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.

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.

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 .

Order–disorder structural transformations in xR2O3·(1 – х)TiO2 (R = Yb, Lu; х = 0.5–0.6) solid solutions with a highly imperfect fluorite-derived structure at 1600°C have been studied using monochromatic synchrotron X-ray diffraction and Raman spectroscopy. The results demonstrate that the synthesis process leads to the formation of two cubic phases identical in composition: a disordered fluorite-like (F) phase (Fm3m) and an ordered pyrochlore-like (P) phase (Fd3m), which is coherent with the disordered phase and consists of nanoscale (<100 Å) and nanocrystalline domains. The lattice parameters of these phases have been determined. The stability range of the solid solutions in the systems studied is 0.5 ≤ x ≤ 0.55. In the samples containing 0.55Yb2O3 and 0.5Lu2O3, the P-phase consists of nanodomains. The Raman spectra of the Yb2TiO5- and Lu2TiO5-based solid solutions contain broad bands at low and high frequencies, with peaks at 101, 175, 290, 346, 384, and 727 (115, 176, 320, and 745) cm–1, which correspond to the P- and F-phases, respectively. The formation of pyrochlore-like phases with different degrees of order in a fluorite matrix is due to the internal stress induced by the high density of structural defects in their unit cells. The materials obtained in this study have a large specific surface area and can be used as catalysts and catalyst supports.

Potential of mechanochemical methods has been studied as applied to the development of promising materials of electrodes for supercapacitors with MeO x /C composition (Me = Mo, Mn, V, Bi, Sb, Tb, Cd, and Pb; C = graphite). Optimal procedures have been elaborated for mechanical activation and pressing of pellets. The properties of activated systems have been studied by several methods: X-ray diffraction, adsorption (determination of BET specific surface area and porosity), electron microscopy, and measurements of conductivity and capacity. It has been shown that, under the selected activation conditions, metal oxide/carbon nanosized composites are formed with a large specific surface area (as large as 100 m 2 /g) and a rather high porosity. At the same time, no chemical reactions occur between the components. The large values of the specific surface area and porosity remain preserved when pellets are pressed. MoO 3 /5С, MnO 2 /5С, and V 2 O 5 /5С systems have turned out to be most promising. The resistance of these systems amounts to several tens and hundreds ohms per centimeter. For the MoO 3 /5С system, cyclic voltammetric measurements carried out in an electrochemical cell in the presence of H 2 SO 4 as an electrolyte have yielded the specific capacity of the material equal to 36 µF/cm 2 .

Stabilized fluorites and pyrochlores with different types of substitution in the (Nd2O3 - ZrO2 system (NdZrO)) have been studied by Raman spectroscopy, X-ray diffraction, and impedance spectroscopy methods. Ionic and proton conductivity maps for 9 compositions in the NdZrO system are presented. Oxygen partial pressure measurements show a typical ionic conductor behavior, with a significantly increase of conductivity for (Nd2-xZrx)Zr2O7+x/2 (x = 1.27) fluorite and (Nd2-xZrx)Zr2O7+x/2 ( x = 0.4, 0.2) pyrochlores. For the same compositions the strong luminescence was observed. Strong luminescence and high oxygen ion conductivity of these solid solutions can be associated with the presence of phases with the different degree of structural disorder (tetragonal phase, fluorite) in local nanodomains in fluorite or pyrochlore matrix in the ZrO2 - Nd2Zr2O7 region. Really, Raman spectra of (Nd2-xZrx)Zr2O7+x/2 ( x = 0.5 - 0.2) solid solutions demonstrated the fluorite + pyrochlore structural type in the short-range order. Fluorite nanodomains in pyrochlore matrix (ZrO2 - Nd2Zr2O region) resulting from an order-disorder transition can only be detected at a radiation wavelength comparable to the fluorite nanodomain size. Thus, using Raman spectroscopy the broad isomorphism range in theZrO(2) - Nd2Zr2O system has been shown to be nonuniform. (c) 2021 Elsevier Ltd. All rights reserved.

