Ivana Radosavljevic Evans

The TOPAS symbolic computation system

The computer program TOPAS V5 (Bruker AXS, 2011) is primarily a nonlinear least-squares optimization program used in the field of crystallography. It is written in the cþþ programming language and at its core is a symbolic computation system. Symbolic computation is a powerful tool as it extends program functionality at run time. Equally important is the simplicity it offers at program development stage by hiding (not eliminating) underlying complexity. Often cited against the use of symbolic computation is slowness in computation. This is largely overcome by calculating only what has been changed at the symbolic equation node level. Optimization programs generally receive parameter input in the form of numeric values, and these values are modified during the course of optimization. Invariably input flags are used to convey information such as whether to treat a particular parameter value as a constant or whether to modify the parameter value during optimization. TOPAS is similar, except that information to the optimization routines is passed through symbolic algebraic equations which we will call the TOPAS symbolic system. These equations can describe parameter dependencies or they can define functionality at key points in the model function. The extent to which functionality can be extended is dependent on the design of the model function, written in cþþ, and the aspects of the model function that are exposed to the symbolic system. Consider the case of the generalized model function in TOPAS called “fit_obj.” It is written in cþþ code and the only symbolic variable it manages is the abscissa x; this type of variable will be called a multivalued variable.The symbolic system defines the model itself at run time. For example, a normalized Gaussian peak profile P(x) can be defined by writing in the symbolic form (1).

Anion binding by Ag(I) complexes of urea-substituted pyridyl ligands

A series of Ag(I) complexes of ureidopyridyl ligands 1 and 2 have been prepared from oxo-anion salts. In all cases the new materials contain the AgL2+ cation interacting with oxo-anions via the urea moiety. The complexes containing the para ligand 2: [Ag(2)2]CF3SO3·2H2O (3), [Ag(2)2]CH3CO2·1.33H2O·MeOH (4) and [Ag(2)2]NO3·H2O (5), all exhibit remarkably similar chain-like structures based around a linear Ag(I) centre, despite the change in the counter-ion. A recurring R22(8) hydrogen-bonding ring motif between the urea group and the oxo-anion is observed in almost all cases. An exception to this trend is the anhydrous nitrate structure [Ag(2)]NO3 (6) in which the nitrate is coordinated in a bridging position between two silver centres, which adopt distorted trigonal pyramidal geometries. Structures containing the ligand 1, [Ag(1)2]CF3SO3·0.5H2O (7), [Ag(1)2]CF3SO3·H2O·MeCN (8), [Ag(1)2]2SO4 (9), [Ag(1)2]NO3·MeOH (10) and [Ag(1)2]NO3·0.5MeOH·0.5MeNO2 (11), display very different geometries, although the R22(8) is observed to persist throughout. The most notable of these structures are 10 and 11 in which the nitrate anion is chelated within a ‘pincer’ arrangement by the silver complex. The nitrate anion is situated asymmetrically within the cavity of the host complex. This discrete nitrate complex persists in solution with strong nitrate binding by the [Ag(1)2]+ host compared to other anions being observed.

Zinc glycolate: a precursor to ZnO.

A simple and versatile solvent-growth process using ethylene glycol has been demonstrated for the synthesis of novel faceted bipyramidal zinc glycolate. Upon thermal treatment in air, this structure can be converted into a ZnO hexagonal phase with wurtzite structure via solid-state transformation. The morphology, microstructure, and crystallinity of the products before and after thermal treatment have been characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and solid-state (13)C NMR measurements. In addition, the room-temperature photoluminescence of the resulting ZnO has also been investigated.

A study of the thermal and light induced spin transition in [FeL2](BF4)2 and [FeL2](ClO4)2 L=2,6-di(3-methylpyrazol-1-yl)pyrazine.

