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Leucites are tetrahedrally coordinated silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. These structures have general formulae A2BSi5O12 and ACSi2O6; where A is a monovalent alkali metal cation, B is a divalent cation, and C is a trivalent cation. In this paper, we report the Rietveld refinements of three more synthetic leucite analogues with stoichiometries of Cs2NiSi5O12, RbGaSi2O6, and CsGaSi2O6. Cs2NiSi5O12 is Ia$\bar{3}$d cubic and is isostructural with Cs2CuSi5O12. RbGaSi2O6 is I41/a tetragonal and is isostructural with KGaSi2O6. CsGaSi2O6 is $I\bar{4}3d$ cubic and is isostructural with RbBSi2O6.

Hydrothermally synthesised K2ZnSi5O12 has a polymerised framework structure with the same topology as leucite (KAlSi2O6, tetragonal I41/a), which has two tetrahedrally coordinated Al3+ cations replaced by Zn2+ and Si4+. At 293 K it has a cation-ordered framework P21/c monoclinic structure with lattice parameters a = 13.1773(2) Å, b = 13.6106(2) Å, c = 13.0248(2) Å and β = 91.6981(9)°. This structure is isostructural with K2MgSi5O12, the first cation-ordered leucite analogue characterised. With increasing temperature, the P21/c structure transforms reversibly to cation-ordered framework orthorhombic Pbca. This transition takes place over the temperature range 848−863 K where both phases coexist; there is an ~1.2% increase in unit cell volume between 843 K (P21/c) and 868 K (Pbca), characteristic of a first-order, displacive, ferroelastic phase transition. Spontaneous strain analysis defines the symmetry- and non-symmetry related changes and shows that the mechanism is weakly first order; the two-phase region is consistent with the mechanism being a strain-related martensitic transition.

Crystallization, mechanical properties, and workability are all important for the commercialization and optimization of silicate glass compositions. However, the inter‐relations of these properties as a function of glass composition have received little investigation. Soda‐lime‐silica glasses with Na2O‐MgO‐CaO‐Al2O3‐SiO2 compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X‐Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro‐Vicker's indentation, and single‐edge‐notched bend testing methods were used to measure Young's moduli, hardness, and fracture toughness, respectively. It is shown that it is possible to design lower‐melting soda‐lime‐silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young's modulus, brittleness is highly responsive to the composition in soda‐lime‐silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures indicate that soda‐lime‐silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite, or their combinations.

Pure (BNT) and iron-doped bismuth sodium titanate (Fe-BNT) ceramics were produced according to the formula Bi0.5Na0.5Ti1−xFexO3−0.5x, where x = 0 to 0.1. The addition of Fe2O3 enables decreasing the sintering temperature to 900 °C in comparison with 1075 °C for pure BNT, whilst also achieving lower porosities and greater densities. This is attributed to oxygen vacancy generation arising from substitution of Fe3+ onto the Ti4+ site of the BNT perovskite structure, and the resulting increase in mass transport that this enables during sintering. X-ray diffraction (XRD) analysis of Fe-BNT samples shows single-phase BNT with no secondary phases for all studied Fe contents, confirming complete solid solution of Fe. Rietveld refinement of XRD data revealed a pseudocubic perovskite symmetry (Pm-3m), and unit cell lengths increased with increasing Fe content. Scanning electron microscopy (SEM) showed that average grain size increases with increasing Fe content from an average grain size of ~ 0.5 μm in (x = 0) pure BNT to ~ 5 μm in (x = 0.1) Fe-doped BNT. Increasing Fe content also led to decreasing porosity, with relative density increasing to a maximum > 97% of its theoretical value at x = 0.07 to 0.1. The addition of Fe to BNT ceramics significantly affects electrical properties, reducing the remnant polarization, coercive field, strain and desirable ferroelectric properties compared with those of pure densified BNT. At room temperature, a high relative permittivity (ɛ′) of 1050 (x = 0.07) at an applied frequency of 1 kHz and a lower loss factor (tanδ) of 0.006 (x = 0.1) at an applied frequency of 300 kHz were observed by comparison with pure BNT ceramics.

Leucites are silicate framework structures with some of the silicon framework cations partially replaced by divalent or trivalent cations. A monovalent extraframework alkali metal cation is also incorporated to balance the charges. We have previously reported Pbca leucite structures with the stoichiometries Cs2X2+Si5O12 (X = Mg, Mn, Co, Ni, Cu, Zn, Cd) and Rb2X2+Si5O12 (X = Mg, Mn, Ni, Cd). These orthorhombic leucite structures have all the silicon and non-silicon framework cations completely ordered onto separate crystallographic sites. This structure has five distinct Si sites and 1 X site; there are also two distinct sites for the extra-framework Cs or Rb. We have recently synthesised leucite analogues with two different extra-framework cations, these have the stoichiometry RbCsX2+Si5O12 (X = Mg, Ni, Cd). The initial Rietveld refinements assumed 50% Cs and 50% Rb on each of the two extra-framework cation sites. The refined structures for X = Ni and Cd have (within error limits) complete extra-framework cation site disorder. However, for X = Mg there is partial ordering of the extra-framework cation sites, the site occupancies are:- Cs1 0.37(3), Rb1 0.63(3), Cs2 0.63(3), Rb2 0.37(3).

