Fractional Site Occupancy Research Articles

Adlayer formation on C-plane (0001) and R-plane Al2O3 surfaces.

Adlayers on C-plane (0001) and R-plane terminated surfaces of corundum phase aluminum oxide were synthesized by annealing mixtures of two oxide powders, aluminum oxide with an additive. Using high-angle annular dark field scanning transmission electron microscopy, the adsorbed layers were characterized, and image simulations aided interpretation of the results. The adlayers were pseudomorphic, one atomic layer thick and with a fractional site occupancy. Atomic positions of the adlayer atoms relaxed and changed relative to the bulk structure, where there is evidence that the magnitude of the relaxation is sensitive to the ionic radius of the adsorbate. The pseudomorphic adlayer structure formed for different elements including, but not limited to, the lanthanides (i.e., Ge, Ba and Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm).

Cation Distributions and Magnetic Properties of Ferrispinel MgFeMnO4.

The crystal structure and magnetic properties of the cubic spinel MgFeMnO4 were studied by using a series of in-house techniques along with large-scale neutron diffraction and muon spin rotation spectroscopy in the temperature range between 1.5 and 500 K. The detailed crystal structure is successfully refined by using a cubic spinel structure described by the space group Fd3̅m. Cations within tetrahedral A and octahedral B sites of the spinel were found to be in a disordered state. The extracted fractional site occupancies confirm the presence of antisite defects, which are of importance for the electrochemical performance of MgFeMnO4 and related battery materials. Neutron diffraction and muon spin spectroscopy reveal a ferrimagnetic order below TC = 394.2 K, having a collinear spin arrangement with antiparallel spins at the A and B sites, respectively. Our findings provide new and improved understanding of the fundamental properties of the ferrispinel materials and of their potential applications within future spintronics and battery devices.

Li0.17Na5.83Mo11O36

The title mixed-alkali-metal molybdenium oxide, hexa­kis­(lithium/sodium) undeca­molybdate, was synthesized by solid-state reaction at 793 K. Its [Mo11O36]6− framework is built up from MoO6 octa­hedra and MoO5 pyramids linked together by edges and vertices. The framework delimits two types of inter­secting tunnels running along [100] and [001], where the sodium and lithium ions are located. Two of the sodium ions and the lithium ion have fractional site occupancies. One of the Mo atoms has site symmetry 2, one sodium ion has site symmetry 2 and one has site symmetry -1, and the Li+ ion has site symmetry 2. Structural relationships between the title compound and the anatase and Na6Mo11O36 structures are discussed.

Effects of Hydration during Strontium Exchange into Nanoporous Hydrogen Niobium Titanium Silicate

A Nb-substituted titanium silicate with the sitinakite (NbTS) topology was exchanged with Sr(2+) to determine the mechanisms and pathways of ion diffusion through this mixed polyhedral nanoporous framework. The refined structural models yield unit cell parameters and atomic positions of Sr(2+) and suggest that there was a two-step process during cation diffusion. The starting material of the exchange experiment was the H(+)-exchanged material, H(1.4)Nb(0.6)Ti(1.4)SiO(7)·1.9H(2)O, with space group P4(2)/mcm. In the beginning of the exchange process, Sr filled the 8-membered-ring channel near the 4(2) axis in the center. Once the Sr(2+) fractional occupancy reached approximate 0.11, Sr positions and extra-framework H(2)O molecules shifted away from the central 8-membered-ring toward the framework, and an increase in Sr hydration and framework bonding was observed. The new H(2)O positions resulted in a lowering of symmetry to the P ̅42m space group, and it is thought that the Sr migration served to enhance Sr(2+) ion diffusion capacity into the channels of NbTS since the exchange rate briefly accelerated after the 0.11 fractional occupancy level was passed. Exchange of Sr(2+) into the nanoporous material reached maximum fractional site occupancy of approximately 0.20 using a 10.0 mM SrCl(2) solution.

LaZn12.37 (1), a zinc-deficient variant of the NaZn13structure type

The title compound (lanthanum dodecazinc), LaZn12.37 (1), is confirmed to be a nonstoichiometric (zinc-deficient) modification of the NaZn13 structure type, in which one Zn atom (Wyckoff site 8b, site symmetry m ) has a fractional site occupancy of 0.372 (11). The other Zn atom (96i, m) and the La atom (8a, 432) are fully occupied. The coordination polyhedra of the Zn atoms are distorted icosa­hedra, whereas the La atoms are surrounded by 24 Zn atoms, forming pseudo-Frank–Kasper polyhedra. Electronic structure calculations indicate that Zn—Zn bonding is much stronger than La—Zn bonding.

