Andrew N. Fitch

Crystal structure of the new FeSe1−x superconductor

The newly discovered superconductor FeSe(1-x) (x approximately 0.08, T(c)(onset) approximately 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure-the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO(1-x)F(x) (RE = rare earth) superconductors.

Vaterite Crystals Contain Two Interspersed Crystal Structures

Double Vision Vaterite is the least stable form of anhydrous crystalline calcium carbonate. While rarely found in geological contexts, it is an important biological precursor and occurs as a minor component in the shells of some organisms. The crystal structure of vaterite has long been debated with no model able to explain all the experimentally observed diffraction spots. Kabalah-Amitai et al. (p. 454) show that vaterite contains two coexisting crystallographic structures that form a pseudo-single crystal. Electron microscopy reveals that vaterite, a calcium carbonate polymorph, comprises at least two distinct crystal structures. Calcite, aragonite, and vaterite are the three anhydrous polymorphs of calcium carbonate, in order of decreasing thermodynamic stability. Although vaterite is not commonly found in geological settings, it is an important precursor in several carbonate-forming systems and can be found in biological settings. Because of difficulties in obtaining large, pure, single crystals, the crystal structure of vaterite has been elusive for almost a century. Using aberration-corrected high-resolution transmission electron microscopy, we found that vaterite is actually composed of at least two different crystallographic structures that coexist within a pseudo–single crystal. The major structure exhibits hexagonal symmetry; the minor structure, existing as nanodomains within the major matrix, is still unknown.

A two-circle powder diffractometer for synchrotron radiation with a closed loop encoder feedback system

A high-angular-resolution two-circle powder diffractometer equipped with long diffracted-beam collimators has been built at Daresbury Laboratory. The diffractometer has encoders mounted directly on the 2θ and ω axes. These give a nominal angular resolution of 0.1 and 1.0 mdeg respectively. Repeated scans of single powder peaks have demonstrated a reproducibility of 0.1 mdeg 2θ. Measurements on five independent peaks of tungsten give a self consistency of 1(1) × 10−5 A. An example data set from synthetic olivine Mg2SiO4 has been refined using the Rietveld method and the results compare very well with single-crystal structure refinements.

Temperature-induced valence transition and associated lattice collapse in samarium fulleride

The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-Tc) copper oxide superconductors. Metal fulleride salts—metal intercalation compounds of C60—and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system—they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + ε) + to nearly 2 + . The negative thermal expansion—heating from 4.2 to 32 K leads to contraction rather than expansion—occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm2+ towards the smaller Sm(2+ε)+, analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).

Irreversible network transformation in a dynamic porous host catalyzed by sulfur dioxide.

Porous NOTT-202a shows exceptionally high uptake of SO2, 13.6 mmol g(-1) (87.0 wt %) at 268 K and 1.0 bar, representing the highest value reported to date for a framework material. NOTT-202a undergoes a distinct irreversible framework phase transition upon SO2 uptake at 268-283 K to give NOTT-202b which has enhanced stability due to the formation of strong π···π interactions between interpenetrated networks.

