeffects by 70°C for a m/a mixture of oxides. At the same time, the study of the mechanism of phase formation by isothermal exposure at different temperatures reveals the main advantages of ceramic synthesis from an activated oxide mixture. The total conductivity of Sm2MoO6 with a scheelite structure, which turned out to be of the p-type (1 × 10−6 S/cm at 600°C), is studied.

Tungstates Ln14W4O33 (Ln = Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb) with a pseudorhombohedral structure have been synthesized by a short high temperature annealing (1450-1600 degrees C) from mechanically activated mixtures of oxides. The resulting ceramics were studied by X-ray diffraction, Raman and SEM analyses. The conductivity of ceramics was measured by the 4-probe direct current method in dry and wet atmospheres (oxidizing and reducing) and by impedance spectroscopy. In order to estimate the ionic and electronic contributions to the total conductivity, the dependences of the conductivity on the oxygen partial pressure (pO2) were determined at temperatures between 700 and 900 degrees C. Ln14W4O33 (Ln = Nd, Sm, Gd) tungstates have a wide electrolytic range (10- 15-1 atm) above 700 degrees C. It should be noted that under oxidizing conditions the contribution of hole con-ductivity is absent in these compounds. Among the pseudorhombohedral phases of Ln14W4O33 (Ln = Nd, Sm, Gd), Nd14W4O33 exhibits the highest oxygen-ion conductivity (4 x 10-4 S/cm at 700 degrees C). The electrolytic domain of the second half of the REE Ln14W4O33 (Ln = Dy, Ho, Er, Tm, Yb) series becomes narrower (10- 10-10- 5 atm) and at the same time has a slight slope indicating the presence of impurity electronic charge carriers. In-vestigations in a reducing atmosphere confirmed that the reduction tendency increases with decreasing lanthanide ionic radius in the Ln14W4O33 tungsten series. The variations of the total conductivity in the series of pseudorhombohedral phases Ln14W4O33 (Ln = Dy, Ho, Er, Tm, Yb) are associated with local changes in the short -range order structure (Raman spectroscopy). When Ln14W4O33 (Ln = Nd, Gd) is exposed to water at room temperature for an extended period of time, the total conductivity decreases due to the diffusion of the Nd and Gd cations into the water.

Recent directions concerning hydrogen production, storage and utilization result in growing the demand for efficient technologies, a huge part of which is connected with the application of materials based on complex oxides as efficient catalysts for the fuel reforming, permselective membranes, components of solid oxide fuel cells and electrolyzers. These materials are generally based on the oxides with perovskite and fluorite structures or their derivatives. On the other hand, other classes of materials are recently considered as promising materials for these applications due to their peculiar functional characteristics or an outstanding potential of their improvement. They include a variety of types of rare earth element tungstates/molybdates, Co and Ba cobalitites with layered structure as well as other materials including langasites, magnetoplumbites, swedenborgites, etc., and composites based on them. In this work, structural, transport features and performance of such modern materials are reviewed.

The phase formation of neodymium tungstate Nd2WO6 from mechanically activated oxides was studied in a wide temperature range: 25–1600°C. Conditions for the formation of various polymorphic modifications were determined: low-temperature orthorhombic β-Nd2WO6 and δ-Nd2WO6 (P212121 (no. 19)); high-temperature monoclinic Nd2WO6 (C12/c1 (no. 15)). Optical absorption spectra were studied for polymorphic ceramics with a nominal composition of Nd2WO6. Differences in the spectra of δ-Nd2WO6 and monoclinic Nd2WO6 were detected. Both modifications (δ-Nd2WO6 and monoclinic Nd2WO6) showed proton conductivity with activation energies of 1.05 and 1.06 eV, respectively. For the Ca-containing solid solution with a monoclinic structure (Nd _1 - x Cax)2WO _6-δ (x = 0.01), whose overall conductivity increases compared to that of pure monoclinic Nd2WO6, hole conductivity predominates in air.

Using mechanical activation of an oxide mixture containing 0.2 wt% hexagonal boron nitride, followed by high-temperature firing (1400-1500 degrees C), we prepared single-phase (La0.2Nd0.2Ho0.2Lu0.2Y0.2)(2)ZrO5 ceramic, a high-entropy oxide (HEO) analog of Gd2ZrO5, whereas we failed to obtain single-phase Gd2ZrO5 under similar conditions: the material consisted of two phases, with the fluorite and bixbyite structures, like in the case of conventional synthesis or coprecipitation in previous work. Thus, the use of the HEOs allowed us to obtain a phase-pure compound with the fluorite structure at a markedly lower synthesis temperature. An important role was played by 0.2 wt% hyperstoichiometric hexagonal BN additions, which ensured considerable amorphization of the starting oxides. The start powders and resulting ceramics were studied by X-ray diffraction, SEM analyses and conductivity was measured by impedance spectroscopy method in dry and wet air. The highest proton conductivity, similar to 2.5 x 10(-5) S/cm at 630 degrees C, was offered by the (La0.2Nd0.2Ho0.2Lu0.2Y0.2)(2)ZrO5 HEO ceramic. The use of HEO analogs of various compounds can be helpful for not only assessing thermodynamic properties of ceramics (reduction in synthesis and polymorphic transformation temperatures), but also preparing proton conductors when it is necessary to enhance hydrating properties of the cation sublattice.

