Ann-Kristin Larsson

Elucidation of the crystal structure of oxyapatite by high-resolution electron microscopy.

High-resolution electron-microscopy (HREM) images from different hydroxyapatite (OHAp) samples showed p3 projection symmetry along [001] instead of the p6 projection symmetry compatible with the space group P6_3/m of OHAp. Image processing was used to establish without ambiguity that threefold symmetry dominates the images along [001]. OHAp crystals decompose in the transmission electron microscope and it is concluded that the threefold symmetry observed corresponds to an early step in the decomposition process, the dehydration of OHAp to oxyapatite (OAp). A structural model for OAp where every second O atom along the 6(3) axis in OHAp is removed has the maximal space-group symmetry P{\bar 6}. This is compatible with the p3 projection symmetry observed. Atomic shifts in this OAp model compared to the OHAp structure were estimated using the HREM images and geometric optimizations of the atomic structure. No refinements of the atomic coordinates against diffraction data were possible but the simulated HREM images of this crude model fit well with the experimental images.

On the question of commensurability : the Nowotny chimney-ladder structures revisited

A composite approach was used for the structure solution and refinement of two new Nowotny chimney-ladder phases, (Mo/Rh)11Ge18 and (Mo/Rh)13Ge22, found during investigations in the Mo/Rh/Ge system. The descriptions of the phases with conventional models (in I¯42d(No. 122) with a=5.885(2) Å, c=52.62(2) Å for (Mo/Rh)11Ge18 and a=5.922(1) Å, c=62.88(4) Å for (Mo/Rh)13Ge22) led to high numbers of unobserved reflections, high parameter/reflection ratios and an unstable refinement in the case of (Mo/Rh)13Ge22. Microscopy studies showed that the phases were in fact incommensurately modulated. Therefore, a composite model consisting of a rather stable diamond-like transition-metal substructure and a helical main-group-metal arrangement was developed. For the refinement of both phases, (Mo/Rh)11Ge18 as well as (Mo/Rh)13Ge22, the same model – same space group pair (W: P4/nnc:q¯1q1 P: I41/amd: 1¯1ss), same atom positions and modulation waves – was used. The refinements converged rapidly, showing that the presented structure model is very general, and may well be applicable to any Nowotny chimney-ladder phase.

On the carbothermal vapour–liquid–solid (VLS) mechanism for TaC, TiC, and TaxTi1–xC whisker growth

The growth of TaC, TiC and TaxTi1-xC whiskers has been studied in some detail. The whiskers were synthesised via a vapour-liquid-solid (VLS) growth mechanism in the temperature range 1220-1400 degr ...

An electron diffraction study of modulated Ni1+xGe B8 type phases

Abstract Structural modulations of NiAs-Ni 2 In type structures within the B8 phase field of the Ni-Ge system have been investigated with electron diffraction. The modulations of the average B8 substructure (caused by ordering of interstitial Ni atoms in trigonal bipyramidal sites) can in reciprocal space be described by H = G +m q , where G are the Bragg reflections corresponding to the P6 3 /mmc average substructure, m is an integer and q is a primary modulation-wave vector of the form [h,h,2h,l]*. Ni 5 Ge 3 is described by q = 1 / 3 [1122]* which corresponds to a =11.682(6) A, b =6.737(3) A, c =6.264(3) A, b =52.11(1)° and space group C2/c. Ni 19 Ge 12 and Ni 17 Ge 12 are both described by q = 1 / 6 [2243]*. They are each others antistructure with respect to the interstitial sites and have hence identical primary modulation-wave vectors. Ni 7 Ge 4 exhibits a complicated diffuse distribution partly condensed out at q 1 ≈0.18[2243]* and q 2 ≈0.27[1122]*. The diffuse scattering resembles the undulating channels along c * previously observed for Co 1+ x Sn and Ni 1+x Sn but in Ni 7 Ge 4 two independent sets, cutting through q 1 and q 2 respectively, are present.

Meditation on an Engraving of Fricke and Klein (The Modular Group and Geometrical Chemistry)

A non-technical account of the links between two-dimensional (2D) hyperbolic and three-dimensional (3D) euclidean symmetric patterns is presented, with a number of examples from both spaces. A simple working hypothesis is used throughout the survey: simple, highly symmetric patterns traced in hyperbolic space lead to chemically relevant structures in euclidean space. The prime examples in the former space are derived from Felix Kleins engraving of the modular group structure within the hyperbolic plane; these include various tilings, networks and trees. Disc packings are also derived. The euclidean examples are relevant to condensed atomic and molecular materials in solid-state chemistry and soft-matter structural science. They include extended nets of relevance to covalent frameworks, simple (lattice) sphere packings, and interpenetrating extended frameworks (related to novel coordination polymers). Limited discussion of the projection process from 2D hyperbolic to 3D euclidean space via mapping onto triply periodic minimal surfaces is presented.

On twinning and microstructures in calcite and dolomite

Abstract Apparent superlattice reflections obtained in electron diffraction patterns of modulated calcite and dolomite have been ascribed to local domains of various different metastable Ca-Mg ordering schemes that are not known as macroscopic phases. We show that the type “c” reflections in diffraction patterns of supposed superstructures can in fact be produced by superposition of diffraction from the host crystal and that from domains that are in an orientation related to the host by twinning on (104). From details of the additional reflections present, we deduce that the carbonate anions are orientationally disordered in the twin nanodomains, which have the R3̅m space group of high-temperature disordered calcite. This twinning can explain the diffraction ascribed to type “γ”/”μ”/”ν” superstructures, and resolves controversies over the occurrence of these purportedly different superstructures. The relationships between composition, orientational order of the carbonates, molar volume and known macroscopic structures, and the possibility of interfacial strain reduction by static disorder in the twin domain, are discussed. We stress the importance of checking for presence of twinned nanodomains using microdiffraction before attributing an apparent superstructure modulation to local cation ordering. High-resolution imaging may not be diagnostic, since overlap of small domains and host matrix can give Moiré patterns that resemble superlattice fringes.