The spin crossover compounds [FeL2](BF4)2, L=2,6-di(3-methylpyrazol-1-yl)pyrazine and [FeL2](ClO4)2 have very unusual two stage spin transitions which are initially steep and then become more gradual. A detailed variable temperature single crystal X-ray diffraction study has shown that the course of the spin transition is controlled by an order-disorder transition in the counter anions. The high and low spin states both crystallise in the tetragonal space group I4, the structures of the high and low spin states are presented at 290 and 30 K, respectively. The title compounds are shown to undergo LIESST (Light Induced Excited Spin State Trapping) under irradiation with either red or green laser light with wavelengths of 632.8 and 532.06 nm, respectively, at 30 K. The cell parameters for the tetragonal photo-induced metastable high spin state at this temperature are a= 9.169(6), c= 17.77(1) A for [FeL2](ClO4)2 with an increase in unit cell volume of 21 A3, and a= 9.11(1), c= 17.75(2) A and an increase in volume of 42.8 A3 for [FeL2](BF4)2.

The R21(6) hydrogen-bonded synthon in neutral urea and metal-bound halide systems

The recurrence of the R21(6) motif between urea and metal bound halides is explored through studies in the CSD, including examples of the closely related thiourea and guanidine-based structures. A series of compounds containing the isomeric pyridyl–urea ligands 1 and 2 attached to trans-dihalide metal units have been prepared and their X-ray crystal structures determined. A hydrogen-bonding motif between the metal-bound halide ligands and the urea groups on the pyridyl ligands is observed in most cases which takes the form of an R21(6) ring. Three copper(II) complexes, [Cu(2)2Cl2]·2MeOH 3, [Cu(2)2Br2(MeOH)] 4 and [Cu(2)2Cl2(EtOH)] 5 all retain this supramolecular synthon despite a changing coordination environment and alteration of the halide ligands. A series of solvates of the complex [Pd(1)2Cl2] (6–8) shows that the R21(6) synthon does not occur in the presence of strongly hydrogen-bond accepting solvents such as DMF (6) and MeCN (7) but is observed in the presence of the hydrogen bond donor solvent methanol (8). The motif also occurs within an octahedral cadmium system [Cd(1)4Cl2]·2H2O·2MeCN 9. In all cases in which the R21(6) synthon is observed chain-like structures form between the metal complexes. Two contrasting structures based around tetrahedral zinc(II) centres [Zn(1)2Cl2] (10) and [Zn(1)2I2]·0.5MeOH (11) do not display the bifurcated synthon, showing instead an R22(8) motif in the case of 10 or no ring-type halide interaction in 11.

The spin-states and spin-crossover behaviour of iron(II) complexes of 2,6-dipyrazol-1-ylpyrazine derivatives

The syntheses of [FeL2]X2 (L = 2,6-dipyrazol-1-ylpyrazine [L2H], 2,6-bis{3-methylpyrazol-1-yl}pyrazine [L2Me], 2,6-bis{3,5-dimethylpyrazol-1-yl}pyrazine [L2Me2] or 2,6-bis{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyrazine [L2Mes]; X− = BF4− or ClO4−) are described. Solvent-free [Fe(L2H)2][BF4]2 and [Fe(L2H)2][ClO4]2 exhibit very similar abrupt spin-state transitions at 223 K and 208 K respectively, which show hysteresis loops of 3–5 K. Powder diffraction measurements afforded related, but not identical, unit cells for these two compounds, and imply that [Fe(L2H)2][ClO4]2 is isomorphous with [Fe(L1H)2][BF4]2 (L1H = 2,6-dipyrazol-1-ylpyridine). The single crystalline solvate [Fe(L2H)2][BF4]2·3CH3NO2 undergoes a similarly abrupt spin-state transition at 198 K. Polycrystalline [Fe(L2Me)2][BF4]2 and [Fe(L2Me)2][ClO4]2 are isomorphous with each other and also exhibit spin-state transitions at low temperature, although these are very different in form. In contrast, both salts of [Fe(L2Me2)2]2+ and [Fe(L2Mes)2]2+ are fully low-spin at 295 K. Single crystal structures of [Fe(L2Me2)2][BF4]2·0.5{CH3}2CO·0.1H2O and [Fe(L2Mes)2][BF4]2·5CH3NO2 show low-spin complex dications, and imply that [Fe(L2Me2)2][BF4]2 is low-spin as a result of intra-ligand steric repulsions involving the pyrazole 5-methyl substituents. NMR and UV/vis data in MeCN and MeNO2 show that the spin states of all four complex dications are similar in solution and the solid state except for [Fe(L2Me2)2]2+, which exists as a mixture of high- and low-spin species in these solvents.

