Search

Refinement of time-of-flight high-resolution neutron powder diffraction data for lueshite (Na, Ca)(Nb, Ta, Ti)O3, the natural analogue of synthetic NaNbO3, demonstrates that lueshite at room temperature (298 K) adopts an orthorhombic structure with a 2ap × 2ap × 4ap superlattice described by space group Pmmn [#59: a = 7.8032(4) Å; b = 7.8193(4) Å; c = 15.6156(9) Å]. This structure is analogous to that of phase S of synthetic NaNbO3 observed at 753–783 K (480–510 °C). In common with synthetic NaNbO3, lueshite exhibits a series of phase transitions with decreasing temperature from a cubic (Pm3¯m) aristotype through tetragonal (P4/mbm) and orthorhombic (Cmcm) structures. However, the further sequence of phase transitions differs in that for lueshite the series terminates with the room temperature S (Pmmn) phase, and the R (Pmmn or Pnma) and P (Pbcm) phases of NaNbO3 are not observed. The appearance of the S phase in lueshite at a lower temperature, relative to that of NaNbO3, is attributable to the effects of solid solution of Ti, Ta and Ca in lueshite. [ABSTRACT FROM AUTHOR]

Since being discovered initially in mixed-cation systems, a method of forming end-member MgSO4·9H2O has been found. We have obtained powder diffraction data from protonated analogues (using X-rays) and deuterated analogues (using neutrons) of this compound over a range of temperatures and pressures. From these data we have determined the crystal structure, including all hydrogen positions, the thermal expansion over the range 9-260 K at ambient pressure, the incompressibility over the range 0-1.1 GPa at 240 K and studied the transitions to other stable and metastable phases. MgSO4·9D2O is monoclinic, space group P21/c, Z = 4, with unit-cell parameters at 9 K, a = 6.72764 (6), b = 11.91154 (9), c = 14.6424 (1) Å, α = 95.2046 (7)° and V = 1168.55 (1) Å 3. The structure consists of two symmetry-inequivalent Mg(D2O)6 octahedra on sites of ¯1 symmetry. These are directly joined by a water-water hydrogen bond to form chains of octahedra parallel with the b axis at a = 0. Three interstitial water molecules bridge the Mg(D2O)6 octahedra to the SO42- tetrahedral oxyanion. These tetrahedra sit at a ≃ 0.5 and are linked by two of the three interstitial water molecules in a pentagonal motif to form ribbons parallel with b. The temperature dependences of the lattice parameters from 9 to 260 K have been fitted with a modified Einstein oscillator model, which was used to obtain the coefficients of the thermal expansion tensor. The volume thermal expansion coefficient, αV, is substantially larger than that of either MgSO4·7D2O (epsomite) or MgSO4·11D2O (meridianiite), being ∼ 110 × 10-6 K-1 at 240 K. Fitting to a Murnaghan integrated linear equation of state gave a zero-pressure bulk modulus for MgSO4·9D2O at 240 K, K0 = 19.5 (3) GPa, with the first pressure derivative of the bulk modulus, K' = 3.8 (4). The bulk modulus is virtually identical to meridianiite and only ∼ 14% smaller than that of epsomite. Above ∼ 1 GPa at 240 K the bulk modulus begins to decrease with pressure; this elastic softening may indicate a phase transition at a pressure above ∼ 2 GPa. Synthesis of MgSO4·9H2O from cation-pure aqueous solutions requires quench-freezing of small droplets, a situation that may be relevant to spraying of MgSO4-rich cryomagmas into the surface environments of icy satellites in the outer solar system. However, serendipitously, we obtained a mixture of MgSO4·9H2O, mirabilite (Na2SO4·10H2O) and ice by simply leaving a bottle of mid-winter brine from Spotted Lake (Mg/Na ratio = 3), British Columbia, in a domestic freezer for a few hours. This suggests that MgSO4·9H2O can occur naturally - albeit on a transient basis - in certain terrestrial and extraterrestrial environments. [ABSTRACT FROM AUTHOR]

The crystal structures of the simple monoclinic halogen-bonded complexes pyridine-ICl and pyridine-IBr have been redetermined using modern X-ray and neutron diffraction techniques. The representation quadric surfaces for the thermal expansion tensor have been found to consist of a hyperboloid of one sheet. Negative thermal expansion occurs parallel to the crystallographic b axis, whilst there is colossal thermal expansion in a direction approximately parallel to the a* direction. These effects arise because of a decrease in the strength of an already weak C-H...X (X = Br, Cl) hydrogen bond with increasing temperature. The decrease in strength of the hydrogen bonding originates because of the reduction in the strength of the halogen bond formed between the nitrogen atom and the IX moiety with increasing temperature. Thus since at 298 K the I-X bonds are shorter than at 110 K, there will be a smaller partial negative charge on the X halogen at 298 K, leading to a weaker C-H...X hydrogen bond. [ABSTRACT FROM AUTHOR]