Alkali feldspar is routinely used in retrospective dosimetry using luminescence methods. However there is a signal loss over time, termed ‘anomalous fading’, which results in age underestimation if uncorrected. Although significant improvements have been made in recent years, luminescence dating of feldspars remains challenging. This paper investigates the relationships between chemistry, structural state and the scale of exsolution with thermoluminescence (TL) emission spectra and infrared stimulated luminescence (IRSL) fading rates. We measure TL emission spectra, where possible linking the recombination site to physical features of the feldspar crystals. We show that fading rates are lowest in ordered end-member Na- and K-feldspars but significantly greater in disordered end-members, showing that Al–Si order influences fading. As well as having very low fading rates, ordered end-member samples have distinctive TL emission spectra, with the yellow-green emission dominant, while all other samples have a dominant blue emission. Perthite, i.e. exsolved members of the (Na,K)-feldspar solid solution, show greater fading than disordered end-members and fading is greatest in semi-coherent macroperthite. We propose that the state of Al–Si-order, and the occurrence of defects and dislocations at the perthite lamellar interfaces influence anomalous fading rates in feldspar.

European journal of chemistry 12(1), 60 - 63 (2021). doi:10.5155/eurjchem.12.1.60-63.2089, The synthetic leucite silicate framework mineral analogues Cs$_{2}$XSi$_{5}$O$_{12}$ (X = Cu, Cd, Zn) were prepared by high-temperature solid-state synthesis. The results of Rietveld refinement, using 18 keV synchrotron X-ray powder diffraction data collected at low temperatures (8K X = Cu, Zn; 10K X = Cd) show that the title compounds crystallize in the space group Pbca and are isostructural with the ambient temperature structures of these analogues. The structures consist of tetrahedrally coordinated SiO$_{4}$ and XO$_{4}$ sharing corners to form a partially substituted silicate framework. Extraframework Cs cations sit in channels in the framework. All atoms occupy the 8c general position for this space group. In these refined structures, silicon and X atoms are ordered onto separate tetrahedrally coordinated sites (T-sites)., Published by Univ., Tucker, Ga.

Feldspars are ubiquitous natural dosimeters widely used in luminescence dating. Despite decades of research, the lattice defects as well as the mechanisms involved in luminescence production in feldspars remain poorly understood. The recently developed method of infrared photoluminescence (IRPL) excitation-emission spectroscopy has revealed the presence of two electron trapping centres emitting IRPL at 1.3 eV and 1.41 eV (IRPL1.3 and IRPL1.41 centres), and it enables detailed investigations into the ground and excited state energies of these centres. Here we make measurements of a range of single crystal alkali feldspars to understand the effects of feldspar chemical composition, crystal structure and framework disorder on the physical characteristics of IRPL1.3 and IRPL1.41 electron trapping centres. Measurements of our sample suite reveals IRPL and IR-radioluminescence (RL) emissions at 1.41 eV, 1.3 eV and, for the first time, at 1.2 eV. Our results show that whilst the IRPL1.3 trapping centre is unaffected by the M site cation occupancy, the presence of IRPL1.41 trapping centres seems to be linked to the presence of K+ ions on M sites. However, no clear trends in IRPL and IR-RL emission energies and signal intensities with chemical composition of the samples were found. Exploring the effect of framework disorder on IRPL1.3 and IRPL1.41 emissions revealed no significant changes to IRPL and IR-RL emission energies or ground state energies of the trapping centres, suggesting that the corresponding defects are not located on bridging O ions. Variations in ground state energies across the whole sample suite range from 2.04 eV to 2.20 eV for the IRPL1.3 centre and from 2.16 eV to 2.46 eV for the IRPL1.41 centre. Variations in trap depth seem to be driven by other factors than sample chemistry, degree of Al3+disorder and number of phases present in a single crystal feldspar. Interestingly, the IR resonance peak is invariant between samples. Regarding the use of IRPL in luminescence dating, we show that optical resetting differs for the three different emissions, with the emission at ∼1.41 eV not being reset in some samples even after 18 h of solar bleaching 2021 IOP Publishing Ltd.

Reproducible preparation of lithium tetraborate fused beads for XRF analysis of glass and mineral samples is of paramount importance for analytical repeatability. However, as with all glass melting processes, losses due to volatilisation must be taken into account and their effects are not negligible. Here the effects of fused bead melting time have been studied for four Certified Reference Materials (CRM&rsquo, s: three feldspars, one silicate glass), in terms of their effects on analytical variability and volatilisation losses arising from fused bead preparation. At melting temperatures of 1065 &deg, C, and for feldspar samples, fused bead melting times shorter than approximately 25 min generally gave rise to a greater deviation of the XRF-analysed composition from the certified composition. This variation might be due to incomplete fusion and/or fused bead inhomogeneity but further research is needed. In contrast, the shortest fused bead melting time for the silicate glass CRM gave an XRF-analysed composition closer to the certified values than longer melting times. This may suggest a faster rate of glass-in-glass dissolution and homogenization during fused bead preparation. For all samples, longer melting times gave rise to greater volatilisation losses (including sulphates and halides) during fusion. This was demonstrated by a linear relationship between SO3 mass loss and time1/2, as predicted by a simple diffusion-based model. Iodine volatilisation displays a more complex relationship, suggestive of diffusion plus additional mechanisms. This conclusion may have implications for vitrification of iodine-bearing radioactive wastes. Our research demonstrates that the nature of the sample material impacts on the most appropriate fusion times. For feldspars no less than ~25 min and no more than ~60 min of fusion at 1065 &deg, C, using Li2B4O7 as the fusion medium and in the context of feldspar samples and the automatic fusion equipment used here, strikes an acceptable (albeit non-ideal) balance between the competing factors of fused bead quality, analytical consistency and mitigating volatilisation losses. Conversely, for the silicate glass sample, shorter fusion times of less than ~30 min under the same conditions provided more accurate analyses whilst limiting volatile losses.