Incommensurately modulated Nowotny chimney-ladder phases : Cr1-xMoxGe~1.75 with x=0.65 and 0.84

Abstract The crystal structures of two new ternary, incommensurately modulated phases in the Cr/Mo/Ge system, Cr1-xMoxGe∼1.75 with x = 0.65 and 0.84, have been refined using the same composite approach as for the earlier reported compounds, (Mo/Rh)11Ge18 and (Mo/Rh)13Ge22, from the Mo/Rh/Ge system. The composite model consists of a rather stable diamond-like transition metal substructure and a helical main group element arrangement. The same super space group pair, W:P4/nnc: q1̅q1 P:I41/amd: 11̅ss, the same positions and modulation waves were used. The refinements of the new phases in the Cr/Mo/Ge system converged rapidly to R-values below 5% with GOF of 2.53 and 3.71, respectively. Each structure is built by two sub lattices with the parameters - a, cx and the modulation vector qx - for Ge and a, cT and qT for the transition metals. The refined data for x = 0.65 are found to be a = 5.935(2), cx = 2.770(1), cT = 4.839(1), qx = 0.428 [001]*, qT = 0.253 [001]* and for x = 0.84, a = 5.963(1), cx = 2.786(1), cT = 4.875(1), qx = 0.430 [001]*, qT = 0.264 [001]*. It is clearly shown the composite approach is applicable to different chimney-ladder compounds, even though they have quite different compositions and hence q-vectors.

Tegengrenite, a new, rhombohedral spinel-related Sb mineral from the Jakobsberg Fe-Mn deposit, Värmland, Sweden

Abstract Tegengrenite, forming octahedra up to 150 μm in size, occurs associated with hausmannite, calcite, brucite, dolomite, clinohumite, kinoshitalite, native copper, barytocalcite, bindheimite, and cerussite in Mn ore from Jakobsberg, Filipstad district, Värmland, Sweden. The mineral is deep red and translucent, with a sub-adamantine luster. In reflected light it is gray and nearly isotropic, with measured reflectance values (R%) 10.4 (λ = 470 nm), 10.0 (546 nm), 9.9 (589 nm), and 9.8 (650 nm). The refractive index at λ = 589 nm is 1.92(2). There is no cleavage; fracture is conchoidal. Dcalc is 4.58(1) g/cm3. Electron-microprobe analyses gave (average of 35 points; in wt%) MgO 21.83, Al2O3 0.76, SiO2 1.70, Sb2O5 36.13, TiO2 1.40, Fe2O3 0.78, MnO 25.74, Mn2O3 (calculated from stoichiometry) 8.14, ZnO 2.66, sum 99.14, yielding the empirical formula Mg2+1.22Mn2+ 0.82Zn0.07 (Sb0.50Mn3+ 0.23Si0.06Ti0.04Al0.03Fe0.02)O4. Combined X-ray and electron diffraction studies show that tegengrenite is rhombohedral, space group R3̄ or R3, and pseudocubic. The individual tegengrenite octahedra consist of eight twin domains that give rise to complex diffraction patterns and make them unsuitable for a conventional structure determination. Tegengrenite, like the chemically related mineral filipstadite, bears a close structural relationship to spinel. The eight strongest lines in the X-ray diffraction pattern are [d (in angstroms) (I/I0)(hkl)]: 4.98(20)(211, 003), 4.32(19)(122), 4.24(18)(113), 3.052(33)(140, 214), 2.608(100)(241, 143, 125), 2.162(28)(244), 1.665(30)(363, 075), 1.531(26)(820) and 1.527(29)(428). The refined unit-cell parameters (hexagonal setting) are a = 16.196(1) and c = 14.948(2) Å with Z = 42; for the cubic spinel-type subcell A = c/√3 - = 8.63 Å. The new species is named for Felix Tegengren (1884-1980).

The structure of Nb5O12F

Abstract The title compound was synthesized according to Andersson Acta Chem. Scand. 18 (1964) 2233. The structure is an incommensurately modulated variation of the UO3 structure, in a (3+1)-dimensional super space description isostructural to the composite phases (1−x)Ta2O5•xWO3 Acta Crystallogr., Sect. B 52 (1996) 223 and Acta Crystallogr., Sect. B 51 (1995) 709. The compound crystallizes in the orthorhombic system with cell parameters 3.936(3), 6.153(2), 3.656(1) A. The superspace group is X:Pmmm(00γ):1s 1 , where X refers to a non-standard superspace centering with the lattice vector (0 1 2 1 2 1 2 ) , and γ=1.61 is the modulation wave vector.

Twinning and defects in N-Nb2O5

N-Nb(2)O(5) [C2/m (No. 12), a = 28.51, b = 3.830 and c = 17.48 A, and beta = 124.8 degrees ] has been investigated by means of selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). N-Nb(2)O(5) is domain twinned, with the twin plane perpendicular to the c* axis. The domains are rather small and the domain twinning can sometimes be best explained as stacking faults. A second type of coherent twinning at an angle of 90 degrees to the other two domain directions was also found. These domains are linked together by areas containing blocks of different sizes, similar to the disordered block arrangement observed in M-Nb(2)O(5).