Certain complex structures are logically regarded as intergrowths of chemically or topologically discrete modules. When the proportions of these components vary systematically a polysomatic series is created, whose construction provides a e basis for understanding defects, symmetry alternation and trends in physical properties. Here, we describe the polysomatic family A5NB3NO9N+6XNδ (2 ⩽ N ⩽ ∞) that is built by condensing N apatite modules (A5B3O18Xδ) in configurations to create BnO3n+1(1 ⩽ n ⩽ ∞) tetrahedral chains. Hydroxyapatite [Ca10(PO4)6(OH)2] typifies a widely studied polysome where N = 2 and the tetrahedra are isolated in A10(BO4)6X2 compounds, but N = 3 A15(B2O7)3(BO4)3X3 (ganomalite) and N = 4 A20(B2O7)6X4 (nasonite) are also known, with the X site untenanted or partially occupied as required for charge balance. The apatite modules, while topologically identical, are often compositionally or symmetrically distinct, and an infinite number of polysomes is feasible, generally with the restriction being that an A:B = 5:3 cation ratio be maintained. The end-members are the N = 2 polysome with all tetrahedra separated, and N = ∞, in which the hypothetical compound A5B3O9X contains infinite, corner- connected tetrahedral strings. The principal characteristics of a polysome are summarized using the nomenclature apatite- (A B X)-NS, where AIBIX are the most abundant species in these sites, N is the number of modules in the crystallographic repeat, and S is the symmetry symbol (usually H, T, M or A). This article examines the state-of-the-art in polysomatic apatite synthesis and crystallochemical design. It also presents X-ray and neutron powder diffraction investigations for several polysome chemical series and examines the prevalence of stacking disorder by electron microscopy. These insights into the structure-building principles of apatite polysomes will guide their development as functional materials. [ABSTRACT FROM AUTHOR]

Abstract: The skutterudites Yb x Fe2Ni2Sb12 (0≤x≤0.4) have been prepared by solid-state reaction and characterised by powder X-ray diffraction. The compounds crystallise in the cubic space group Im (a≈9.1Å) with Yb atoms partially filling the voids in the skutterudite framework. A neutron time-of-flight diffraction experiment for Fe2Ni2Sb12 confirms the disorder of Fe and Ni atoms on the transition-metal site. Electrical resistivity, Seebeck coefficient and thermal conductivity measurements indicate that the thermoelectric performance of the skutterudites shows a marked dependence on the Yb content. Magnetic measurements over the temperature range 2≤T/K≤300 show paramagnetic behaviour for all compounds. Decomposition studies under an oxidising atmosphere at elevated temperatures have also been carried out by thermogravimetric analysis. [Copyright &y& Elsevier]

The importance of sample history on the Pbnm-Cmcm phase coexistence in NaTaO 3 has been investigated using high resolution neutron and synchrotron X-ray diffraction methods. These results reveal that the phase coexistence, associated with the first order Pbnm-Cmcm phase transition, is not significantly affected by the annealing conditions used to prepare the sample. Rather it is found that compositional inhomogeneities or defects play a significant role in stabilising the high temperature Cmcm phase at room temperature. [ABSTRACT FROM AUTHOR]

In this article we comment on the results published by Thompson et al. (, J. Solid State Chem. 219 (2014) 173–178) on the crystal structure of SrFeO 2 F, who claim the compound to crystallize in the cubic space group Pm -3 m . We give a more detailed explanation of the determination of our previously reported structural model with Imma symmetry (Clemens et al., J. Solid State Chem. 206 (2013) 158–169), with addition of variable temperature XRD measurements with high counting time to provide unambiguous evidence for the Imma model being correct for our sample. [ABSTRACT FROM AUTHOR]

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group P21/c [a = 6.9753(1) Å, b = 11.7281(2) Å, c = 23.4507(4) Å, β = 98.813(1)°, and Z = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba7(FeO4/2)1(FeO3/2O1/1)3(FeO5/2)2(FeO6/2)1 = Ba7Fe[6]1Fe[5]2Fe[4]4O17.5 and is in agreement with the 57Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of "G-type" antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements. [ABSTRACT FROM AUTHOR]

The compounds 15R-BaFeO2F and 15R-BaFeO2.27F0.5 have been synthesised by the low temperature fluorination of 15R-BaFeO3−dF0.2 using polyvinylidenedifluoride (PVDF) as a fluorination agent. The materials have been structurally characterised by Rietveld analysis of the X-ray- and HRPD-powder neutron diffraction data. A detailed analysis of bond valence sums suggests that the oxide and fluoride ions order on the different anion sites. A reinvestigation of our recently published structure (Clemens et al., 2013) [34] of 6H-BaFeO2F is also reported and incorporation of fluoride in h-type layers is also confirmed in this compound. The magnetic moments for 15R-BaFeO2F and 15R-BaFeO2.25F0.5 align in the a/b-plane with antiferromagnetic alignment of the moments between adjacent layers, and are flipped by 90° as compared to the precursor compound. 15R-BaFeO2F exhibits very robust antiferromagnetism with a Néel temperature between 300 and 400°C. [ABSTRACT FROM AUTHOR]

A correction to the article "Determination of structural chirality of berlinite and quartz using resonant x-ray diffraction with circularly polarized x-rays," which was published in the December 16, 2011 issue is presented.