Thermogravimetry and calorimetry in combination with mass spectroscopy, as well as X-ray diffraction, have been employed to study thermal transformations in mechanically activated MoO 3 /C, MnO 2 /C, Bi 2 O 3 /C, and V 2 O 5 /C systems, which are promising materials for electrodes of supercapacitors and ion batteries. It has been found that the crystalline structure of activated highly dispersed nanocomposites is stable up to 250–350°C depending on the nature of an oxide. Reactivity has been analyzed for “weakly bound” oxygen formed in different oxides due to reduction with carbon during mechanical activation. In the cases of МоО 3 and MnO 2 , the onset temperature of oxide reduction with carbon is substantially decreased due to the lower temperatures of oxide decomposition with oxygen liberation as a result of the activation. The interactions of the mechanically activated oxides with carbon, aluminum, and CO have been compared. It has appeared that the presence of carbon decreases the temperature of MnO 2 reduction, has almost no effect on the reaction rate in the cases of МоO 3 and Bi 2 O 3 , and increases the temperature of V 2 O 5 transformation.

The phase composition, microstructure, electrical and thermal conductivity of Nd2 +/- xHf2 +/- xO7 +/-delta ceramic materials have been studied. (Nd2-xHfx)Hf2O7+x/2 (x = 0.2, 0.32, 0.39) composites, containing 4-16 wt% HfO2, have the highest conductivity and the brightest narrow-band luminescence among materials under study. The (Nd2-xHfx)Hf2O7+x/2 (x = 0.2) composite, containing similar to 4 wt% HfO2, is shown to have the lowest thermal conductivity: 1.52 W m(-1) K-1 at 300 K. XRD and Raman spectroscopy results show that (Nd2-xHfx)Hf2O7+x/2 (x = 0.2, 0.32, 0.39, 1.14) are composites consisting of a mixture of pyrochlore solid solutions with Hf on the Nd site and two HfO2 polymorphs (m - HfO2 and gamma(2) - HfO2 nanodomains). The properties of the (Nd2-xHfx)Hf2O7+x/2 (x = 0.2, 0.32, 0.39, 1.14) composites are compared to that of the Nd-2(Hf2-xNdx)O7-x/2 (x = 0, 0.1) pyrochlores, and the best properties were shown by the (Nd2-xHfx)Hf2O7+x/2 (x = 0.2, 0.32) composites.

Ln(2)(Hf(2-x)Ln(x))O7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 degrees C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf1-xEuxO6 octahedra in pyrochlore Eu-2(Hf2-xEux)O7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 degrees C for 24h in air reduces the total conductivity of the Ln(2)(Hf(2-x)Ln(x))O7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5-6, due to the decrease in the concentrations of oxygen vacancies and Ln(2+) ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu-2(Hf2-xEux)O7-x/2 (x = 0.1) at similar to 800 degrees C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.

We have studied the structure and proton conductivity of rhombohedral La6-xMoO12-delta (x = 0.5, 0.6, 0.7, 1) lanthanum molybdates prepared via mechanical activation of lanthanum and molybdenum oxides, followed by thermal annealing at 1650 degrees C. The La6-xMoO12-delta (x = 0.5, 0.6) materials were phase-pure and had a complex rhombohedral structure (R1). An increase in the molybdenum concentration leads to a decrease in the degree of rhombohedral distortion and proton conductivity in the La6-xMoO12-delta (x = 0.5, 0.6, 0.7, 1) series. The proton conductivity at the optimal composition La6-xMoO12-delta (x = 0.5) is similar to 4.0 x 10(-5) S/cm at 500 degrees C in wet air. A comparative analysis shows that, in the Ln(6-x)MoO(12-delta) (Ln = La, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb; x = 0-1) series, proton conductivity decreases with the Ln ionic radii decreasing regardless of the structural type. Because of this, the high proton conductivity is demostrated by the stable La6-xMoO12-delta (x = 0.5, 0.6) materials, with an inherently deficient oxygen sublattice, which crystallize in a large-volume, complex rhombohedral cell (R1). (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