The leucite tectosilicate mineral analogues K2 X 2+Si5O12 (X = Fe2+, Co, Zn) and Rb2 X 2+Si5O12 (X = Mn) have been synthesized at elevated temperatures both dry at atmospheric pressure and at controlled water vapour pressure; for X = Co and Zn both dry and hydrothermally synthesized samples are available. Rietveld refinement of X-ray data for hydrothermal K2 X 2+Si5O12 (X = Fe2+, Co, Zn) samples shows that they crystallize in the monoclinic space group P21/c and have tetrahedral cations (Si and X) ordered onto distinct framework sites [cf. hydrothermal K2MgSi5O12; Bell et al. (1994a), Acta Cryst. B50, 560–566]. Dry-synthesized K2 X 2+Si5O12 (X = Co, Zn) and Rb2 X 2+Si5O12 (X = Mn) samples crystallize in the cubic space group Ia{\overline 3}d and with Si and X cations disordered in the tetrahedral framework sites as typified by dry K2MgSi5O12. Both structure types have tetrahedrally coordinated SiO4 and XO4 sharing corners to form a partially substituted silicate framework. Extraframework K+ and Rb+ cations occupy large channels in the framework. Structural data for the ordered samples show that mean tetrahedral Si—O and X—O bond lengths cover the ranges 1.60 Å (Si—O) to 2.24 Å (Fe2+—O) and show an inverse relationship with the intertetrahedral angles (T—O—T) which range from 144.7° (Si—O—Si) to 124.6° (Si—O—Fe2+). For the compositions with both disordered and ordered tetrahedral cation structures (K2MgSi5O12, K2CoSi5O12, K2ZnSi5O12, Rb2MnSi5O12 and Cs2CuSi5O12 leucites) the disordered polymorphs always have larger unit-cell volumes, larger intertetrahedral T—O—T angles and smaller mean T—O distances than their isochemical ordered polymorphs. The ordered samples clearly have more flexible frameworks than the disordered structures which allow the former to undergo a greater degree of tetrahedral collapse around the interframework cavity cations. Multivariant linear regression has been used to develop equations to predict intertetrahedral T—O—T angle variation depending on the independent variables Si—O and X—O bond lengths, cavity cation ideal radius, intratetrahedral (O—T—O) angle variance, and X cation electronegativity.

Unit cell data for synthetic samples of I41/a or Ia-3d AAlSi2O6 (A=K,Rb,Tl,Cs) leucite/pollucites are compared to those for feldspar-stoichiometry leucite analogues (Rb,Tl,Cs)0.75□0.25Al0.75Si2.25O6. I41/a solid solutions between KAlSi2O6 and KAlSi3O8 (orthoclase feldspar, Or) and between K(Al0.85Fe0.15)Si2O6 and Or both show increasing volumes and decreasing c/a with increasing Or. Phase relations are given for KAlSi3O8-RbAlSi3O8; monoclinic feldspars occur at low temperatures and I41/a leucites at higher temperatures; volumes of leucite solid solutions increase with increasing Rb. The cell volume of Rb0.75□0.25Al0.75Si2.25O6 leucite is larger than that of RbAlSi2O6 leucite. Published leucite/pollucite structures belonging to different Space Group symmetries are treated as having the cubic Ia-3d topology with a ‘pseudocell’ edge V1/3 (V = unit cell volume); inequivalent site bond lengths and angles provide mean structural parameters for the cubic ‘pseudocells’. Multivariant linear regression equations using mean A – O distances as the independent variable were obtained using other structural parameters as dependent variables. Only two independent variables (V and mean tetrahedral ionic radius) are required to account for >90% of the variation. These regression equations can be used to predict the sizes of the cavity cation sites in leucite/pollucite-type phases of any stoichiometry.

Optimal processing conditions for fabrication of dense single-phase 6L-Ba

Optimal processing conditions for fabrication of dense single-phase 6L-Ba2CoTeO6 ceramics via the solid-state reaction method were determined. These ceramics possess a room-temperature crystal structure described by the centrosymmetric P[3 with combining macron]m1 space group. Polarized Raman spectroscopy enabled the observation of all the 25 predicted Raman modes and assignment of their symmetries. On cooling, BCTO ceramics exhibit two antiferromagnetic transitions at 3 K and 12.5 K, in broad agreement with a recent single-crystal study [P. Chanlert, N. Kurita, H. Tanaka, D. Goto, A. Matsuo and K. Kindo, Phys. Rev. B: Condens. Matter Mater. Phys., 2016, 93, 094420]. Low temperature Fourier-transform infrared reflectivity analyses suggest the antiferromagnetic phase transitions to be driven by small distortions of the CoO6 octahedra, lowering locally their C3v symmetry. This causes splitting of the associated vibrational modes, but without a long-range structural change. AC impedance spectroscopy revealed BCTO ceramics to be leaky insulators with an activation energy for conduction of ∼0.15−0.25 eV, which suggests electron hopping between mixed oxidation states of Co.

