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Journal logoSTRUCTURAL SCIENCE
CRYSTAL ENGINEERING
MATERIALS
ISSN: 2052-5206

June 2006 issue

Highlighted illustration

Cover illustration: Room-temperature structure of the cation-deficient perovskite, Nd0.7Ti0.9Al0.1O3 [Zhang et al. (2006). Acta Cryst. B62, 60-67>]. The superlattice peaks associated with out-of-phase octahedral tilting (shown) have intensities consistent with tilting around the x and y axes (sum of a- and c-), but tilting around the y axis (contribution b-) evidently does not occur. The tilt system in Glazer's notation is thus a-b0c-, and the space group and structure are found from the tree displayed. Two views of the structure are shown - the (TiAl)O6 are shown as octahedra, the Ti/Al atom being visible within them, and the darker and lighter spheres outside these octahedra represent fully and partly occupied layers of Nd, respectively. The octahedral tilting, around both x and y axes, is clearly seen.

feature articles


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An integrated and comparative study of more than 20 enantiotropic solid-state phase transitions shows that these are all first-order and proceed by a `nucleation and growth' mechanism in accordance with the description set out especially by Yuri Mnyukh.

research papers


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The crystal structures of six A2M3+M5+O6 ordered perovskites as determined from X-ray and neutron powder diffraction data are reported here. These samples show a high degree of pseudosymmetry that complicates the interpretation of the powder diffraction data. Correct space-group assignments were made through the systematic analysis of the supercell reflections, splitting of the fundamental perovskite peaks and crystal chemical considerations.

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The capabilities of the program SPuDS have been extended to predict crystal structures of A2MMX6 ordered double perovskites. Its accuracy and potential applications are discussed.

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Expectation values for size differences between pairs of `ions' (atoms in oxidation states ≠ 0) embedded into similar coordination environments are calculated from bond-valence parameter and ionic radii tables.

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Experimentally determined, halide-ion size differences are compared with expected sizes in the three series of isotypic bismuth chalcogenide halide compounds KBi6O9X, BiOX and BiSX.

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In situ high-pressure X-ray powder diffraction up to 42 GPa was performed in order to explore the compressibilities of the title compounds. The results show that the sialons MYb[Si4−xAlxOxN7−x] (x = 2; M = Sr, Ba) are more compressible than the nitridosilicate SrYb[Si4N7], while all phases are stable up to the highest pressures reached.

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A new approach based on the bond-valence matching relation is developed to predict the detailed structural evolution of GeFeO3-type perovskites at high pressure from knowledge of the room-pressure structure and the high-pressure unit-cell parameters alone.

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A (3 + 1)-dimensional superspace model for the incommensurate structure of Na2Si3O7 is presented and refined from twinned X-ray diffraction data.

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The atomic structures of 222- and 221-polybasite have been solved and refined from X-ray diffraction data from twinned crystals at 100 and 120 K, respectively. A possible mechanism regulating the type of unit cell that is stabilized is proposed.

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A comparative analysis of simple anhydrous salts My(LO3)z (L = S, Se, Te, Cl, Br, I) showed their relationships to six types of binary compounds (NaCl, NiAs, PoCl2, Tl2S2, ZnTe, rutile). It has been established that in 36 out of the 61 salts at least one ion array has the topology of close packing or the body-centred cubic lattice.

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The crystal structures of M(SO3CF3)2 (M = Mg, Ca, Ba, Zn, Cu) were determined using high-resolution X-ray powder diffraction data.

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The crystal structure of trans-PtCl2(dms)2 (dms is dimethyl sulfide) has been determined from single-crystal diffraction data and the pressure dependence of its unit-cell dimensions has been studied with diamond–anvil cells up to 8.0 GPa using powder diffraction. trans-PtCl2(dms)2 adopts the molecular point group Ci, which is the most frequently occurring point group for trans-PtX2L2 complexes (78%) as observed in the Cambridge Structural Database (CSD), followed by C1 (16%). No preference for either trans- or cis-PtX2L2 is found in the CSD.

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Schiff base structures containing pyridoxal (PL) and their 1:1 acid–base adducts are reported and serve as models for the coenzyme pyridoxal-5′-phosphate (PLP) in the dependent enzymes. They allow the study of the intramolecular OHN hydrogen bond of PL/PLP Schiff bases and the H-acceptor properties of their pyridine rings.

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Analyses of the crystal structures of enantiopure S,S-1,2-dicyclohexylethane-1,2-diol and its racemic compound suggest a reason why crystals of any enantiopure material might be expected to be less dense than crystals of the corresponding racemic compound and to be more likely to have Z′ > 1.

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The structures of anhydrous ephedrine and ephedrine hemihydrate together with 17 salts of the base are reported. It is evident that the observed packings are in most cases related to those of the neutral molecule, incorporating both bilayer and conformational elements.

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The single-crystal incommensurately modulated structure of 3,4-diphenyl-2a,5a,6,7,8,8a,8b-heptahydrofuro[4,3,2-de]chromen-2-one, C22H20O3, is presented.

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Atomic interactions in the pentaerythritol crystal based on the experimental electron density at 15 K, and theoretical calculations based on the experimental molecular geometries obtained at room and low (15 K) temperatures, as well as at high pressure (1.15 GPa) have been analyzed and compared.
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