A facile nonaqueous route for fabricating titania nanorods and their viability in quasi-solid-state dye-sensitized solar cells

A facile and simple process has been detailed for the synthesis of titanium glycerolate nanofibers using glycerol as both a solvent and a chelating agent. This complex has then been successfully converted to a high surface area anatase phase of titanium dioxide (TiO2) through solid state transformation without alteration in the overall fiber morphology. The structure, crystallinity and morphology of the products before and after transformation have been characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), solid-state 13C NMR and thermogravimetric analysis (TGA) measurements. As a demonstration of a potential application, these anatase nanorods (NRs) have been used as a photoanode to fabricate a dye-sensitized solar cell (DSSC) using a gel polymer electrolyte. Devices with efficiencies of 2.8% and 4.4% were recorded under light intensity of 100 mW/cm2 and 10 mW/cm2 illumination respectively.

α-Bi2Sn2O7– a 176 atom crystal structure from powder diffraction data

Pyrochlore-type bismuth tin oxide, Bi2Sn2O7, is a technologically important material used in applications such as catalysis and gas sensing. Its room temperature structure has been solved by a method of simulated annealing of combined X-ray and neutron diffraction data, followed by Rietveld refinement. With 176 crystallographically independent atoms in the asymmetric unit, it is one of the largest structures solved to-date from powder diffraction data. In addition to the number of unique atoms, additional crystallographic difficulty stems from the fact that the true symmetry of this structure is lower than the apparent metric symmetry of the unit cell.

Remarkably High Oxide Ion Conductivity at Low Temperature in an Ordered Fluorite-Type Superstructure†