Crystalline and glassy phases in the ‘leucite’ systems (K,Rb,Cs)Fe 3 + Si 2 O 6 and (K,Rb,Cs) 2 (Mg,Mn,Fe,Co,Ni,Cu,Zn) 2 + – Si 5 O 12 have been studied using K-edge X-ray Absorption Spectroscopy (XAS) for 3d transition elements and Mg. Crystalline samples of known structure are used as model compounds for deducing the local structures of the equivalent glasses. ‘Ideal’ tetrahedral framework X 2 + –O distances are: Mn 2 + –O 2.02, Fe 2 + –O 1.98; Co 2 + –O 1.96, Ni 2 + –O 1.95, Cu 2 + –O 1.92, Zn 2 + –O 1.93 and Mg 2 + –O 1.93 A. The quenched glasses have Extended X-ray Absorption Fine Structure (EXAFS) first-shell distances and coordination numbers consistent with Mg and 3d elements occurring in tetrahedral coordination in most cases. The X-ray Absorption Near Edge Structure (XANES) spectra of the glasses are basically the same as those for the crystalline samples showing that they have similar medium range order, consistent with most of the Mg and 3d elements acting as network formers along with the associated alkalis as X 2 Y 2 + O 2 complexes. Anhydrous glasses in the system CaO–MgO–Al 2 O 3 –SiO 2 together with a basalt glass from Hawaii show examples of Mg in octahedral- (2.07 A, basalt), tetrahedral- (~ 1.87 A, siliceous glass) and 5-coordinated (2.01–2.04 A) sites. Although most Earth scientists assume that Mg and Fe 2 + act as network modifiers in natural magmas, ultra-potassic, peralkaline compositions ((Na + K)/(Al + Fe 3 + ) > 1) could have had some Fe 2 + and Mg complexes with alkalis in the melt network as K 2 (Mg,Fe 2 + )O 2 and the implications of this to melt density and viscosity are considered.

Synthetic leucite KGaSi2O6 at 298K is I41/a tetragonal and is isostructural with natural leucite (KAlSi2O6); with unit cell parameters of a ​= ​13.1099 (4), c ​= ​13.8100 (4) A, V ​= ​2373.50 (12) A3. With increasing temperature it undergoes a reversible, displacive phase transition from I41/a to cubic I a 3 ‾ d ; this well-studied phase transition in KAlSi2O6 occurs at ~930K. However for KGaSi2O6 it is smeared out from 673 to ~970K where it consists of a mixture of the low- and high-temperature polymorphs. The proportion of the cubic phase increases with temperature; the cubic phase volume is ~1% larger than the coexisting tetragonal polymorph. At a fixed temperature within this ‘region of coexistence’ phase proportions do not change. Such features are characteristic of 1st order, diffusionless, strain-meditated, martensitic-type phase transitions. It seems that the phase transition for synthetic KGaSi2O6 is close to being purely ferroelastic in character.

This paper presents the results of analytical work which aimed to explore potential\ud sources for the lime mortar used in the Hadrianic fort baths and a third-century\ud repair to Hadrian's Wall at Wallsend, UK. It is generally assumed that quick lime for\ud mortar is produced close to the source, however, as yet, no archaeological evidence\ud of kilns has been found in the Wallsend area. After extensive analysis the mortars\ud were found to be very different in characteristics and suggest variable sources for\ud the quarried limestone and for the aggregates used to manufacture the mortar.\ud Precedence exists in other locations for quicklime to be sourced from kilns set at\ud some considerable distance from Roman construction sites. It is only at\ud Housesteads and Vindolanda, sited close to Carboniferous Limestone outcrops, that\ud Roman lime kilns have been discovered to date. Therefore the investigation\ud included a number of potential sources in the vicinity of Housesteads as well as\ud Permian limestones at Trow Point which is geographically the closest source of\ud limestone. Results suggest that Carboniferous limestone was the most likely source\ud for some of the mortars analysed which may suggest that areas for lime production\ud are less numerous than previously thought and may even have been concentrated\ud around one area.

Glasses with nominal molar composition 20B2O3 – (80-x)Bi2O3 – xFe2O3 (where x = 0–40) were successfully prepared by melt-quenching. These glasses were characterised by multiple techniques including density, X-ray diffraction (XRD), X-Ray fluorescence (XRF), Raman, FT-IR and Mossbauer spectroscopies, dilatometry and differential thermal analysis (DTA). Partial replacement of Bi2O3 by Fe2O3 leads to decreasing density and molar volume and a substantial increase in thermal stability, as measured by several parameters, with maximum improvements achieved when x = 10-20. These improvements are accompanied by modest increases in dilatometric softening point. FT-IR and Raman spectra confirm the presence of BO3 and BiO6 structural units in all glasses, with glass structure apparently little affected by Fe2O3. Mossbauer spectroscopy confirms that iron is present partly as 4-fold coordinated Fe3+ in all glasses, with some 5- and / or 6- coordinated Fe3+ sites also present.

