The structural and associated ferroic properties of the stuffed tridymite-type compounds BaAl2O4 and SrAl2O4 have been investigated by means of ab initio calculations. Structures and energy landscapes have been analyzed in terms of symmetry-adapted distortion modes. Despite their rather different room-temperature symmetries, a triply-degenerate unstable antiferrodistortive rigid unit mode (RUM) of the Al2O4 tetrahedral framework is shown to be the dominant instability for both compounds. An orthorhombic configuration resulting from a single wave (1q) distortion mode competes with a hexagonal configuration resulting from three superposed such waves (3q). The very small energy difference between these two configurations in the case of BaAl2O4 would explain the recent electron microscope observation of orthorhombic symmetry on the nanoscale. A second unstable mode of polar character (also a RUM for the tetrahedral framework) is also present. While this second instability is too weak to condense in BaAl2O4, in the case of SrAl2O4 it is fundamental to make the 1q configuration prevail. The condensation of this additional instability is the cause of the symmetry reduction in SrAl2O4 to monoclinic. In contrast with previous literature, SrAl2O4 is a proper ferroelectric and a pseudoproper ferroelastic, with an uncommon bilinear coupling between its spontaneous polarization and shear monoclinic strain. One expects direct switching of spontaneous polarization through a shear stress, and conversely switching of the ferroelastic spontaneous shear strain through an electric field. This property must be related to the elasticoluminescent and electroluminiscent properties reported in Eu-doped SrAl2O4. The calculated energy maps have been systematically compared with variations in the so-called global instability index, introduced within the empirical bond valence model. The similarity of the variation in both quantities (for the most unstable distortion modes) reported in other systems is also observed here.

The P63 (a=2ap, b=2bp, c=cp) crystal structure reported for BaAl2O4 at room temperature has been carefully re-investigated by a combined transmission electron microscopy and neutron powder diffraction study. It is shown that the poor fit of this P63 (a=2ap, b=2bp, c=cp) structure model for BaAl2O4 to neutron powder diffraction data is primarily due to the failure to take into account coherent scattering between different domains related by enantiomorphic twinning of the P6322 parent sub-structure. Fast Fourier transformation of [001] lattice images from small localized real space regions (∼10nm in diameter) are used to show that the P63 (a=2ap, b=2bp, c=cp) crystal structure reported for BaAl2O4 is not correct on the local scale. The correct local symmetry of the very small nano-domains is most likely orthorhombic or monoclinic.

The (3+1)-d incommensurately modulated structures of four members of the NiGe1-xPx solid solution field have been successfully refined from X-ray powder diffraction data (Rw(all)/Rwp=2.13/3.92; 1.52/4.25; 1.27/3.44 and 2.00/4.03 for x=0.4, 0.5, 0.6 and 0.7, respectively). The 4-d superspace group symmetry is Amam(00γ)s00 (Z=4; a=5.0468(2), 5.0188(2), 4.9796(2) and 4.9651(1)Å; b=6.0636(3), 6.0576(2), 6.0183(2) and 6.0031(1)Å; c=3.4877(2), 3.4812(2), 3.4593(1) and 3.45442(7)Å; γ=0.7769(2), 0.7467(1), 0.7241(1) and 0.7046(1) for x=0.4, 0.5, 0.6 and 0.7, respectively). The underlying average structure is of NiAs type while the (in general) incommensurate primary modulation wave-vector, γc*, varies continuously and smoothly with composition. The two largest amplitude displacive atomic modulation functions (AMFs), for all samples, were the Ni displacement along b AMF and the Ge/P displacement along a AMF. The refined amplitude of the former was found to systematically increase with P content from 0.215Å for NiGe0.6P0.4 to 0.294Å for NiGe0.3P0.7 while the magnitude of the latter was found to increase with P content from 0.177Å for NiGe0.6P0.4 to 0.253Å for NiGe0.3P0.7. These displacive shifts significantly modulate the local crystal chemistry i.e. the local interatomic distances and co-ordination polyhedra. This continuously variable, incommensurately modulated, intermediate structure type is shown to provide a natural link or bridge between the two extreme end-member structures i.e. NiGe (of MnP structure type) and NiP by simply choosing the commensurate options with γ=1 and 12 respectively.

The commensurate superstructures of a NiAs/Ni2In type parent structure, Ni3.32InTe2 and Ni3.12In0.86Te2.14 (q = gamma[0 0 1]*, gamma = 2/3) as well as one dimensionally incommensurate structure of Ni31nTe2 (gamma = 0.71) were refined from neutron powder diffraction data (R-wp = 4.77%, 4.53% and 4.91 % for the three structures, respectively, at 298 K). The commensurate structures were refined in the P6(3)/mmc space group (c = 3c(NiAs)). The stacking sequence at the hcp array is -In/Te/Te/- and the trigonal bipyrainidal site within the In layer, Ni(2), is partially occupied while it is empty in the Te layers. The octahedral position in between the In and Te layers, Ni(1a), is fully occupied while the octahedral position in between two adjacent Te layers, Ni(1b), is partially occupied. With decreasing In and Ni content, the modulation wave vector,, was found to increase continuously until gamma = 1. From this, crenel functions to describe the whole homogeneity range of the solid solution were constructed with the length of the atomic domains Delta(Te) = gamma (and hence Delta(In) = Delta(Ni) = 1-gamma) and Delta(Ni(1b)) = gamma/2 (and hence Delta(Ni(1a)) = 1-gamma/2) which were then used for the refinement of the incommensurate structure of Ni3InTe2. The corresponding effect in real space is that the single In layers separating double layers of Te, occur less frequent when y in increasing until at gamma = 1 the CdI2 type structure of Ni1+xTe2 is reached. (C) 2007 Elsevier Inc. All rights reserved.