Oxide ion conductors are technologically important materials because of their potential applications in oxygen sensors and pumps, as dense membranes for oxygen permeation, catalysts, and as electrolytes for solid oxide fuel cells (SOFCs). To be efficient in various applications, candidate materials should possess a conductivity of at least 10 2 S cm 1 at deviceoperating temperatures; currently commercially used yttriastabilized zirconia (YSZ) reaches this target at 700 8C. Given the drive towards lowering device-operating temperatures, there is a strong impetus and a great challenge for materials chemists to develop materials with enhanced ionic mobility and superior low-temperature oxide ion conductivity. 6] A better understanding of generic structural features and pathways which facilitate ionic mobility at lower temperature is a key step in reaching this goal. Here we report a remarkably high oxide ion conductivity at low temperatures (300–500 8C) in an ordered pseudo-cubic 3 3 3 d-Bi2O3 superstructure with composition Bi1 xVxO1.5+x (x = 0.087 and 0.095). Its conductivity is the highest we know of in a singly substituted d-Bi2O3-based material and comparable to the unstable Bi0.85Pr0.105V0.045O1.545 [7] and Bi12.5La1.5ReO24.5 on their first use, that is, before their conversion into a stable tetragonal form and an associated drop of conductivity of almost two orders of magnitude. 9] By contrast and unusually, our materials crystallize as stable ordered superstructures, and do not undergo phase transitions to lower symmetry and lower conductivity polymorphs. Our ab initio molecular dynamics (AIMD) simulations reveal the structural features and mechanisms which facilitate the high oxide ion mobility at low temperatures, and provide conceptual insight readily applicable to other materials and structure types. The high-temperature cubic fluorite-type bismuth oxide, d-Bi2O3, with intrinsic oxygen vacancies, shows the highest oxide ion conductivity measured in any material (around 1 Scm 1 at 750 8C); however, it is only thermodynamically stable in the narrow range between 730 and 824 8C. There has been considerable interest in stabilizing the highly conducting d-Bi2O3 phase by isovalent or aliovalent cation substitution to preserve oxide ion conductivity at lower temperatures. For example, 20% substitution of Er into Bi2O3 results in oxide ion conductivity of 2 10 2 Scm 1 at 500 8C and 0.4 S cm 1 at 700 8C. Double cation substitution has yielded even higher conductivities at low temperatures (300– 500 8C); the best examples include Dy-W, Pr-V, and the recently reported La-Re co-substitutions. On first use, Bi0.85Pr0.105V0.045O1.545 [7] and Bi12.5La1.5ReO24.5 [8] show the highest oxide ion conductivity among the doped d-Bi2O3 materials, with s 10 –10 2 Scm 1 at 300–400 8C, approaching the Cu-doped layered Bi2VO5.5 (BICUVOX), which itself has the disadvantage of two-dimensional, anisotropic conductivity. Although the relative chemical instability of Bi oxides under reducing conditions has so far hampered their applications in SOFCs, the use of bilayer electrolytes can overcome this issue. In addition to high oxide ion conductivity, bismuthbased oxides show electrocatalytic activity and therefore also have great potential for applications in electrochemical oxygen separation. A common structural feature in the best d-Bi2O3-based oxide ion conductors reported so far is that doping stabilizes simple cubic structures with a 5.5 and space group Fm 3m. 13] By comparison, doped d-Bi2O3 materials which possess long-range superstructures usually have lower conductivities. Simple cubic doped materials, however, are often only metastable and convert to lower symmetry forms with significantly lower conductivities, which is a major obstacle to their practical use. Initial X-ray and electron diffraction studies of the 3 3 3 fluorite superstructures in the Bi2O3-V2O5 system carried out by Zhou, suggested the existence of a phase with composition Bi18V2O32 (Bi1 xVxO1.5+x with x = 0.100); the closely related Bi16V2O29 (x = 0.111) was also found to be a 3 3 3 fluorite supercell, but distinguishable from Bi18V2O32 based on peak positions in its diffraction pattern. In our syntheses (see Methods in ESI), single-phase materials were formed for compositions with x = 0.087 and 0.095 (Bi0.913V0.087O1.587 and Bi0.905V0.095O1.595), by firing the starting oxides at 825 8C for 12 h, after initial calcinations at 700, 750, and 800 8C (for 12 h at each temperature with intermediate grinding); we will occasionally refer to these two very similar compositions jointly as “Bi18V2O32”. Bi16V2O29 started appearing as a second phase for 0.095< x< 0.100 and V-doped gBi2O3 was present for 0.074< x< 0.083 (see Figure S1 in the Supporting Information). Impedance measurements on Bi0.905V0.095O1.595 and Bi0.913V0.087O1.587 (Figure 1) were carried out on heating to [*] X. Kuang, J. L. Payne, I. Radosavljevic Evans Department of Chemistry, University of Durham Science Site, Durham DH1 3LE (UK) E-mail: ivana.radosavljevic@durham.ac.uk

An X-ray powder diffraction study of the spin-crossover transition and structure of bis(2,6-dipyrazol-1-ylpyrazine)iron(II) perchlorate

The crystal structure of the iron(II) spin-crossover compound [Fe(C(10)H(8)N(6))(2)](ClO(4))(2) in the high-spin state has been solved from powder X-ray diffraction data using the DASH program and refined using Rietveld refinement. The thermal spin transition has been monitored by following the change in unit-cell parameters with temperature. The title compound has been found to undergo a crystallographic phase change, involving a doubling of the crystallographic a axis, on undergoing the spin transition.