Reinhard X. Fischer

Refractive Index and Dispersion of Fluorides and Oxides

The refractive indices of 509 oxides and 55 fluorides were analyzed using two forms of a one-term Sellmeier equation: (1) 1/(n2−1)=−A/λ2+B, where A, the slope of the plot of (n2−1)−1 versus λ−2 in units of 10−16 m2, gives a measure of dispersion and B, the intercept of the plot at λ=∞, gives n∞=(1+1/B)1/2 and (2) n2−1=EdEo/(Eo2−(ℏω)2), where ℏω=the photon energy, Eo=the average single oscillator (Sellmeier) energy gap, and Ed=the average oscillator strength, which measures the strength of interband optical transitions. Form (1) was used to calculate n at λ=589.3 nm (nD) and n at λ=∞ (n∞), and the dispersion constant A. The total mean polarizabilility for each compound was calculated using the Lorenz–Lorentz equation: αe=3/4π [(Vm) (n∞2−1)/(n∞2+2)], where Vm is the molar volume in A3. Provided for each compound are: nD, n∞, Vm, 〈αe〉, 〈A〉, 〈B〉, 〈Ed〉, 〈Eo〉, the literature reference, the method of measurement of n and estimated errors in n. Results obtained by prism, infrared reflectivity, ellipsometry, and i...

Formation of aluminum rich 9:1 mullite and its transformation to low alumina mullite upon heating

The formation of aluminum rich mullites Al4 + 2x Si2 − 2xO10 − x with x > 23 has been studied at annealing temperatures between 700 and 1650 °C. Calcination of the amorphous precursor at 700 °C yields a mullite with 88 mol% Al2O3 corresponding to an x-value of 0·809. Simultaneously, a γ-alumina phase is formed. Further increase of the annealing temperature yields an increase in the aluminum incorporation up to 92·1 mol% Al2O3 at 1000 °C derived from the refined lattice constants. This is the highest amount of Al observed so far in a mullite except the supposed end member ι-Al2O3 which, however, has not yet been established unambiguously. Above 1000 °C, the aluminum content in mullite is reduced. This is accompanied by a transformation of the spinel-type phase to a superstructure of a θ-alumina like phase. The final product at 1650 °C consists of 34 mol% of a ‘normal’ mullite with x = 0·32 and 66 mol% corundum.

INVESTIGATION OF THE CLAY FRACTION (<2 μm) OF THE CLAY MINERALS SOCIETY REFERENCE CLAYS

We studied a set of 15 reference clays from The Clay Minerals Society (CMS) Source Clays repository. Our aim was to use them as reference materials in our version of the QUAX mineral database. The QUAX software (Quantitative Phase-Analysis with X-ray Powder Diffraction) has been used successfully at the KTB site (German Continental Deep Drillling) to determine mineral assemblages quickly, in an automatic fashion, on a large number of samples (∼40,000). It was also applied to Quaternary marine sediments of the Japan Sea. Our current research focuses on marine and lacrustrine sediments from the Arctic Ocean and Siberia.QUAX is a full-pattern method using a reference materials database. The quality of a particular quantification depends on the availability of the relevant mineral phases in the database. Our aim is to extend and improve the database continuously with new data from our current projects, particularly from clay and feldspar minerals.A reference material in the QUAX software must be monomineralic. Before X-ray diffraction (XRD) patterns of CMS clays could be added to the database, quantification of any impurities was necessary. After measuring the bulk material by XRD, the <2 µm fraction was separated because we assumed it would contain the smallest amount of impurities. Here we present grain-size data, XRD data and X-ray fluorescence (XRF) data for this clay-sized fraction. The results of chemical and mineralogical preparation techniques and (elemental) analysis methods were combined. For XRD, random and oriented clay-aggregate samples as well as pressed pellets for QUAX analysis were prepared. Semi-quantitative clay mineral determinations were run for comparison.