The Drosophila Gap Gene Network Is Composed of Two Parallel Toggle Switches

Drosophila “gap” genes provide the first response to maternal gradients in the early fly embryo. Gap genes are expressed in a series of broad bands across the embryo during first hours of development. The gene network controlling the gap gene expression patterns includes inputs from maternal gradients and mutual repression between the gap genes themselves. In this study we propose a modular design for the gap gene network, involving two relatively independent network domains. The core of each network domain includes a toggle switch corresponding to a pair of mutually repressive gap genes, operated in space by maternal inputs. The toggle switches present in the gap network are evocative of the phage lambda switch, but they are operated positionally (in space) by the maternal gradients, so the synthesis rates for the competing components change along the embryo anterior-posterior axis. Dynamic model, constructed based on the proposed principle, with elements of fractional site occupancy, required 5–7 parameters to fit quantitative spatial expression data for gap gradients. The identified model solutions (parameter combinations) reproduced major dynamic features of the gap gradient system and explained gap expression in a variety of segmentation mutants.

μ-Bis(di-o-tolylphosphanyl)methane-1:2κ2P:P′]decacarbonyl-1κ3C,2κ3C,3κ4C-triangulo-triruthenium(0)–methanol (8/1)

The asymmetric unit of the title compound, [Ru3(C29H30P2)(CO)10].1/8CH3OH, contains two triangulo-triruthenium mol­ecules with similar configurations and a methanol solvent mol­ecule (fractional site occupancy for the latter = 0.25). The bis­(di-o-tolyl­phosphan­yl)methane ligand bridges an Ru—Ru bond and its P atoms are equatorial with respect to the Ru3 triangle. The phosphine-substituted Ru atoms each bear one equatorial and two axial terminal carbonyl ligands whereas the unsubstituted Ru atom carries two equatorial and two axial terminal carbonyl ligands. The dihedral angles between the two benzene rings attached to each P atom are 76.26 (13) and 74.76 (15)° for the first mol­ecule and 77.21 (13) and 75.68 (14)° for the second. In the crystal, mol­ecules are linked into [001] chains via inter­molecular C—H⋯O hydrogen bonds. Weak inter­molecular C—H⋯π inter­actions also occur.

μ-Bis(di-o-tolylphosphanyl)methane-1:2κ2P:P′]nonacarbonyl-1κ3C,2κ3C,3κ3C-[diphenyl(phenylsulfanylmethyl)phosphane-3κP]-triangulo-triruthenium(0) dichloromethane 0.25-solvate

In the title compound, [Ru3(C29H30P2)(C19H17PS)(CO)9]·0.25CH2Cl2, the atoms of the dichloro­methane solvent mol­ecule have a fractional site occupancy of 0.25; the dichloro­methane mol­ecule is disordered about an inversion centre. The bis­(di-o-tolyl­phosphan­yl)methane ligand bridges an Ru—Ru bond and the monodentate phosphane ligand bonds to the third Ru atom; its S-bonded phenyl ring is disordered over two orientations in a 0.53 (4):0.47 (4) ratio. All the P atoms are equatorial with respect to the Ru3 triangle: each Ru atom also bears one equatorial and two axial terminal carbonyl ligands. The dihedral angles between the two benzene rings attached to each P atom of the diphenyl­phosphanyl ligand are 68.4 (2) and 71.5 (2)°. In the crystal, mol­ecules are linked into [001] chains via inter­molecular C—H⋯O hydrogen bonds. Weak inter­molecular C—H⋯π inter­actions also occur.