Lanthanide orthoferrites have wide-ranging industrial uses including solar, catalytic and electronic applications. Here a series of lanthanide orthoferrite perovskites, LnFeO3 (Ln = La; Nd; Sm; Eu; Gd), prepared through a standard stoichiometric wet ball milling route using oxide precursors, has been studied. Characterisation through X-ray diffraction and X-ray fluorescence confirmed the synthesis of phase-pure or near-pure LnFeO3 compounds. 57Fe Mossbauer spectroscopy was performed over a temperature range of 10 K to 293 K to observe hyperfine structure and to enable calculation of the recoil-free fraction and Debye temperature (θD) of each orthoferrite. Debye temperatures (Ln = La 474 K; Nd 459 K; Sm 457 K; Eu 452 K; Gd 473 K) and recoil-free fractions (Ln = La 0.827; Nd 0.817; Sm 0.816; Eu 0.812; Gd 0.826) were approximated through minimising the difference in the temperature dependent experimental Centre Shift (CS) and theoretical Isomer Shift (IS), by allowing the Debye temperature and Isomer Shift values to vary. This method of minimising the difference between theoretical and actual values yields Debye temperatures consistent with results from other studies determined through thermal analysis methods. This displays the ability of variable-temperature Mossbauer spectroscopy to approximate Debye temperatures and recoil-free fractions, whilst observing temperature induced transitions over the temperature range observed. X-ray diffraction and Rietveld refinement show an inverse relationship between FeO6 octahedral volume and approximated Debye temperatures. Raman spectroscopy show an increase in the band positions attributed to soft modes of Ag symmetry, Ag(3) and Ag(5) from La to GdFeO3 corresponding to octahedral rotations and tilts in the [010] and [101] planes respectively.

The leucite tectosilicate mineral analogues K

A series of isostructural trigonal molybdates CsMZr0.5(MoO4)3 with M = Al, Ga, In, Sc, Cr, V, Fe, and CsCrTi0.5(MoO4)3 was synthesized by solid state reactions and characterized by synchrotron powder diffraction, differential scanning calorimetry, thermal conductivity measurements, and magnetization measurements for compounds with a magnetic 3d transition metal cation. The NASICON-type structure of these compounds represents a three-dimensional framework of corner-sharing trigonal MoO4-pyramids and MO6-octahedra, in which M and Zr (Ti) ions are randomly distributed. The Cs ions occupy large channels along the c-axis. A combination of phenomena like a drastic increase of thermal conductivity, endothermic signals in the DSC curve and anomalies in the thermal expansion coefficients, observed for the compounds with M = Cr, V, Fe, and CsCrTi0.5(MoO4)3 above 700 K, allows one to propose a structural phase transition. The crystal structure of the high-temperature phase is very close to the low-temperature one. A...

Kinetics of the amorphous–anatase phase transition in single immobilized 5-nm TiO2 nanoparticles has been studied at temperatures between 400 and 1000 °C at the DORIS-III synchrotron source of Hasylab. The measurements indicate the anatase phase appearance at temperatures above 500 °C. No formation of rutile phase was observed. The transformation kinetics for each fixed temperature is rapid and completes within 5 min. The crystalline mass progressively increases with temperature and the full transformation into anatase polymorph is achieved above 800 °C. The results suggest higher surface energy of the anatase phase compared to amorphous one, which delays the crystallization process.

The room-temperature crystal structure of the brownmillerite SrCaMnGaO5+δ (δ = 0.035) has been refined from neutron powder diffraction data; space group Ima2, a = 15.7817(6), b = 5.4925(2), c = 5.3196(2) Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μB per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6T, increasing to 90% in 13 T. © 2002 Elsevier Science (USA).

Collisions of high Mach number flows occur frequently in astrophysics, and the resulting shock waves are responsible for the properties of many astrophysical phenomena, such as supernova remnants, Gamma Ray Bursts and jets from Active Galactic Nuclei. Because of the low density of astrophysical plasmas, the mean free path due to Coulomb collisions is typically very large. Therefore, most shock waves in astrophysics are “collisionless”, since they form due to plasma instabilities and self-generated magnetic fields. Laboratory experiments at the laser facilities can achieve the conditions necessary for the formation of collisionless shocks, and will provide a unique avenue for studying the nonlinear physics of collisionless shock waves. We are performing a series of experiments at the Omega and Omega-EP lasers, in Rochester, NY, with the goal of generating collisionless shock conditions by the collision of two high-speed plasma flows resulting from laser ablation of solid targets using ∼10 16 W/cm 2 laser irradiation. The experiments will aim to answer several questions of relevance to collisionless shock physics: the importance of the electromagnetic filamentation (Weibel) instabilities in shock formation, the self-generation of magnetic fields in shocks, the influence of external magnetic fields on shock formation, and the signatures of particle acceleration in shocks. Our first experiments using Thomson scattering diagnostics studied the plasma state from a single foil and from double foils whose flows collide “head-on”. Our data showed that the flow velocity and electron density were 10 8 cm/s and 10 19 cm −3 , respectively, where the Coulomb mean free path is much larger than the size of the interaction region. Simulations of our experimental conditions show that weak Weibel mediated current filamentation and magnetic field generation were likely starting to occur. This paper presents the results from these first Omega experiments.

The title compounds are prepared by melting and quenching of stoichiometric amounts of Rb2CO3, SiO2, and NiO or MnO (1573 K, 1.5 h for the Ni compound; 1673 K, 2 h for the Mn compound) followed by crystallization at 1193 K (12 h).

