Strukturen und Phasen des beta-Eukryptits sowie die Sammlung von Beugungsdaten mit axialen q-Scans

Der 'gestopfte Hochquarz' ß-Eukryptit (LiAlSiO4) ist bekannt für seine außergewöhnliche anisotrope Li-Ionenleitfähigkeit und die nahe Null liegende thermische Ausdehnung.Untersucht wurde die temperaturabhängige ß-Eukryptit-Phasenabfolge, insbesondere die modulierte Phase. Deren Satellitenreflexe sind gegenüber den normalen Reflexen erheblich verbreitert, überlappen miteinander sowie mit den dazwischen liegenden 'a-Reflexen' zu Tripletts. Für die Separation der Triplett-Intensitäten waren bisherige Standardverfahren zur Beugungsdatensammlung ungeeignet. Mit 'axialen q-Scans' wurde ein neuartiges Verfahren entwickelt. Intensitäten wurden mit dem neu-entwickelten least squares-Programm GKLS aus 2000 Profilen seriell und automatisch gewonnen und erfolgreich auf Standarddaten skaliert. Die Verwendung verbreiterter Reflexprofile erwies sich als zulässig.Die Verbreiterung wurde auf eine verminderte Fernordnung der Modulation von 11 bis 16 Perioden zurückgeführt (Analyse mit der Gitterfunktion), womit ein ungewöhnliches beugungswinkelabhängiges Verhalten der Reflexbreiten korrespondiert und mit typischen Antiphasendomänendurchmessern (andere Autoren) korreliert.Eine verminderte Si-/ Al-Ordnung wird als ursächlich für geringe Domänengrößen und Fernordnung angesehen, sowie für Eigenschaften wie z.B. a/c-Verhältnisse, Ausdehnungskoeffizienten, Ionenleitfähigkeit, Strukturtyp und Umwandlungstemperaturen. Änderungen des SiO2-Gehaltes, der Temperatur oder der Si- /Al-Ordnung zeitigen für einige Eigenschaften ähnliche Wirkungen.Die gemittelte Struktur der modulierten Phase wurde erstmals zuverlässig bestimmt, die Rolle des Li charakterisiert, Zweifel an der hexagonalen Symmetrie des ß-Eukryptits wurden ausgeräumt und die Bestimmung der modulierten Struktur wurde weitgehend vorbereitet.The 'stuffed high quartz' ß-eucryptite (LiAlSiO4) is known for its anisotropic Li-ionic conductivity and its zero thermal expansion.The temperature driven phase sequence of ß-eucryptite was studied, with emphasis on the modulated structure. The satellite reflections are heavily broadened with respect to normal reflections. They overlap with each other and 'a-reflections' in between. Such 'triplett-intensities' could not be separated by standard data collection techniques. Thus 'axial q-scans' have been developped.Intensities were drawn from 2000 profiles in a serial automated procedure with the newly-developped least squares program GKLS and were successfully scaled to standard data. Using broadened reflections is shown to be legitimate.The broadening obviously resulted from a decreased long range order of the modulation to 11 to 16 periods (analysis using the lattice function). It corresponds to an extraordinary theta-dependent behaviour of the reflection widths and correlates with typical anti-phase domain diameters (other authors).A decrease in Si/Al order seems to determine decreased domain sizes and long range order as well as properties such as a/c-ratios, thermal expansion coefficients, ionic conductivity, structure type and phase transition temperatures. Changes in SiO2-content, temperature or the Si/Al order invoke comparable changes in certain properties.The average structure of the modulated phase has been determined. The role of Li has been characterized. Doubts about the hexagonal symmetry have been destroyed and the solution of the modulated structure has been prepared

Effect of varying Bi content on the temperature-dependent mechanical, dielectric, and structural properties of nominal Na1/2_{1/2}Bi1/2_{1/2}TiO3_3

