Hannes Krüger

Incommensurately modulated ordering of tetra­hedral chains in Ca2Fe2O5 at elevated temperatures

The basic building units of brownmillerite-type A2B2O5 structures are perovskite-like layers of corner-sharing BO6 octahedra and zweier single chains of BO4 tetrahedra. A three-dimensional framework is formed by alternate stacking of octahedral layers and sheets of tetrahedral chains. The compound Ca2Fe2O5 is known to have Pnma symmetry at ambient conditions. The space group Imma was reported to be evident above 963 K. New high-temperature single-crystal X-ray diffraction experiments at 1100 K revealed that Ca2Fe2O5 forms an incommensurately modulated structure adopting the superspace group Imma(00gamma)s00, with gamma = 0.588 (2). The modulation affects the sequence of the enantiomorphic (right- and left-handed) oriented tetrahedral chains within the layer, breaking the lattice periodicity along c. This ordering can be modelled with crenel occupation modulation functions for the tetrahedrally coordinated Fe, as well as for the O atom interconnecting the tetrahedra.

Incommensurate structure of Ca2Al2O5 at high temperatures – structure investigation and Raman spectroscopy

A high-temperature X-ray diffraction study revealed that brownmillerite-type Ca(2)Al(2)O(5) transforms to an incommensurately modulated structure at elevated temperatures. Single crystals of Ca(2)Al(2)O(5) were synthesized in an end-loaded piston cylinder press at 2.5 GPa and 1273 K. The diffraction pattern observed at 1090 (10) K by in situ single-crystal diffraction experiments can be indexed by an I-centred orthorhombic cell and a modulation wavevector of q = 0.595 (1)c(*). A (3 + 1)-dimensional model in superspace group Imma(00gamma)s00 was used to refine the modulated structure. The structure is assembled from two building units: (i) layers of corner-sharing [AlO(6)] octahedra, stacked along b alternate with (ii) layers of zweier single chains of [AlO(4)] tetrahedra running along a. The modulated structure arises from an aperiodic sequence of two different configurations of the chains within the tetrahedral layers. The modulated high-temperature phase of Ca(2)Al(2)O(5) is isotypic to the modulated high-temperature modification of Ca(2)Fe(2)O(5). A large hysteresis was found in the phase-transition temperature. On heating, the transition occurs at ca 1075 (10) K; on cooling, satellite reflections can be observed down to 975 (10) K. The characterization of Ca(2)Al(2)O(5) is completed by Raman spectroscopy, including a partial interpretation of the spectra.

Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars

The structural variation of monoclinic C2/m alkali feldspars has been analysed, based on 50 previously-determined structures and 10 new structure refinements including three determinations of the structure of an Or90 orthoclase at temperatures of 600 K, 900 K and 1075 K. The influence of temperature, composition and state of Al,Si order on the overall average tetrahedral bond lengths of these structures is statistically insignificant. Analysis of the structures in terms of the tilts of the tetrahedra shows that the tilts evolve uniformly with unit-cell volume irrespective of whether the volume is changed by temperature or exchange of the extra-framework cation. There is a small but systematic decrease in the distortion of the T1 tetrahedron with increasing unit-cell volume. Comparisons of the refined structures with the results of geometric modelling show that volume changes are driven by exchange of the extra-framework cation, or by an increase in its thermal vibrations upon heating. The extreme anisotropy of the changes in the unit-cell parameters of monoclinic feldspars is not due to anisotropic interaction of the extra-framework cation with the anions of the framework, but due to the tilting of the tetrahedra controlled in part by O-O interactions. The anisotropy and tilting is not significantly modified by either Al,Si ordering or the distortions of the tetrahedra provided that the latter remain constant. The monoclinic feldspars show a slightly reduced anisotropy of strains as the unit-cell volume increases as a result of a decrease in the angular distortion of the T1 tetrahedron. The general conclusion is drawn that the pattern of anisotropy of the elastic properties (thermal expansion, compressibility, elastic compliances) and the cell-parameter changes of a tetrahedral framework structure with changing extra-framework species are intrinsic to the topology of the framework. The relative insensitivity of the anisotropy of the strains induced by volume changes to distortions of the tetrahedra also means that framework models which incorporate regular tetrahedra can be safely used to predict anisotropy, provided that the tetrahedral distortions do not change. If, as in the monoclinic feldspars, such a model does not reproduce exactly the observed anisotropy of the real structures, then this immediately indicates that there is a significant change in the distortion of the framework tetrahedra.

Ca5.45Li3.55[SiO4]3O0.45Fi1.55 and Ca7K[SiO4]3F3: single-crystal synthesis and structures of two trigonal oxyfluorides

Abstract Single crystals of the two title compounds have been synthesised utilising fluoride-based fluxes. X-ray diffraction (XRD) revealed that Ca5.45Li3.55[SiO4]3O0.45F1.55 crystallises in space group R-3m (a = 7.1365(6), c = 41.459(3)Å) and Ca7K[SiO4]3F3 exhibits space group P31m (a = 10.8161(5), b = 6.6469(3)Å). Both compounds show a non-stoichiometric composition, related to a coupled substitution of mono- and divalent cations compensated by the oxygen–fluorine contents. The Ca/Li ratio in the first compound is derived from the structural refinement, as well as from bond-valence sum (BVS) calculations. Electron microprobe analysis was performed to reveal the K/Ca ratio in the second structure, which shows a slight enrichment in potassium compared to the ideal stoichiometry. The distribution of K and Ca over two cation sites was studied with BVS calculations. Both structures belong to the group of nesosilicates (monosilicates) with additional anions. Furthermore, the structures are discussed in the context of related materials.