The mechanism of phase formation from (1) the initial and (2) the mechanically activated mixture of Sm2O3 + + MoO3 oxides has been studied by DSC in an oxygen atmosphere. It is shown that different mechanisms of samarium oxymolybdate synthesis are realised in these two cases. As a result of the mechanochemical action at room temperature, a nano-sized mixture of Sm2(MoO4)3 and Sm2O3 was obtained. Upon heating, the first stage is the crystallisation of Sm2(MoO4)3, whose interaction with Sm2O3 in the second stage at 900 °C leads to the synthesis of oxymolybdate Sm2MoO6 with the scheelite structure, and this structure type is stable up to 1400 °C. The kinetic experiment in a DSC cell shows only an apparent similarity of the phase formation mechanism with a decrease of the main exoeffects by 70 °C for a mechanically activated mixture of oxides. At the same time, the study of the mechanism of phase formation by isothermal exposure at different temperatures reveals the main advantages of ceramic synthesis from an activated oxide mixture: partially mechanosynthesis of the intermediate compound Sm2(MoO4)3 takes place at room temperature; the high degree of interaction between the mechanically activated oxides allows single phase ceramics to be synthesised in a single step over a wide temperature range. The total conductivity of Sm2MoO6 with a scheelite structure, which turned out to be p-type (1 · 10−6 S/cm at 600 °C), was studied.

In this work, a high-density ceramics Ln 2 Hf 2 O 7 ( Ln = La, Nd, Sm, Eu, Gd) were synthesized by mechanical activation followed by high-temperature synthesis at 1600°C (3–10 h) and their transport properties were compared with those of Ln 2.1 Hf 1.9 O 6.95 ( Ln = La, Nd, Sm, Eu) doped solid solutions. The total conductivity of ceramics was studied using impedance spectroscopy and dc four-probe method; for Ln 2 Hf 2 O 7 ( Ln = Sm, Eu), by determining the total conductivity as a function of oxygen partial pressure. The maximum oxygen-ion conductivity was observed for Gd 2 Hf 2 O 7 (~1 × 10 –3 S/cm at 700°C); it was shown to approach the conductivity of Gd 2 Zr 2 O 7 (~2 × 10 –3 S/cm at 700°C) for the first time. Thus, the gadolinium hafnate can be a promising material for further doping in order to obtain highly conductive electrolytes. Among pure rare-earth hafnates, the proton conductivity was reliably observed for Nd 2 Hf 2 O 7 only; however, ac measurements detected low-temperature proton conductivity in the Gd 2 Hf 2 O 7 up to 450°С as well. With a decrease in the lanthanide ionic radius, the oxygen-ion conductivity increased in the Ln 2 Hf 2 O 7 ( Ln = La, Nd, Sm, Gd) series. Although the conductivity of samarium hafnate is an order of magnitude lower than that of Gd 2 Hf 2 O 7 , it has a wide range of oxygen-ion conductivity (~10 –18 –1 atm at 700, 800°C); there is no contribution from hole conductivity in air, in contrast to Eu 2 Hf 2 O 7 . Among doped Ln 2.1 Hf 1.9 O 6.95 pyrochlore solid solutions ( Ln = La, Nd, Sm, Eu), the proton conductivity of ~8 × 10 −5 S/cm at 700°C was shown in Ln 2.1 Hf 1.9 O 6.95 ( Ln = La, Nd). With a decrease in the lanthanide ionic radius, the proton conductivity disappeared; the oxygen-ion one, increased.

The search for new materials with pronounced proton or oxygen-ion conductivities is of great importance for the development of solid state ionic and electrochemistry fields. Here, we studied the structure, phase transitions, and ionic (oxygen-ion and proton) conductivity of the pure and Nd containing gamma-La6W2O15-based composites and pseudorhombohedral La14-xNdxW4O33 (x = 12, 14) solid solutions. The proton conductor La14W4O33 (5 x 10(-5) S/cm at 600 & DEG;C) was found to be a two-phase material consisting of an anion-deficient La10W2O21 fluorite-related phase and the gamma-La6W2O15 orthorhombic phase. The phase content of the pure La10W2O21 cubic phase was similar to 18 wt% for the gamma-La6W2O15-based composite. A high degree of Nd content in gamma-La6W2O15-based composite leads to formation of solid solutions based on a pseudorhombohedral phase in La14-xNdxW4O33 with x = 12 and 14. The Nd-containing gamma-La6W2O15-based composites exhibited proton conductivity, which gradually decreased with increasing Nd content, whereas La14-xNdxW4O33 (x = 12, 14) pseudorhombohedral solid solutions were identified as oxygen-ion conductors. Nd14W4O33 has the oxygen-ion conductivity of similar to 4 x 10(-4) S/cm at 700 & DEG;C (1.0 x 10(-3) S/cm at 900 & DEG;C). In contrast to the gamma-La6W2O15 phase, the gamma-La6W2O15-based composite undergoes only a single reversible phase transition at around 910 & DEG;C, which can, however, initiate cracks in ceramics. According to DSC and SEM data, the phase transition near 910 & DEG;C can be suppressed by introducing Nd into the gamma-La6W2O15-based composites. The cracking process is enhanced by evaporation of tungsten oxide at T > 1450 & DEG;C. (c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