Na$_{1/2}$Bi$_{1/2}$TiO$_3$ (NBT) with varying Bi content has gained significant interest as a potential new material for solid-oxide fuel cells and oxygen separation membranes because of its excellent oxygen-ion conductivity. In this work, the effect of varying Bi content in NBT ceramics of compositions Na$_{1/2}$BixTiO$_{2.25+1.5x}$, where x = 0.485–0.510, on the temperature-dependent mechanical and dielectric properties and the crystal structure has been investigated, as these applications expose the components to high thermal and mechanical fields. The effects of Bi variation on phase compositions and structural transitions were systematically investigated by scanning electron microscopy-energy dispersive x-ray analyses and neutron diffraction at room temperature, in situ high-temperature x-ray diffraction, dielectric permittivity, and mechanical measurements. In-depth analysis of the temperature-dependent data shows that the Bi content of the samples does not alter the average crystal structure of the NBT; however, the temperature-dependent behavior of the latter depend on variations in Bi content and the associated oxygen vacancy concentration. This change in phase transition temperature displays a good correlation with the temperature-dependent ferroelastic response and with the Bi content

Erratum: Natural mullites : Chemical composition, crystal structure, and optical properties (European Journal of Mineralogy (2019) 31 (353-367) DOI: 10.1127/ejm/2019/0031-2812)

[ no abstract available

A new mineral from the Bellerberg, Eifel, Germany, intermediate between mullite and sillimanite

A mineral intermediate between sillimanite and mullite, tentatively designated as sillimullite, was studied by electron microprobe analyses and single-crystal X-ray diffraction methods. The chemical compositions derived from the microprobe results and the crystal-structure refinement are Al7.84Fe0.18 Ti0.03Mg0.03Si3.92O19.96 and Al8.28Fe0.20Si3.52O19.76 (Fe is Fe3+) corresponding to x-values of 0.02 and 0.12, respectively, in the solid-solution series Al8+4xSi4-4xO20-2x assigning Fe3+, Ti, and Mg to the Al site. The composition derived from microprobe analysis is very close to a stoichiometric sillimanite (with Fe3+,Ti, and Mg assigned to Al sites), while the composition derived from diffraction data is midway between sillimanite and Si-rich mullites. The discrepancy is assumed to be caused by the occurrence of amorphous nano-sized SiO2 inclusions in the aluminosilicate phase not affecting the diffraction data but detected in the microprobe analysis. Sillimullite crystallizes in the orthorhombic space group Pnam with a = 7.5127(4), b = 7.6823(4), c = 5.785(3) angstrom, V = 333.88(4) angstrom(3), Z = 1. It has a complete Si/Al ordering at tetrahedral sites like sillimanite but with neighboring double chains of SiO4 and AlO4 tetrahedra being offset by 1/2 unit cell parallel to c relative to each other causing the change of the space-group setting from Pbnm (sillimanite) to Pnam. Difference Fourier calculations and refinements with anisotropic displacement parameters revealed the formation of oxygen vacancies and triclusters as known in the crystal structures of mullite. Final refinements converged at R1 = 5.9% for 1024 unique reflections with F-o > 4 sigma(F-o). Fe was found to reside predominantly in the octahedral site and with minor amounts in one of the T* sites. Mg and Ti were not considered in the refinements. The crystal studied here is considered to represent a new mineral intermediate between sillimanite and mullite, named sillimullite

Decrease of the required dopant concentration for δ-Bi2O3 stabilization through thermal quenching during single-step flame spray pyrolysis synthesis