Ca2NaSiO4F: a new monoclinic polymorph

Abstract The crystal structure of a new monoclinic Ca2NaSiO4F polymorph was determined by single crystal X-ray diffraction methods. It crystallises in space group P21/c, unit cell parameters are a = 13.5385(8), b = 7.1338(4), c = 10.6689(6) Å and β = 113.231(4)°, with Z = 8. The crystals show twinning by pseudo merohedry. Monoclinic dicalcium sodium fluoride silicate is a superstructure of a previously known orthorhombic compound of the same composition. A re-investigation of the orthorhombic Ca2NaSiO4F revealed weak diffuse X-ray scattering, which shows a relation to the monoclinic superstructure. Furthermore, both compounds exhibit additional twinning by partial merohedry interrelated with a hexagonal pseudo symmetry. The sixfold pseudo symmetry originates from chains of face-sharing anion-centred coordination polyhedra of fluorine atoms, which are octahedrally coordinated by mixed Ca/Na sites. Main deviations from hexagonal symmetry are caused by the arrangement of the isolated silicate tetrahedra.

Thermal expansion of Li3Na3In2F12 garnet

The structural behaviour of Li3Na3In2F12 garnet (, Z = 8) has been studied as a function of temperature (100 K≤T≤579 K) using single-crystal and powder x-ray diffraction at atmospheric conditions. The temperature dependence of the coefficient of thermal expansion is α (K−1) = 2.0 × 10−5+2.4 × 10−8 T. The unit-cell volume change on cooling the Li3Na3In2F12 crystal from 298 to 100 K is equivalent to compressing it to about 0.5 GPa at room temperature. The effect of low temperature on the crystal structure is examined on the basis of structural parameters obtained from the refinements of the single-crystal data. The large coefficient of thermal expansion and the small bulk modulus of Li3Na3In2F12, in comparison with the oxide analogues, are due to massive changes in the In–F–Li interpolyhedral angle in the garnet framework at non-ambient conditions.

Superstructure of Mullite-type KAl9O14.

Large whiskers of a new KAl9O14 polymorph with mullite-type structure were synthesized. The chemical composition of the crystals was confirmed by energy-dispersive X-ray spectroscopy, and the structure was determined using single-crystal X-ray diffraction. Nanosized twin domains and one-dimensional diffuse scattering were observed utilizing transmission electron microscopy. The compound crystallizes in space group P21/n (a = 8.1880(8), b = 7.6760(7), c = 8.7944(9) Å, β = 110.570(8)°, V = 517.50(9) Å3, Z = 2). Crystals of KAl9O14 exhibit a mullite-type structure with linear edge-sharing AlO6 octahedral chains connected with groups of two AlO4 tetrahedra and one AlO5 trigonal bipyramid. Additionally, disproportionation of KAl9O14 into K β-alumina and corundum was observed using in situ high-temperature optical microscopy and Raman spectroscopy.

Monoclinic structure and nonstoichiometry of 'KAlSiO4-O1'.

Crystals of KAlSiO4-O1 (potassium aluminium silicate) were synthesized using a flux method and analysed utilizing single-crystal X-ray diffraction and electron microprobe analysis. Both methods confirm that the crystals are nonstoichiometric according to K(1-x)Al(1-x)Si(1+x)O4 with x = 0.04 (1). KAlSiO4-O1 is closely related to the stuffed derivatives of tridymite, although the topology of the Si/Al-ordered framework is different. Six-membered rings of UUDDUD and UUUDDD (U = up and D = down; ratio 2:1) configurations are present in layers parallel to the ab plane. In contrast, the framework of tridymite exhibits UDUDUD rings. The crystals are affected by inversion, pseudo-orthorhombic and pseudo-hexagonal twinning.

Na2Si3O7: an incommensurate structure with crenel-type modulation functions, refined from a twinned crystal

The structure of metastable, incommensurately modulated Na(2)Si(3)O(7) has been determined from single-crystal X-ray diffraction data. In contrast to previous investigations which stated that the compound crystallizes in an orthorhombic space group, this study shows that the compound is monoclinic with a pseudo-orthorhombic cell and is affected by twinning. The structure is described in the (3 + 1)-dimensional superspace. Crenel-type modulation functions are used to account for an aperiodic sequence of right- and left-handed zweier single chains of silicate tetrahedra. The modulation mainly affects one of the two symmetrically independent tetrahedral chains, which are connected to build up [Si(3)O(7)](2-) layers. Sodium cations are coordinated by five oxygen ligands and provide linkage between adjacent tetrahedral sheets. Distortions of the silicate tetrahedra and crystal chemical relationships of the title compound to sodium and lithium di- and metasilicates are discussed in detail.

Modulated structure and phase transitions of Sr10Ga6O19.

The crystal structure of Sr10Ga6O19 was investigated by in situ single-crystal X-ray diffraction in the temperature range 298-673 K. At ambient conditions the compound shows a (3 + 1)-dimensional modulated structure in the superspace group C2/c(0beta0)s0 [a = 34.9145 (13), b = 7.9369 (2), c = 15.9150 (7) A and beta = 103.551 (3) degrees] with a modulation wavevector of q = 0.4288 (2)b*. Whereas the presented structural model uses first-order harmonic modulation functions only, some features of the modulations are discussed utilizing an electron density derived by the maximum entropy method. Furthermore, two phase transitions were identified: between 453 and 503 K the incommensurate superstructure is replaced by a doubling of the a and b lattice constants, and between 503 and 673 K a phase with the basic cell is formed, identical to alpha-Sr10Ga6O19. Under some cooling conditions crystals showing a combined diffraction pattern of both superstructures can be obtained. The relation of these results to alpha-Sr10Ga6O19 [Kahlenberg (2001). J. Solid State Chem. 160, 421-429] is discussed.