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Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB3–x, which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of α-SiB3–x and β-SiB3 and analyze their relative thermodynamic stabilities. α-SiB3–x is metastable (with respect to β-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200–1300 °C. At the same time, α-SiB3–x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable β-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:1–1:4 at temperatures 1175–1200 °C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of β-SiB3 was seen at 1100 °C for samples pressurized to 5.5–8 GPa. β-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 °C. The highly ordered nature of β-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of α-SiB3–x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered α-SiB3–x were ascertained by a combination of single-crystal X-ray diffraction and 29Si magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T ≤ 1200 °C) and single crystal samples (obtained from Si-rich molten Si–B mixtures at T > 1400 °C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of α-SiB3–x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B12 icosahedron. Accordingly, α-SiB3–x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of α-SiB3–x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition β-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that α-SiB3–x only becomes more stable than β-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that α-SiB3–x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise α-SiB2.5 and β-SiB3, whereas α-SiB3 represents a p-type conductor.

We investigated the hydride reduction of tetragonal BaTiO3 using the metal hydrides CaH2, NaH, MgH2, NaBH4, and NaAlH4. The reactions employed molar BaTiO3/H ratios of up to 1.8 and temperatures near 600 °C. The air-stable reduced products were characterized by powder X-ray diffraction (PXRD), transmission electron microscopy, thermogravimetric analysis (TGA), and 1H magic angle spinning (MAS) NMR spectroscopy. PXRD showed the formation of cubic products—indicative of the formation of BaTiO3–xHx—except for NaH. Lattice parameters were in a range between 4.005 Å (for NaBH4-reduced samples) and 4.033 Å (for MgH2-reduced samples). With increasing H/BaTiO3 ratio, CaH2-, NaAlH4-, and MgH2-reduced samples were afforded as two-phase mixtures. TGA in air flow showed significant weight increases of up to 3.5% for reduced BaTiO3, suggesting that metal hydride reduction yielded oxyhydrides BaTiO3–xHx with x values larger than 0.5. 1H MAS NMR spectroscopy, however, revealed rather low concentrations of H and thus a simultaneous presence of O vacancies in reduced BaTiO3. It has to be concluded that hydride reduction of BaTiO3 yields complex disordered materials BaTiO3–xHy□(x–y) with x up to 0.6 and y in a range 0.04–0.25, rather than homogeneous solid solutions BaTiO3–xHx. Resonances of (hydridic) H substituting O in the cubic perovskite structure appear in the −2 to −60 ppm spectral region. The large range of negative chemical shifts and breadth of the signals signifies metallic conductivity and structural disorder in BaTiO3–xHy□(x–y). Sintering of BaTiO3–xHy□(x–y) in a gaseous H2 atmosphere resulted in more ordered materials, as indicated by considerably sharper 1H resonances.

Perovskite-related Ln2NiO4+δ (Ln = La, Pr, Nd) nickelates with layered Ruddlesden-Popper combine redox stability with noticeable oxygen stoichiometry changes, yielding enhanced mixed transport and electrocatalytic properties. These unique features are promising for applications as oxygen electrodes with good electrochemical performance in reversible SOFC/SOEC (solid oxide fuel/electrolysis cell) systems. To date, most efforts were focused on oxygen-hyperstoichiometric Ln2NiO4+δ-based phases, whereas nickelates with oxygen-deficient lattice remain poorly explored. Recent studies demonstrated that the highest electrical conductivity in (Ln2-xSrx)2NiO4±δ series at elevated temperatures is observed for the compositions containing ~ 60 at.% of strontium in A sublattice [1,2]. The present work was focused on the characterization of (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates for the possible use as materials for reversible oxygen electrodes. The ceramic materials were prepared by Pechini method with repeated annealings at 650-1200°C and sintered at 1250-1300°C for 5 h under oxygen atmosphere. Variable-temperature XRD studies confirmed that all studied compositions retain tetragonal K2NiF4-type structure in the temperature range 25-900°C. The results of thermogravimetric analysis showed that the prepared nickelates has oxygen-deficient lattice under oxidizing conditions at temperatures above 700°C. Partial substitution of nickel by cobalt or iron results in a decrease of p-type electronic conductivity and the concentration of oxygen vacancies in the lattice, but also suppresses dimensional changes associated with microcracking effects (due to anisotropic thermal expansion of tetragonal lattice). Electrochemical performance of porous (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ electrodes in contact with Ce0.9Gd0.1O2-δ solid electrolyte was evaluated at 600-800°C employing electrochemical impedance spectroscopy and steady-state polarization (anodic and cathodic) measurements. published

Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric alpha-SiB3-x, which relates to the alpha-rhombohedral phase of boron, and as strictly ordered and stoichiometric beta-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of alpha-SiB3-x and beta-SiB3 and analyze their relative thermodynamic stabilities. alpha-SiB3-x is metastable (with respect to beta-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200-1300 degrees C. At the same time, alpha-SiB3-x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable beta-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:11:4 at temperatures 1175-1200 degrees C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of beta-SiB3 was seen at 1100 degrees C for samples pressurized to 5.5-8 GPa. beta-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 degrees C. The highly ordered nature of beta-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of alpha-SiB3-x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered alpha-SiB3-x were ascertained by a combination of single-crystal X-ray diffraction and Si-29 magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T amp;lt;= 1200 degrees C) and single crystal samples (obtained from Si-rich molten Si-B mixtures at T amp;gt; 1400 degrees C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B-12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of alpha-SiB3-x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B-12 icosahedron. Accordingly, alpha-SiB3-x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of alpha-SiB3-x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition beta-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that alpha-SiB3-x only becomes more stable than beta-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that alpha-SiB3-x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise alpha-SiB2.5 and beta-SiB3, whereas alpha-SiB3 represents a p-type conductor. Funding Agencies|Swedish Research CouncilSwedish Research Council [2016-03441]; National Science FoundationNational Science Foundation (NSF) [DMR-1007557]; Deutsche ForschungsgemeinschaftGerman Research Foundation (DFG) [SCHE 487/12-1]; U.S. Department of EnergyUnited States Department of Energy (DOE) [DE-SC0014506]; Ratchadaphiseksomphot Endowment Fund, Chulalongkorn UniversityChulalongkorn University; Swedish Foundation for Strategic Research (SSF) through the Future Research Leaders program; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Compounds Sr3&minus, xPrxFe1.25Ni0.75O7&minus, &delta, with 0 &le, x &le, 0.4 and Ruddlesden&ndash, Popper n = 2 type structures were synthesized and investigated by X-ray and neutron powder diffraction, thermogravimetry, and M&ouml, ssbauer spectroscopy. Both samples, prepared at 1300 &deg, C under N2(g) flow and samples subsequently air-annealed at 900 &deg, C, were studied. The structures contained oxygen vacancies in the perovskite layers, and the Fe/Ni cations had an average coordination number less than six. The oxygen content was considerably higher for air-annealed samples than for samples prepared under N2, 7 &minus, = ~6.6 and ~5.6 per formula unit, respectively. M&ouml, ssbauer data collected at 7 K, below magnetic ordering temperatures, were consistent with X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) results. The electrical conductivity was considerably higher for the air-annealed samples and was for x = 0.1~30 S&middot, cm&minus, 1 at 500 &deg, C. The thermal expansion coefficients were measured in air between room temperature and 900 &deg, C and was found to be 20&ndash, 24 ppm&middot, K&minus, 1 overall.

Previously unexplored oxygen-deficient RuddLesden-Popper Nd2-xSrxNiO4-delta (x = 1.0-1.6) nickelates were evaluated for potential use as oxygen electrode materials for solid oxide fuel and electrolysis ceRs, with emphasis on structural stability, oxygen nonstoichiometry, dimensional changes, and electrical properties. Nd2-xSrxNiO4-delta ceramics possess the K2NiF4-type tetragonal structure under oxidizing conditions at 25-1000 degrees C. Acceptor-type substitution by strontium is compensated by the generation of eLectron-hoLes and oxygen vacancies. Oxygen deficiency increases with temperature and strontium doping reaching -1/8 of oxygen sites for x = 1.6 at 1000 degrees C in air. Strongly anisotropic expansion of the tetragonal Lattice on heating correlated with oxygen nonstoichiometry changes results in an anomalous dilatometric behavior of Nd2-xSrxNiO4-delta ceramics under oxidizing conditions. Moderate thermal expansion coefficients, (11-14) x 10(-6) K-1, ensure however thermomechanical compatibility with common solid electrolytes. Reduction in inert atmosphere induces oxygen vacancy ordering accompanied by a contraction of the Lattice and a decrease of its symmetry to orthorhombic. Nd2-xSrxNiO4-delta ceramics exhibit a p-type metallic-Like electrical conductivity at 500-1000 degrees C under oxidizing conditions, with the highest conductivity (290 S cm(-1) at 900 degrees C in air) observed for x = 1.2. The high Lev\& of oxygen deficiency in Sr-rich Nd2-xSrxNiO4 impLies enhanced mixed ionic-electronic transport favorable for electrode applications.