— Dense Gd 2 Zr 2 O 7 -based ceramics with the composition Gd 2 Zr 1.9 Be 0.1 O 6.9 , containing heterovalently substituted beryllium on the zirconium site, have been prepared for the first time via mechanical activation of an oxide mixture, followed by annealing of green compacts at 1500°C. The use of mechanical activation has been shown to enable the preparation of dense ceramics (relative density of 88%) by annealing for 5 min and gas-tight ceramics (relative density of 97.3%) by annealing for 4 h. Gd 2 Zr 1.9 Be 0.1 O 6.9 has a pure oxygen ion conductivity of 4.0 × 10 –3 S/cm at 800°C. The main advantage of partial beryllium substitution on the zirconium site in Gd 2 Zr 2 O 7 is the formation of a material having negligible electron and hole conductivities in wide ranges of oxygen partial pressures and temperatures.

Seven binary mixed oxides of V 2 O 5 , MoO 3 , TiO 2 , B 2 O 3 , Bi 2 O 3 , In 2 O 3 , and Tm 2 O 3 , in which V 2 O 5 is a permanent component, were prepared by the method of mechanochemical activation (MCA). All composites have been investigated using EPR spectroscopy, X-ray diffraction, BET analysis and EPR spectra calculations. The results obtained were compared with those for individual V 2 O 5 powder. The kinetic of structural transformations occurring in these binary mixtures under MCA were quantitatively characterized using a developed program of EPR spectra analysis. These structural rearrangements are fitted well by the first-order rate constants. Influence of the oxide nature mixed with vanadium pentoxide, the ratio of the components and time of milling on these transformations are discussed.

— Ln 2 O 3 + HfO 2 (Ln = Nd, Dy) powders and ceramics have been studied in an oxidizing (O 2 ) and a mild reducing (He) atmosphere using differential scanning calorimetry (DSC), thermogravimetry, mass spectrometric analysis of released gases, X-ray diffraction, IR spectroscopy, and Raman spectroscopy. The results demonstrate that both a mechanically activated oxide mixture of appropriate composition and the powders and ceramics prepared by heat-treating the mixture contain carbon-containing compounds (basic rare-earth carbonates and hydroxycarbonates) and/or at least 0.2–0.5 wt % carbon (X-ray amorphous or crystalline). As a result, during heating in an oxidizing atmosphere all of the samples release CO 2 in the same temperature ranges (250–600 and 750–1200°C), which is accompanied by exothermic peaks in their DSC curves. The CO 2 release in the range 250–600°C is due to the onset of decomposition of the basic rare-earth carbonates and hydroxycarbonates, which are present in small amounts in the starting mixture, powders, and ceramics. The CO 2 release in the range 750–1200°C is due to the burnout of strongly bonded carbon and thermally stable carbon-containing compounds (rare-earth dioxymonocarbonates, Ln 2 O 2 CO 3 ). The exothermic peaks in the DSC curve are due to fluorite LnHfO 4 – δ (Ln = Nd, Dy) crystallization processes. We believe that synthesis in air, involving the formation of X-ray amorphous (fine-particle and nanocrystalline) precursors containing rare-earth oxides, which tend to form basic rare-earth carbonates and hydroxycarbonates in air, will always yield high-temperature ceramics containing carbon compounds and at least 0.5 wt % X-ray amorphous carbon and/or graphite. The amount of carbon and carbon-containing compounds in the dysprosium-containing ceramics is markedly smaller (~0.2%) than that in the neodymium-containing ceramics. The crystallization of the rare-earth hafnates is a rather slow process that can begin at temperatures as low as 550°C. The formation of Nd 2 Hf 2 O 7 with the pyrochlore structure involves fluorite NdHfO 4 – δ formation as an intermediate step, and a single-phase product can only be obtained by high-temperature firing at ~1600°C. Phase-pure DyHfO 4 – δ with the fluorite structure can be obtained by firing at 1200°C.

Structure, conductivity and magnetism of orthorhombic and fluorite Ln10Mo2O21 (Ln = Gd, Dy, Ho) polymorphs.