Hartmut Schneider

Crystal chemistry of borates and borosilicates with mullite-type structures: a review

Boron compounds are included in the family of mullite-type structures if they contain chains of edge-sharing M O 6 octahedra ( M are the octahedrally coordinated cations in the chains) similar to those in mullite. The crystal structures of all boron compounds in the mullite family are derived from a tetragonal aristotype and should obey the criteria defining the limits for the deviation from the root structure. The compounds complying with these criteria for mullite-type crystal structures have an observed maximum deviation from orthogonality by Δγ′ = 4° (ominelite: γ ′ = 86.0°, mullite: 90°), a range of octahedral tilting angles ω between 61.5° (boromullite) and 90° (for 2:1 mullite it is 59.8°), a range of distances between neighboring chains normalized to the ionic radii of the central atoms between Q r = 8.2 % (PbCrBO 4 ) and 13.8 % (PbMnBO 4 , for 2:1 mullite it is 9.9 %), and a maximum deviation from tetragonal metric ( a = b , Q a = 1) with Q a = 0.78 (PbMnBO 4 ; for 2:1 mullite Q a = 0.986). The average value of all mean B–O distances in BO 3 groups is 1.383 A, which compares well with known trigonally coordinated B–O distances. It is recommended to designate all boron compounds with the characteristic mullite-type M O 4 chains of M O 6 octahedra as “mullite-type boron compounds” and to use the term “boron-mullite” or “B-mullite”, initially introduced by Werding & Schreyer (1984) (Geochim. Cosmochim. Acta, 48 , 1331–1344) for the subgroup of Al borates and Al borosilicates with mullite-type structures. The name “boromullite” is reserved for a natural mineral with sillimanite-like and Al 5 BO 9 -like modules.

Crystal structure of synthetic Al4B2O9 : a member of the mullite family closely related to boralsilite

Abstract The crystal structure of Al4B2O9, synthesized from Al(NO)3·9H2O and B(OH)3 via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with 11B and 27Al MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al32B16O72 is closely related to the boralsilite (Al32B12Si4O74) structure with Si replaced by B and to mullite (Al4+2xSi2-2xO10-x). It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) Å, b = 5.5413(2) Å, c = 15.0531(6) Å, β = 90.913(2)°, Z = 8 for Al4B2O9. The main structural units are isolated chains of edge-sharing AlO6-octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO4, AlO5, BO3, and BO4 groups with two B atoms and one O atom (O5′) disordered on interstitial positions. 27Al and 11B NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold:[fourfold + fivefold] = ~50%:50%) and of threefold and fourfold coordinated B (~80%:20%).

Crystal chemistry and properties of mullite-type Bi2M4O9: An overview

Abstract Bi2M4O9 (M = Al3+, Ga3+, Fe3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M2O7 tetrahedral groups and by BiO polyhedra. The Bi3+ cations in Bi2M4O9 contain stereo-chemically active 6s2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi2Mn4O10 are similar to those of Bi2M4O9, Bi2Mn4O10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s2 LEPs of Bi3+ in Bi2Mn4O10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi2(Al1–xFex)4O9 and Bi2(Ga1–xFex)4O9 (x = 0–1). Things are more complex in the case of the Bi2(Fe1–xMnx)4O9+y mixed crystals, where a miscibility gap occurs between x = 0.25–0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe3+ favorably replaces Mn3+ in the pyramids. The crystal structure of Bi2M4O9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature-induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi3+ is substituted by Sr2+ in Bi2Al4O9 corresponding to a composition of (Bi0.94Sr0.06)2Al4O8.94. Sr2+ for Bi3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 · 10−2 S m−1 at 1073 K, 800°C). An explanation could be the low thermal stability of Sr-doped Bi2Al4O9, especially in coexistence with liquid Bi2O3. Therefore, Bi2Al4O9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca2+, Sr2+) have not been produced yet. Thus the question whether or not M2+-doped Bi2M4O9 is an oxygen conducting material is still open.

First-principles study on variation of lattice parameters of mullite Al4+2xSi2−2xO10−x (x = 0.125, 0.250, 0.375)

Abstract Mullite (Al4+2xSi2-2xO10-x) is known to exhibit a very distinct compositional variation in which the lattice constant in the a direction expands linearly, while that in the b direction slightly contracts with an increase of the x-value. In this study, first-principles density functional theory (DFT) simulations were applied to examine the cause. The atomic structure and charge density were examined. We found that the local charge redistribution due to a newly formed vacancy leads to the onset and recurrence of localized atomic relaxation. The charge redistribution and atomic relaxation causes the clockwise and counterclockwise rotations of the neighboring octahedral units. These rotations contribute to expansion in the a-axis and contraction in the b-axis. The mechanism is supported by the previous experimental measurements of the Al-O2 bond lengths projecting in the (001) plane. We conclude that the rotations of the octahedral units are the fundamental mechanism responsible for compositionally induced variations of mullite. Results derived from simulations also provide evidence for a preferred occurrence of oxygen vacancies parallel to the crystallographic b axis. It thus supports earlier findings of a partial ordering in mullite.

Electrical conductivity of synthetic mullite single crystals

Abstract The electrical conductivity of 2/1-mullite (approximate composition 2Al2O3·SiO2) was measured using plane parallel, polished plates cut perpendicular to [100], [010], and [001] from a large single crystal grown by the Czochralski method. Impedance spectra were recorded in the 1 Hz to 1 MHz frequency range at temperatures from 550 to 1400 °C in air. The conductivity vs. temperature curves display changes of their slope between 850 and 950 °C depending on the crystallographical direction. The low-temperature region (T < 850 °C) of conductivity is characterized by low-electrical conductivities (σav ≈ 5.4 × 10-9 Ω-1cm-1, average conductivity at 550 °C) with σ[010] > σ[100] > σ[001] and low-activation energies (≈0.66 eV, average value). In the high-temperature region (T > 950 °C) the electrical conductivity is significantly higher (σav ≈ 1.1 × 10-5 W-1cm-1, average conductivity at 1400 °C) with σ[001] > σ- ≈ σ[010], and with higher activation energies (≈1.6 eV). While the conductivity in the low-temperature region essentially is electronic, ion conductivity dominates the conductivity in the high-temperature region. We believe that the ionic conductivity is essentially due to hopping of O atoms from structural sites linking the tetrahedral double chains in mullite toward adjacent oxygen vacancies especially in c-axis direction. These oxygen hoppings are associated with complex structural re-arrangements, which control and slow down the velocity of the processes. Thus the electrical conductivity of mullite at high temperature is much lower than, e.g., that of Y-doped zirconia, but is significantly higher than that of a-alumina.

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

Abstract 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) Å, V = 333.88(4) Å3, 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 ½ 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 Fo > 4s(Fo). 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.”

WHIPOX all oxide ceramic matrix composites

WHIPOX is a fiber-reinforced porous matrix CMC developed at the German Aerospace Center (DLR). The composite consists of commercial alumina or alumino silicate fibers and mullite, mullite/alumina, or alumina matrices. WHIPOX manufacturing is based on a continuous fiber bundle infiltration and winding process. After shaping in the moist stage, green bodies of WHIPOX are sintered pressureless in air. WHIPOX CMCs display quasi-ductile deformation and excellent thermal shock resistance. Depending on fiber and matrix compositions, interlaminate shear strength values of 10 MPa and Young’s moduli of ≃150 GPa can be obtained. Thermal conductivity perpendicular to fiber orientation is about 1W/mK. The simple winding and sintering techniques and the fact that no fiber coating is required makes WHIPOX an inexpensive CMC material.