&delta;-Bi2O3 is one of the best oxygen ion conductors known. However, due to its limited thermal stability and complicated synthesis techniques, the applications are limited. Here, the synthesis of stable nano-sized &delta;-Bi2O3 using versatile and rapid flame spray pyrolysis (FSP) combined with in-situ Ti and/or Mn doping for an enhanced thermal stability is reported for the first time. Exceptionally low Bi replacing cation concentrations (8 at.% Ti) were sufficient to obtain phase-pure &delta;-Bi2O3 which was attributed to the extraordinary high temperature gradient during FSP. The required cation amount for &delta;-phase stabilization was even further reduced by introducing mixtures of Mn and Ti (2.5 at.% Mn + 2.5 at.% Ti). Rietveld analysis revealed that the &delta;-Bi2O3 structure is best represented by the Fm϶m space group containing two closely neighbored 8c and 32f Wyckoff positions. Depending on the amount of Mn/Ti cations, about 25% of the possible oxygen positions remain vacant suggesting high bulk oxygen mobility. The enhanced oxygen mobility was confirmed by temperature programmed reduction (H2-TPR) with bulk reduction for &delta;-Bi2O3 in contrast to exclusive surface reduction for &beta;-Bi2O3.</p

Screening Precursor–Solvent Combinations for Li₄Ti₅O₁₂ Energy Storage Material Using Flame Spray Pyrolysis

The development and industrial application of advanced lithium based energy-storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility, and cost. In the present study, phase pure anode material Li₄Ti₅O₁₂ (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point, and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass %) and homogeneous LTO nanoparticles of size 4–9 nm, attributed to two reasons: (1) stabilization of water sensitive metal alkoxides precursor and (2) formation of volatile carboxylates from lithium nitrate evidenced by attenuated total reflection Fourier transform infrared spectroscopy and single droplet combustion experiments. In contrast, the absence of carboxylic acids resulted in larger inhomogeneous crystalline titanium dioxide (TiO₂) particles with significant reduction of LTO content as low as ∼34 mass %. In-depth particle characterization was performed using X-ray diffraction with Rietveld refinement, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry, gas adsorption, and vibrational spectroscopy. High-resolution transmission electron microscopy of the LTO product revealed excellent quality of the particles obtained at high temperature. In addition, high rate capability and efficient charge reversibility of LTO nanoparticles demonstrate the vast potential of inexpensive gas-phase synthesis for energy-storage materials

Crystal Growth, Crystal Structure, Optical Properties, and Phase Transition of BaCaBO₃F

A crystal of BaCaBO₃F was grown by the Czochralski method. It shows a phase transition at 242.7 °C upon cooling where the hexagonal high-temperature modification is transformed to a monoclinic low-temperature form (space group <i>C</i>2, <i>a</i> = 17.8779(8) Å, <i>b</i> = 9.0596(4) Å, <i>c</i> = 12.9735(5) Å, β = 118.875(13)°, <i>V</i> = 1840.03(22) ų, <i>Z</i> = 18). The monoclinic phase has a symmetry lowered by an index of 18 relative to the hexagonal supergroup. This represents a higher symmetry than the rhombohedral model which has a symmetry of index 36 lower than the hexagonal form determined in a previous study (Li and Zeng, J. Cryst. Growth 2013, 382, 47). The monoclinic system is confirmed by optical investigations with a polarizing microscope showing biaxial negative character and refractive indices <i>n</i>_<i>x</i> = 1.615(1), <i>n</i>_<i>y</i> = 1.664(1), and <i>n</i>_<i>z</i> = 1.665(1), 2V_<i>x</i> = 10.0(2)° as measured on a microrefractometer spindle stage in immersion liquids. Electron microprobe analysis yielded a slight excess of Ba resulting in the composition Ba_1.04Ca_0.96BO₃F corresponding to ^[7]Ba_4.7^[8]Ba₆^[9]Ba₄^[10]­Ba₄^[7]Ca_17.3^[3]B₁₈O₅₄F₁₈ in a crystal-chemical notation for the unit-cell content with the cations in their respective coordination. On the basis of this composition, the mean refractive index <<i>n</i>> can be calculated from individual electronic polarizabilities of the ions resulting in <<i>n</i>_calc> = 1.638 which is close to the observed value of <<i>n</i>_obs> within an error of 0.5%