Methane Adsorption in a Supramolecular Organic Zeolite

Gas storage in solids is an increasingly important research area with applications in the field of energy, environment, biology and medicine. Because it concerns the adsorption of methane, the main component of natural gas fuels, the storage target for any solid to be effective in energy applications and compete with the compressed form is approximately 35 wt % or 180 v/v. Several different types of porous materials such as carbon materials, zeolites, silica gels, MOFs and polymers have been recently examined in addition to organic crystalline microporous solids and nonporous ones, but no ideal candidates have emerged so far. In this paper we wish to report on the methane adsorption properties of a new microporous organic zeolite based on 1,2-dimethoxy-p-tert-butylcalix[4]dihydroquinone (Scheme 1), which has been recently characterised by our research group. The crystalline solid has a cubic structure (a= 36.412(4) ) with networked channels and hydrophobic cages. The channels have minimum and maximum diameter of 3.9 and 8.5 , respectively, excluding van der Waals radii, and they are filled with easily removable water molecules. The hydrophobic cages show a gate diameter of 2.2 and can reach a maximum diameter of 11.2 (excluding van der Waals radii), and the estimated cage volume is about 1000 . Interestingly, the supramolecular framework is also preserved after the removal of channel water molecules. Previous studies showed that the BET surface area corresponds to 230 m g 1 (N2 at 77 K) and that carbon dioxide can be absorbed at room temperature inside its nanochannels with high selectivity with respect to H2 gas, [9] encouraging further studies towards methane adsorption. Methane adsorption measurements were performed by using a home-made volumetric system. Sorption isotherms have been carried out at 298, 278 and 203 K. Data have been collected at initial pressures between 0.5 and 3 atm. The adsorption isotherm at 203 K is reported in Figure 1, (see the Supporting Information for adsorption isotherms at 298 and 278 K). At 1 atm and 298 K our compound adsorbs 0.11 methane molecules for each calixarene molecule. For comparison, p-tert-butylcalix[4]arene adsorbs 0.14 methane molecules for each calixarene molecule under the same conditions. Under the mild conditions of ambient pressure and temperature our sample shows an efficiency for methane storage of 0.29 wt %, whereas in the case of CO2 molecules the value was 0.5 wt %. The adsorption isotherm at 203 K is not of type I. The adsorption step at 2 atm could be interpreted by considering that at first the methane molecules enter the channels, interacting with the most favourite sites, and then upon increasing the pressure the channels are progressively filled. Note that there are 90 methane molecules in the unit cell at 3 atm pressure, which approximately corresponds to a 1:2 ratio between calixarene molecules and methane molecules. [a] Dr. C. Tedesco, Dr. L. Erra, Dr. V. Cipolletti, Dr. C. Gaeta, Prof. P. Neri Department of Chemistry and Nano_MATES Research Center University of Salerno, 84084 Fisciano (Italy) Fax: (+39) 089-969603 E-mail : ctedesco@unisa.it neri@unisa.it [b] Dr. M. Brunelli, Prof. A. N. Fitch European Synchrotron Radiation Facility 38043 Grenoble (France) [c] Prof. J. L. Atwood Department of Chemistry University of Missouri-Columbia Columbia, Missouri 65211 (USA) [d] Dr. M. Brunelli Current address: ILL Institut Laue-Langevin BP 156, 38042 Grenoble Cedex 9 (France) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200903218. Scheme 1. 1,2-Dimethoxy-ptert-butylcalix[4]dihydroquinone.

Selective Adsorption of Sulfur Dioxide in a Robust Metal–Organic Framework Material

Selective adsorption of SO2 is realized in a porous metal-organic framework material, and in-depth structural and spectroscopic investigations using X-rays, infrared, and neutrons define the underlying interactions that cause SO2 to bind more strongly than CO2 and N2 .

Metal-Insulator Transition and Orbital Order in PbRuO3

Anomalous low temperature electronic and structural behavior has been discovered in PbRuO3. The structure [space group Pnma, a=5.563 14(1), b=7.864 68(1), c=5.614 30(1) A] and metallic conductivity at 290 K are similar to those of SrRuO3 and other ruthenate perovskites, but a sharp metal-insulator transition at which the resistivity increases by 4 orders of magnitude is discovered at 90 K. This is accompanied by a first-order structural transition to an Imma phase [a=5.569 62(1), b=7.745 50(1), c=5.662 08(1) A at 25 K] that shows a coupling of Ru4+ 4d orbital order to distortions from Pb2+ 6s6p orbital hybridization. The Pnma to Imma transition is an unconventional reversal of the group-subgroup symmetry relationship. No long range magnetic order is evident down to 1.5 K. Calculations show that Pb 6s6p and Ru 4d orbital hybridization and strong spin-orbit coupling are significant.

Phase separation in carbon-doped MgB2 superconductors

High-resolution synchrotron X-ray powder diffraction measurements have been carried out on superconducting MgB2−xCx (0⩽x⩽0.1) phases. The carbon miscibility in the hexagonal AlB2-type structure is very small, x<0.04. At higher carbon contents, macroscopic phase separation occurs leading to the coexistence of at least two phases, isostructural with the parent MgB2 compound. The coexisting phases differ mainly in their in-plane lattice constants, suggesting that boron is substituted by carbon in the boron layers, but in different concentrations. The multiphase samples still show a clear one-step superconducting transition and full shielding, χ=−1/4π.

Optimized unconventional superconductivity in a molecular Jahn-Teller metal.

A superconductivity dome is created by the electronic structure of the molecular building block of an unconventional superconductor. Understanding the relationship between the superconducting, the neighboring insulating, and the normal metallic state above Tc is a major challenge for all unconventional superconductors. The molecular A3C60 fulleride superconductors have a parent antiferromagnetic insulator in common with the atom-based cuprates, but here, the C603– electronic structure controls the geometry and spin state of the structural building unit via the on-molecule Jahn-Teller effect. We identify the Jahn-Teller metal as a fluctuating microscopically heterogeneous coexistence of both localized Jahn-Teller–active and itinerant electrons that connects the insulating and superconducting states of fullerides. The balance between these molecular and extended lattice features of the electrons at the Fermi level gives a dome-shaped variation of Tc with interfulleride separation, demonstrating molecular electronic structure control of superconductivity.