The crystal structures of two low temperature B82 related superstructure phases in the nickel indium system, Ni13In9 and Ni7−δIn3, have been refined and described in relation to the underlying parent Ni2In structure-type. They are isostructural to In13Pt9 and Cu7In3, respectively and both accommodate the change in stoichiometry from Ni2In by forming “same atom” structural motifs, In-centred indium octahedral units in the case of Ni13In9 and trigonal bipyramidal nickel units in the case of Ni7−δIn3. A sample with the nominal composition Ni5In3, reported to form part of a solid solution in the binary phase diagram, is not stable at 400°C and forms a two-phase mix of Ni7−δIn3 and Ni13In9.

A new compound, Mn8Pd15Si7, is reported to crystallize in a face centered cubic unit cell of dimension a=12.0141(2)Å, space groupFm3¯m, and can thus be classified as a G-phase. The crystal structure was studied by single crystal X-ray diffraction, X-ray and neutron powder diffraction and electron diffraction. A filled Mg6Cu16Si7 type structure was found, corresponding to the Sc11Ir4 type structure. The magnetic properties were investigated by magnetization measurements and Reverse Monte Carlo modeling of low temperature magnetic short-range order (SRO). Dominating near neighbor antiferromagnetic correlations were found between the Mn atoms and geometric frustration in combination with random magnetic interactions via metal sites with partial Mn occupancy were suggested to hinder formation of long-range magnetic order.

Precision-cut lung slices (PCLS) allow comparison of the airway responses of different species under identical experimental conditions. The aim of this study was to establish and characterise PCLS from guinea pigs (GPs) and to compare them with human PCLS. GP PCLS were prepared according to previously published procedures with the exception that the agarose solution and the initial incubation medium contained isoproterenol to avoid post mortem airway contraction. The median effective concentrations (EC50, expressed as nM) for agonist-induced bronchoconstriction in GP and human PCLS, respectively, were: leukotriene D4 (1.8, 5.0); thromboxane (16, 1.3); serotonin (69, unresponsive); histamine (217, 2,170); and methacholine (231, 234). Allergen-induced bronchoconstriction of passively sensitised PCLS was attenuated by histamine or thromboxane-prostanoid receptor antagonists and was almost completely prevented by their combination with leukotriene receptor antagonists. Airways pre-contracted with methacholine were relaxed by the beta-agonist salbutamol or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. Simultaneous studies of airways and vessels are possible with, for example, EC50 values for endothelin-1 of 37 nM (pulmonary arteries), 10 nM (pulmonary veins) and 9.6 nM (airway). When compared with previous findings in rat and mouse, these data show that guinea pig lungs are a more appropriate model for human airway pharmacology than lungs from rats or mice.

The wide-range of non-stoichiometric NiAs-type solid solution Ni1±yGe1−xPx has been studied by means of X-ray powder and electron diffraction. The incommensurately modulated structure of Ni(Ge, P) has been found to exist over a wide compositional range which is limited by the end points ≈NiGe0.8P0.2 and NiGe0.3P0.7 so that the general stoichiometry might be referred to as NiGe1−xPx with 0.2⩽x⩽0.7. The modulation wave vector is of the type q=γ[112¯0]* and its modulus is strongly composition dependent. A possible interpretation is given as a “soft transition”, via an incommensurately modulated structure, between the MnP and the NiP structure types, based on the almost purely displacive origin of the distortion. Further, the crystal structures of Ni5Ge2P3 and Ni2GeP seem to be commensurate approximations of the incommensurate modulated structure of Ni(Ge, P).

The crystal structures of Co3Se4 and Co6.8Se8 have been refined from twinned single crystal and powder X-ray diffraction data. The refined structures are isostructural to the Cr3±xSe4 and Cr7Se8 types, respectively. The structures are described as superstructures of the Cd(OH)2-type structure, where the empty layers of unoccupied octahedra are replaced with alternating Co filled and empty [100] rods and Co filled Kagomè nets for Co3Se4 and Co6.8Se8, respectively. The structure of Co3Se4 is not a substructure of Co7Se8 and the path from Co7Se8 to Co3Se4 by removing 1/4 of the Co atoms in the Kagomè nets involves both gain and loss of symmetry. Yet the refinements of Co6.8Se8 suggest an extended homogeneity field.

We describe self-assembled silica-carbonate aggregates that show a diverse range of morphologies, all of which display complex internal structure, orientational ordering of components, and well-organised, curved global morphologies that bear a strong resemblance to biogenic forms. The internal order is described as a liquid-crystal-like organisation of colloidal particles. We discuss possible causes for the striking morphologies of these inorganic materials, including local nanocrystal packing constraints and global silica membrane templating.