We report a study of nanophases in the La2O3–MO3 (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La2O3 + MO3 (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La15M8.5O48 (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range ~480–520 °C for La15Mo8.5O48 and ~685–760 °C for La15W8.5O48, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La15Mo8.5O48 to tetragonal γ-La2MoO6 and of hexagonal La15W8.5O48 to orthorhombic β-La2WO6. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La18W10O57 (phase I, 6H polytype) ceramic. Low-density La15W8.5O48 and La15Mo8.5O48 (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.

Tm2(Ti2−xTmx)O7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) solid electrolytes have been investigated as potential electrolyte materials for solid oxygen fuel cells (SOFCs), operating in the medium temperature range (600–700 °C). The design of new oxygen-conducting materials is of importance for their possible utilization in the solid oxide fuel cells. The oxygen–ion conductivity of the Tm2(Ti2−xTmx)O7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) “stuffed” pyrochlores ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) in dry and wet air. The synthesis of precursors via co-precipitation and the precipitate decomposition temperature have been shown to be of key importance for obtaining dense and highly conductive ceramics. At ~770 °C, the highest total conductivity, ~3.16 × 10−3 S/cm, is offered by Tm2Ti2O7. The conductivity of the fluorite-like solid solution Tm2(Ti2−xTmx)O7−x/2 (x = 0.74) is an order of magnitude lower. However, for the first time a proton contribution of ~5 × 10−5 S/cm at 600 °C has been found in Tm2(Ti2−xTmx)O7−x/2 (x = 0.74) fluorite. Until now, compositions with proton conductivity were not known for the intermediate and heavy rare earth titanates Ln2(Ti2−xLnx)O7−x/2 (Ln = Ho − Lu) systems. The use of X-ray diffraction (structural analysis with Rietveld refinement), optical spectroscopy and dielectric permittivity data allowed us to follow structural disordering in the solid solution series with increasing thulium oxide content. High and low cooling rates have been shown to have different effects on the properties of the ceramics. Slow cooling initiates’ growth of fluorite nanodomains in a pyrochlore matrix. The fabrication of such nanostructured dense composites is a promising direction in the synthesis of highly conductive solid electrolytes for SOFCs. We assume that high-temperature firing of nanophase precursors helps to obtain lightly doped “stuffed” pyrochlores, which also provide the high oxygen–ion conductivity.

Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)–electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden–Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.

The hexagonal solid solution La 2 W 1 + x O 6 + 3 x ( x ~ 0.11) and the orthorhombic low-temperature phase β-La 2 WO 6 (La 2 W 1 + x O 6 + 3 x ( x = 0)) are synthesized by the methods of preliminary mechanical activation of oxides followed by high-temperature synthesis at 1400°С, 4 h. In addition, by using the method of crystallization from solution, a single crystal of the La 2 W 1 + x O 6 + 3 x ( x ~ 0.22) composition isostructural to La 2 W 1 + x O 6 + 3 x ( x ~ 0.11) is grown. Both ceramics and the single crystal are studied by the methods of Raman spectroscopy, thermal analysis, and thermogravimetry. Their conductivity is studied by impedance spectroscopy in dry and humid air. The hexagonal single crystal La 2 W 1 + x O 6 + 3 x ( x ~ 0.22) demonstrates strong luminescence in the IR region, in contrast to the hexagonal ceramics La 2 W 1 + x O 6 + 3 x ( x ~ 0.11) and the β-La 2 WO 6 ceramics with the orthorhombic structure. The polycrystalline La 2 W 1 + x O 6 + 3 x ( x ~ 0.11) ceramics is found to be more stable under redox conditions as compared with the single crystal. The conductivity of the hexagonal single crystal La 2 W 1 + x O 6 + 3 x ( x ~ 0.22) is of the oxygen-ionic nature and lower than the conductivity of ceramics La 2 W 1 + x O 6 + 3 x ( x ~ 0.11) due to its perfect structure. The contribution of the protonic component of conductivity is absent for both the hexagonal solid solution La 2 W 1 + x O 6 + 3 x ( x ~ 0.11) and the single crystal La 2 W 1 + x O 6 + 3 x ( x ~ 0.22), their conductivity being of the purely ionic nature with the close values of activation energy (0.89 and 1.08 eV, respectively). The synthesized β-La 2 WO 6 ceramics demonstrates a small contribution of the protonic conductivity in humid air equal to ~1 × 10 –6 S/cm at 600°С, which is close to the conductivity of the earlier studied strontium-doped solid solution La 1.96 Sr 0.04 WO 6 – δ based on β-La 2 WO 6 .