Synchrotron X-ray powder diffraction data have been collected on three synthetic leucite analogues with the general formula Cs2XSi5O12(X = Cd, Cu, Zn) between 295 and 1173 K. All three samples have the orthorhombic Pbca leucite structure at room temperature with ordered framework T-site cations. The sample with X = Cd retains the Pbca structure over the whole of the investigated temperature range. The sample with X = Cu also retains the Pbca structure, but there is a transition to a less distorted structure with a larger unit-cell volume at ∼333 K. The sample with X = Zn shows evidence for a transition to a previously unknown Pa cubic structure, with some T-site cation disorder, at 566 K, on heating. This transition is reversible on cooling to 633 K.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The effects of thermo-mechanical auxetic foam conversion parameters on the Young's modulus and Poisson's ratio of open-cell polyurethane foam are related to changes in chemical bonding and cell structure. Applied volumetric compression, conversion temperature, and duration are reported on foam Young's modulus, Poisson's ratio, and structural stability. Fourier transform infrared spectral analysis on samples converted at and above 160 °C strongly indicates a hydrogen bond interaction increase in urea groups (C=O---H-N) and an increase in hydrogen bonding population. Spectral changes inferred soft segment degradation following extensive heat exposure (200 °C, 180 min), specifically a shift and intensity change in CH2 and C-O-C polyol bands and a broad baseline increase between 3600 and 2400 cm−1. These changes are linked to (i) resistance to dimensional recovery over time and during re-heating; (ii) Poisson's ratio becoming negative, then zero in tension or marginally positive in compression; (iii) Young's Modulus reducing then increasing; (iv) mass loss, evidenced by thermogravimetric analysis increasing from 150 °C. The minimum mean values of Poisson's ratio of ≈−0.2 (to 10% compression/tension) are comparable to other studies. All samples that retain their imposed compression over time are isotropic, with near constant Young's moduli and Poisson's ratio (to 10% compression/tension).

In situ, high-temperature energy-dispersive X-ray powder diffraction (EDXRD) data have been collected on synthetic and a natural sample of mercury sulphide (HgS). These measurements were made between temperatures of 295 and 798 K. Synthetic samples of HgS were prepared by reaction between sulphide and mercury in aqueous solution. In a subsequently dried and aged synthetic HgS sample, heated in vacuo, there is a change from a poorly crystalline pseudocubic material into a well crystalline cubic material in the temperature region 583–623 K. At higher temperature (748 K), there is evidence for a partial phase transition to the high temperature hypercinnabar HgS structure. In a neoformed synthetic sample, heated in a sealed Ti container, the initial ‘pseudocubic’ metacinnabar phase partially transforms to a previously unknown phase (XHgS) in the temperature range 467–522 K. This phase disappears at 527 K, and the metacinnabar phase changes to a well crystalline cubic phase; cinnabar develops at 542 K. The proportion of cinnabar continues to increase up to 647 K. Both metacinnabar and cinnabar phases are retained on cooling. No phase transitions were observed for the natural cinnabar sample.

The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined structures for Cs2MgSi5O12 and Rb2MgSi5O12 were determined with complete T-site ordering, but the refined structure for Cs2ZnSi5O12 was determined with partial disorder of Mg and Si over two of the T sites.

The chemical composition oft he natural arsenate-apatite mineral johnbaumite [nominally Ca10(AsO4)6(OH)2] and its alteration product hedyphane [Ca4Pb6(AsO4)6Cl2] have been determined by electron microprobe analysis and the structures ofjohnbaumite and synthetic Sr-, Ba- and Pbarsenate apatites have been studied by As K-edge X-ray absorption spectroscopy and synchrotron X-ray powder diffraction. All samples belong to the holosymmetric apatite space group P63/m with As5+substituted for P5+in the tetrahedral structural site. Johnbaumite contains small amounts ofF and Pb (~0.9 and ~4.4 wt.% respectively) and hedyphane has the ideal composition (formula given above); the compositions of these coexisting phases define the two limbs ofa solvus occurring between Ca- and Pb-arsenate apatite end members. The unit-cell parameters and cation–oxygen bond lengths for the arsenate apatites studied are discussed alongside published data for end-member Ca-, Sr-, Ba- and Pbphosphate apatite analogues with (OH), F, Cl or Br as the anions at the centres of the channels in the apatite structure. This discussion rationalizes the relationships between the two structural sites A(1) and A(2) occupied by divalent cations in terms of the size of the A–O polyhedra and the distortion of the A(1)–O polyhedron as measured by the metaprism twist angle [O(1)–A(1)–O(2) projected onto (001)].

The design and performance characterization of a new light-weight and compact X-ray scintillation detector is presented. The detectors are intended for use on the new I11 powder diffraction beamline at the third-generation Diamond synchrotron facility where X-ray beams of high photon brightness are generated by insertion devices. The performance characteristics of these detection units were measured first using a radioactive source (efficiency of detection and background count rate) and then synchrotron X-rays (peak stability, light yield linearity and response consistency). Here, the results obtained from these tests are reported, and the suitability of the design for the Diamond powder beamline is demonstrated by presenting diffraction data obtained from a silicon powder standard using a prototype multicrystal analyser stage.

The bacterium Geobacter sulfurreducens can produce nanoparticulate magnetite (Fe3O4) by the reduction of amorphous Fe(III) oxyhydroxide coupled to the oxidation of organic matter in the anoxic subsurface as an alternative to oxygen respiration. G. sulfurreducens can transfer electrons to solid Fe(III)-bearing minerals through either direct contact between the cell and the mineral surface or by using an electron shuttling compound. High resolution synchrotron X-ray powder diffraction has been used to study samples taken at different stages of this reaction. This shows that an initial amorphous phase first transforms to goethite (FeO(OH)), before undergoing a further transformation to magnetite. Magnetite is formed faster in the presence of the electron shuttling compound disodium anthraquinone 2,6 disulphonate.

The phase transformations as a function of the temperature of two natural illitic clays were investigated through XRD measurements, ex situ at room temperature with conventional set up and in situ with synchrotron radiation, in order to understand the origin of the corundum phase, which is one of the main characteristics of the red glaze (slip) of Terra Sigillata from south Gaul. These clays were chosen on the basis of their chemical composition and for the quality of vitrification in the firing temperature range of sigillata (1030–1080 °C). Results show that corundum can be formed above 1000 °C if the amount of Mg is not too high. The corundum formation does not result directly from the total decomposition of illite ( 3–4 wt. %, this intermediate phase is not observed and only a spinel phase is formed.

