Kevin H. Stone

Superconductivity in LaFe 1 − x Co x AsO

Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN{approx}145 K). Replacing Fe with Co is expected both to electron dope and introduce disorder in the FeAs layer. For x=0.05 antiferromagnetic order is destroyed and superconductivity is observed at Tconset=11.2 K. For x=0.11 superconductivity is observed at Tconset=14.3 K and for x=0.15 it is observed at Tconset=6.0 K. For x=1, and the material appears to be ferromagnetic as judged by magnetization measurements. We conclude that Co is an effective dopant to induce superconductivity. Somewhat surprisingly, the system appears to tolerate considerable disorder in the FeAs planes.

Third structure determination by powder diffractometry round robin "SDPDRR-3…

The results from a third structure determination by powder diffractometry (SDPD) round robin are discussed. From the 175 potential participants having downloaded the powder data, nine sent a total of 12 solutions (8 and 4 for samples 1 and 2, respectively, a tetrahydrated calcium tartrate and a lanthanum tungstate). Participants used seven different computer programs for structure solution (ESPOIR, EXPO, FOX, PSSP, SHELXS, SUPERFLIP, and TOPAS), applying Patterson, direct methods, direct space methods, and charge flipping approach. It is concluded that solving a structure from powder data remains a challenge, at least one order of magnitude more difficult than solving a problem with similar complexity from single-crystal data. Nevertheless, a few more steps in the direction of increasing the SDPD rate of success were accomplished since the two previous round robins: this time, not only the computer program developers were successful but also some users. No result was obtained from crystal structure prediction experts.

MnII(TCNE)3/2(I3)1/2-A 3D network-structured organic-based magnet and comparison to a 2D analog

Mn{sup II}(TCNE){sub 3/2}(I{sub 3}){sub 1/2} and Mn{sup II}(TCNE)[C{sub 4}(CN){sub 8}]{sub 1/2} [tetracyanoethylene (TCNE)] are organic-based magnets with 3D and 2D extended network structures with vastly different magnetic behavior. They have similar ferrimagnetic coupled layers of Mn{sup II}(TCNE){sup {lg_bullet}-} with different interlayer couplings, which lead, respectively, to net ferrimagnetic (T{sub c} = 171 K) and antiferromagnetic (T{sub c} = 68 K) order.

Implementation and use of robust refinement in powder diffraction in the presence of impurities

A modification to the usual least-squares analysis is implemented for the robust refinement of structural parameters from powder diffraction data in the presence of unmodeled impurities. This is accomplished in the program TOPAS-Academic by an iterative reweighting of the data as the model is refined. The method is tested and characterized using mixtures of known materials, acetaminophen and ibuprofen. The technique is also used to refine two previously unknown structures.

Cadmium and zinc thiolate and selenolate metal–organic frameworks

Metal-organic frameworks based on metal-sulfur or metal-selenium bonds are relatively rare; herein we describe the synthesis and structural characterization of several examples, including, for example, [Cd(en)3][Cd(SC6H4S)2], which contains the anionic two-dimensional square-grid network [Cd(SC6H4S)2]n(2n-).

Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17–xNi0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.Lithium ion battery electrodes employing anion redox exhibit high energy densities but suffer from poor cyclability. Here the authors reveal that the voltage of anion redox is strongly affected by structural changes that occur during battery cycling, explaining its unique electrochemical properties.

Characterization of the Antiferromagnetism in Ag(pyz)2(S2O8) (pyz = Pyrazine) with a Two-Dimensional Square Lattice of Ag2+ Ions

X-ray powder diffraction and magnetic susceptibility measurements show that Ag(pyz)(2)(S(2)O(8)) consists of 2D square nets of Ag(2+) ions resulting from the corner-sharing of axially elongated AgN(4)O(2) octahedra and exhibits characteristic 2D antiferromagnetism. Nevertheless, mu(+)SR measurements indicate that Ag(pyz)(2)(S(2)O(8)) undergoes 3D magnetic ordering below 7.8(3) K.

Synthesis, Characterization, and Calculated Electronic Structure of the Crystalline Metal−Organic Polymers (Hg(SC 6 H 4 S)(en)) n and (Pb(SC 6 H 4 S)(dien)) n

The reaction of Hg(OAc)(2) with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg(SC(6)H(4)S)(en)](n), while the reaction of Pb(OAc)(2) with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb(SC(6)H(4)S)(dien)](n). Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal-organic polymers with -M-SC(6)H(4)S- repeat units. Diffuse reflectance UV-visible spectroscopy indicates band gaps of 2.89 eV for [Hg(SC(6)H(4)S)(en)](n) and 2.54 eV for [Pb(SC(6)H(4)S)(dien)](n), while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb(SC(6)H(4)S)(dien)](n) calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg(SC(6)H(4)S)(en)](n) does have moderate dispersion and a calculated electron effective mass of 0.29·m(e). Hybridization of the empty Hg 6s orbital with SC(6)H(4)S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors.

Superconductivity in LaFe1-xCoxAsO

Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN{approx}145 K). Replacing Fe with Co is expected both to electron dope and introduce disorder in the FeAs layer. For x=0.05 antiferromagnetic order is destroyed and superconductivity is observed at Tconset=11.2 K. For x=0.11 superconductivity is observed at Tconset=14.3 K and for x=0.15 it is observed at Tconset=6.0 K. For x=1, and the material appears to be ferromagnetic as judged by magnetization measurements. We conclude that Co is an effective dopant to induce superconductivity. Somewhat surprisingly, the system appears to tolerate considerable disorder in the FeAs planes.

The study of the polymorphic system of 2-chloro-4-nitrobenzoic acid

The solid-state behaviour of 2-chloro-4-nitrobenzoic (2c4n) acid has been reinvestigated. Infrared spectra, differential scanning calorimetry (DSC), and thermomicroscopy investigations as well as X-ray powder patterns for two modifications of the substance are described. Modification II is the thermodynamically stable crystal form from absolute zero to its transition point which occurs at approximately 97 °C according to the DSC analysis. Above that temperature, the material transforms to modification I which undergoes dramatic evolution of its lattice parameters with increasing temperature. Synchrotron powder diffraction measurements confirm the existence of two polymorphic forms. Both modifications have been structurally characterized by single-crystal X-ray diffraction. The acid molecules in both crystal structures are organized into R22(8) dimer units at the first graph set level in modification I and at the second level in modification II.