Rietveld analysis of dicalcium aluminate (Ca2Al2O5)—A new high pressure phase with the Brownmillerite-type structure

Abstract Dicalcium aluminate (Ca2Al2O5) was prepared in a piston cylinder apparatus at 1250 °C and 2.5 GPa. The compound is orthorhombic with space group symmetry /2mb, a = 5.2281(1) Å, b = 14.4686(2) Å, c = 5.4004(1) Å (Z = 4, Dcalc = 3.481 g/cm3), and belongs to the brownmillerite structure family. Main building units are (1) layers of perovskite type corner connected AlO6-octahedra perpendicular to [010], and (2) zweier single chains of AlO4-tetrahedra running parallel [100]. The alternate stacking of the layers and sheets of single chains results in a three dimensional network in which the calcium ions are incorporated for charge compensation. The present structure is the first example for an alkaline earth aluminate with zweier single chains of tetrahedra.

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.

Symmetry relationships of sodalite (SOD) : type crystal structures

Abstract The mineral sodalite with the ideal composition Na8[Al6Si6O24]Cl2 is the prototype for numerous related crystal structures based on the same topology of the underlying net of strong bonds. This net has been assigned the code SOD by the Structure Commission of the International Zeolite Association. Minerals and synthetic compounds with a SOD-type framework represent the most comprehensive family of zeolite-type compounds with the highest number of published crystal structures. Among these are aluminosilicates (e.g., sodalite, haüyne, helvine, lazurite, nosean, tsaregorodtsevite), beryllosilicates (e.g., danalite, genthelvite, tugtupite), chlorides, sulfides (e.g., tetrahedrite, binnite, freibergite, galkhaite, goldfieldite, tennantite), borates (rhodizite), synthetic phosphates, aluminates, phosphides, nitrides and clathrate hydrates which have been shown by crystal structure analyses to adopt the SOD-type framework. More than 900 SOD-type crystal structures, which represent over 18% of the total number of published zeolite structures, are known. The complete symmetry relationships of SOD-type crystal structures comprising 27 space groups are listed in a Bärnighausen-tree together with the type and index of symmetry reduction. No other zeolite type displays such a rich harvest of symmetry as the sodalite-type. Seven additional space groups reported for SOD-types have not been considered here because they were not well supported by experimental evidence. Of the 26 low-symmetry derivatives six are due to an ordering of tetrahedrally coordinated framework cations. In two cases the lowering of symmetry is caused by some of the framework cations assuming 5- or 6-coordinations instead of the usual 4-coordination. This means that in 18 cases the lower space group symmetries are achieved by the influence of pore-filling matter.

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.

Crystal structures of Na and K aluminate mullites

Abstract Mullite-type alkali aluminates (KyNa1-y)0.67Al6O9.33 were synthesized from amorphous Al and alkali nitrates by sol-gel techniques. Rietveld refinements of six members of the solid solution series (y = 0.0, 0.2, ..., 1.0), together with Fourier syntheses and grid search analyses show that the Na and K atoms reside in the vacant Oc sites, with K at 1/2, 0, 1/2 and Na on a split site off the special position. The number of alkali atoms is restricted to 2/3 atoms per unit cell due to crystal chemical constraints. Consequently, unlike the aluminosilicate mullites, alkali mullites do not form a solid solution series with varying oxygen composition. All compounds studied here crystallize in space group Pbam with lattice constants ranging from a = 7.6819(4) Å, b = 7.6810(4) Å, c = 2.91842(8) Å for the Na aluminate to a = 7.6934(3) Å, b = 7.6727(3) Å, c = 2.93231(7) Å for the K aluminate mullite

Crystal growth and cation distribution in doped dicalcium ferrite Ca2(Fe1-x Mex)2O5 (Me = Al3+,Ga3+)

The crystal structures of Ca 2 (Fe 0.933 Al 0.067 ) 2 O 5 and Ca 2 (Fe 0.722 Ga 0.278 ) 2 O 5 have been determined from single crystals grown from a CaCl 2 flux. Both are isostructural with Ca 2 Fe 2 O 5 (C 2 F), belonging to space group Pnma. Lattice constants are a = 5.420(1) A, b = 14.743(3) A, c = 5.597(1) A and a = 5.420(1) A, b = 14.721(3) A, c = 5.599(1) A, respectively. The Al 3+ ions in the doped C 2 F occupy the tetrahedral 4(c) site of Pnma exclusively. In the gallium doped sample, the Ga 3+ ions preferentially reside on the tetrahedral positions with a significant amount of about 23% of the total gallium content in the octahedrally coordinated 4(a) site.