We have examined alkaline sulfidic (0.5–0.003 M Na 2 S), aqueous solutions of Hg(II)-S complexes (4–370 ppm Hg(II)) by Hg L III edge EXAFS spectroscopy at 296, 348 and 423 K. Data were collected using the ID26 High Brilliance X-ray Spectroscopy beamline at the ESRF. Analysis of these EXAFS spectra shows Hg coordinated by two S atoms at 2.30 A; multiple scattering analyses reveal a linear [–S–Hg–S–] arrangement in the solution complex. These results are in agreement with earlier results on more concentrated solutions of these complexes. There is also evidence in the data for polynuclear sulfide complexes at 296 K and 348 K for samples with the lowest sulfide concentrations although this is complicated by multiple scattering effects.

The focus of this study consists of examining how simultaneous SR-XRD and electrochemical measurements can provide information on the effectiveness of stabilization and storage treatments of copper artefacts in aqueous solution. The electrochemical cell used here was designed for in situ, time resolved SR-XRD studies of corrosion and inhibition studies on cultural heritage materials. Key objectives of the new cell were to monitor corrosion layers on alloys with realistic metallographic structures and to obtain co-incident, time resolved, electrochemical data such as reduction measurements, oxidation measurements and corrosion potential (Ecorr) measurements. Here we present some early results from the cell. Firstly, a correlated SR-XRD and corrosion potential (Ecorr) study of the reduction of nantokite during storage in sodium sesquicarbonate, which shows that the surface chemistry continues to change after Ecorr has stabilized. Secondly, the use of X-ray data to identify specific changes occurring as a function of potential in the forced reduction of a more complex system. Keywords: Spectroelectrochemistry, SR-XRD, Corrosion, Copper, Cultural heritage, Conservation

Neutron powder diffraction data have been collected from a sample of polycrystalline Fe 3 O 4 . High resolution data collected at 60 K are fitted using a simple rhombohedral distortion of the cubic unit cell. This model accounts for all of the peak splittings that occur upon cooling through the Verwey transition temperature ( T V ). Lower resolution data collected between 2 and 280 K are fitted using the same model, which gives T V =110±5 K. These data show a change in the thermal expansion due to the softening of phonons above T V and a change in crystallite extinction arising from twinning at the transition.

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many M II 2 M III O 2 BO 3 and M II M III OBO 3 materials can be grown from borate fluxes. In the Fe II -Fe III -B-O system, flux growth results in Fe 3 O 2 BO 3 crystals, but solid state reaction at higher temperatures has yielded Fe 2 OBO 3 as a polycrystalline powder. This has been characterised by synchrotron and neutron diffraction, electron microscopy, Mossbauer spectroscopy, and conductivity and magnetic measurements. Two notable transitions occur, a broad semiconductor-semiconductor change accompanied by a structural transition at 317 K, and L-type ferrimagnetic order below a Curie temperature of 155 K. An average (Fe 2+ ) 0.5 (Fe 3+ ) 0.5 valence is observed at the two crystallographically distinct sites in Fe 2 OBO 3 , indicating that charge ordering occurs.

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal–insulator transitions1 in manganite perovskites (which show ‘colossal’ magnetoresistances), and the Verwey2 transition in magnetite (in which the material becomes insulating at low temperatures when the conduction electrons freeze into a regular array). Charge-ordered ‘stripes’ are found in some manganites3,4 and copper oxide superconductors5; in the latter case, dynamic fluctuations of the stripes have been proposed6 as a mechanism of high-temperature superconductivity. But an important unresolved issue is whether the charge-ordering in oxides is driven by electrostatic repulsions between the charges (Wigner crystallization7), or by the strains arising from electron–lattice interactions (such as Jahn–Teller distortions) involving different localized electronic states. Here we report measurements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structurally distinct Fesites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown8 to undergo charge-ordering through Jahn–Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement.

The structure of a synthetic end-member wairakite (CaAl2Si4O12·2H2O) has been determined using Rietveld analysis of high-resolution, synchrotron X-ray powder diffraction data, and 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The framework in the synthetic sample is more disordered than that in natural wairakite. Ca is distributed over the cavity cation sites M2, M12A, M12B in the approximate proportions 0.8:0.1:0.1, respectively, with M11 being vacant. 29Si MAS NMR data are consistent with about 80% of the Si occupying tetrahedral T11 and T12 sites linked to two Al atoms [Q4(2Al) silicons]. Tetrahedral and cavity cation site disorder are coupled so that Al mainly occupies T2 sites, with Ca in M12A and M12B being balanced by Al in T12A and T12B; T11A and T11B sites appear to only contain Si, in agreement with the M11 site being vacant. The crystal chemistries of the wide range of stoichiometries which crystallize with the leucite/pollucite structure-type are also reviewed, with particular attention being paid to the tetrahedral ordering configurations present in these phases, and the implications to crystallographic phase transitions.

Structural (a = 3.1688(1), b = 9.3835(3), c = 9.2503(3) �, ? = 90.220(1)�) and magnetic properties of a new mixed-valent oxoborate Fe2+Fe3+OBO3 were studied from 3-337 K. Low-temp. neutron powder diffraction and high-temp. synchrotron x-ray powder diffraction show a change in the symmetry of the warwickite type structure from monoclinic (P21/c) to orthorhombic (Pnma) at 315 K and demonstrate that the 2 Fe sites are occupied by an exact av. Fe2.5 valence at all temps. Magnetization measurements reveal an ordering transition at Tc = 155 K below which there is a small ferromagnetically ordered component. Neutron diffraction measurements below Tc show an apparent antiferromagnetic ordering but the magnetization measurements suggest that this is an L-type ferrimagnetic behavior. Resistivity measurements show a small change in resistivity at 315 K. [on SciFinder(R)]

Sulphosalts in the system CuSbS2-CuBiS2(chalcostibite-emplectite) form a complete solid solution series. Seven compositions with the general formula Cu(SbxBi1–x)S2have been synthesized using dry methods at 310°C. All members of the series are orthorhombic (space groupPnma) and show smoothly increasingaandbcell parameters with substitution of Bi for Sb; theccell parameter increases up to 50% CuBiS2substitution and then becomes constant. DSC experiments on CuBiS2show an endothermic heat effect (2.45 kJ/mol.) at 472°C due to the breakdown reaction to Cu3BiS3(wittichenite) plus Bi2S3(bismuthinite). With the addition of 10% CuSbS2to CuBiS2, the decomposition temperature increases and the endothermic peak is broadened but the energy remains essentially the same (2.53 kJ/mol.). No evidence of this decomposition was observed when the amount of the CuSbS2component was >30%. The local structure and co-ordination of Cu in the samples were studied by EXAFS analysis of the Cu-Kedge but no significant variation occurs in the local Cu environment. The Debye-Waller factor for the first shell of S atoms surrounding Cu in end member CuSbS2tends to be slightly smaller than for the intermediate solid solutions, suggesting that the tetrahedral Cu environments in the intermediate composition samples is somewhat more disordered than in the end-member. The low expansion characteristics alongcappear to be controlled by the linkages between the (CuS3+ BiS2) sheets perpendicular tocbeing relatively inflexible.

Sr2TiO(Si2O7), the Sr analogue of the mineral fresnoite Ba2TiO(Si2O7), is prepared by quenching a stoichiometric melt of SrCO3, TiO2, and SiO2 followed by annealing at 1323 K (7 d).

Experiments investigating the physics of interpenetrating, collisionless, ablated plasma flows have become an important area of research in the high-energy-density field. In order to evaluate the feasibility of designing experiments that will generate a collisionless shock mediated by the Weibel instability on the National Ignition Facility (NIF) laser, computer simulations using the Center for Radiative Shock Hydrodynamics (CRASH) radiation-hydrodynamics model have been carried out. This paper reports assessment of whether the experiment can reach the required scale size while maintaining the low interflow collisionality necessary for the collisionless shock to form. Comparison of simulation results with data from Omega experiments shows the ability of the CRASH code to model these ablated systems. The combined results indicate that experiments on the NIF are capable of reaching the regimes necessary for the formation of a collisionless shock in a laboratory experiment.

The Sr analogue of the mineral fresnoite (Sr2TiSi2O8) is of interest as a potential storage medium for radioactive Sr from nuclear waste. No high or low temperature crystal structure information is known on this phase. Therefore high-resolution synchrotron X-ray powder diffraction measurements have been done on a synthetic sample of Sr-fresnoite in the temperature range 87-1223K. This was done as a test experiment using the HRPD beamline P02.1 at PETRA-III, DESY. Synchrotron X-ray wavelengths of 0.2067(3)Å (293K and 573-1223K) and 0.2079(3)Å (87-499K) were used. Powder diffraction data were collected with a counting time of 30s using a PerkinElmer XRD 1621 flat panel image plate detector. CeO2 was included as an internal standard to calibrate the sample to detector distance. The P4bm tetragonal crystal structure of fresnoite (Ba2TiSi2O8) was used as a starting model for Sr-fresnoite. Small amounts of SrTiO3 and SrSiO3 were also found as impurities in this sample; therefore four-phase Rietveld refinements were done. The P4bm fresnoite structure is retained over the temperature range 87-1223K.

Lattice parameter of metastable high-pressure phase of zinc oxide, rock-salt ZnO was measured in the 10-300 K temperature range using synchrotron X-ray powder diffraction. No phase transition was observed down to 10 K. The lattice parameter of rock-salt ZnO was found to increase from 4.266 {\AA} in the 10-80 K range up to 4.2752(3) {\AA} at 298 K, while the volume thermal expansion coefficient increases from slight negative values below 40 K up to 4.77\times10^-5 K^-1 at 298 K., Comment: 4 pages, 3 figures, 1 table

The fresnoite-type compound Sr2TiO(Si2O7), distrontium oxidotitanium disilicate, has been prepared by high-temperature solid-state synthesis. The results of a Rietveld refinement study, based on high-resolution synchrotron X-ray powder diffraction data, show that the title compound crystallizes in the space group P4bm and adopts the structure of other fresnoite-type mineral samples with general formula A2TiO(Si2O7) (A = alkaline earth metal cation). The structure consists of titanosilicate layers composed of corner-sharing SiO4 tetrahedra (forming Si2O7 disilicate units) and TiO5 square-based pyramids. These layers extend parallel to the ab plane and are stacked along the c axis. Layers of distorted SrO6 octahedra lie between the titanosilicate layers. The Sr2+ ion, the SiO4 tetrahedron and the bridging O atom of the disilicate unit are located on mirror planes whereas the TiO5 square-based pyramid is located on a fourfold rotation axis.