The crystal structures of Hf 3±δNb 4±δAs 3 and Hf 7.2Nb 3.8As 4: Members of a homologous series combining W-type, Mg-type and AlB 2-type building blocks

Two new partially ordered compounds were synthesized in the system Hf–Nb–As by arc melting and subsequent annealing at 1400 °C and they were structurally characterized by means of single crystal X-ray diffraction. The compound Hf 3± δ Nb 4± δ As 3 is isostructural to Zr 3Pd 4P 3 and Hf 3Pd 4P 3 (Space group: Pnma, Pearson symbol oP40) and shows a considerable homogeneity range between the limiting compositions Hf 2.4Nb 4.6As 3 and Hf 4.1Nb 2.9As 3. Mixed occupation of Hf and Nb were found at all seven independent metal sites, with varying Hf/Nb ratios, so the compound is stabilized by differential fractional site occupation (DFSO). The compound Hf 7.2Nb 3.8As 4 is isostructural with Zr 6.45Nb 4.55P 4 ( Immm, oI30) and was found to be metastable at the temperature of annealing (1400 °C). Similar to the type structure, Hf 7.2Nb 3.8As 4 is stabilized by (DFSO). Structural relations of the two compounds with Hf 1.5+ δ Nb 1.5− δ As ( Pnma, oP16) are discussed. All three compounds belong to a homologous series M k +2 l + mX 2 m combining W-type, Mg-type and AlB 2-type building blocks.

Stoichiometry optimization of homoepitaxial oxide thin films using x-ray diffraction

Homoepitaxial SrTiO3 thin films grown by molecular beam epitaxy are analyzed using high-resolution x-ray diffraction and transmission electron microscopy. Measured 00L x-ray scans from stoichiometric and nonstoichiometric films are compared with calculations that account for the effects of film thickness, lattice parameter, fractional site occupancy, and an offset between film and substrate at the interface. It is found that thickness fringes, commonly observed around Bragg reflections even in stoichiometric homoepitaxial SrTiO3 films, arise from a film/substrate interface offset. Transmission electron microscopy studies confirm the presence of strain at those homoepitaxial interfaces that show an offset in x-ray diffraction. The consequences for stoichiometry optimization of homoepitaxial films using high-resolution x-ray diffraction and the quality of regrown oxide interfaces are discussed.

1,1′-Dimethyl-4,4′-(2,4-di-1-naphthylcyclobutane-1,3-diyl)dipyridinium–(E)-1-methyl-4-[2-(1-naphthyl)vinyl]pyridinium–4-aminobenzenesulfonate–water (0.25/1.50/2/2)

In the title compound, 1.5C18H16N+·0.25C36H32N2 2+·2C6H6NO3S−·2H2O, the monocation exists in the E configuration with respect to the ethenyl C=C double bond and is almost planar, the dihedral angles between the pyridinium and the fused six-membered rings being 3.1 (7) and 3.8 (8)°. The dication lies about an inversion centre. In the crystal, the dication occupies almost the same site occupied by monocations at (x, y, z) and (−x, 1 − y, 1 − z). The anions and water mol­ecules are linked into a chain along the a axis by O—H⋯O and N—H⋯O hydrogen bonds. The structure is further stabilized by C—H⋯O hydrogen bonds and π–π inter­actions between pyridinium and benzene rings, with centroid–centroid distances in the range 3.516 (9)–3.553 (8) Å. The crystal is a twin with twin law, TWIN 0 0 0 0 1 0 1. The monocation and dication are disordered with fractional site occupancy ratio of 0.75:0.25.

Crystal structures, site occupations and phase equilibria in the system V–Zr–Ge

The V–Zr–Ge system was studied for two isothermal sections at 900 and 1200 °C. Three ternary compounds VZrGe ( tI12, I4/ mmm, CeScSi-type), V x Zr 5− x Ge 4 ( oP36, Pnma, Sm 5Ge 4-type) and V 4+ x Zr 2− x Ge 5 ( oI44, Ibam, Si 5V 6-type) were structurally characterized. Optical microscopy and powder X-ray diffraction (XRD) were used for initial sample characterization and electron probe microanalysis (EPMA) of the annealed samples was used to determine the exact phase compositions. The variation of the cell parameters of the various ternary solid solutions with the composition was determined. The three ternary phases were structurally characterized by means of single crystal and powder XRD. While VZrGe is almost a line compound, V x Zr 5− x Ge 4 (0.2 ≤ x ≤ 3.0) and V 4+ x Zr 2− x Ge 5 (0.06 ≤ x ≤ 1.2) are forming extended solid solution ranges stabilized by differential fractional site occupancy of V and Zr on the metal sites.

The crystal structure of Hf 1.5+δNb 1.5−δAs and structure–composition relations in the section Hf 3As–Nb 3As

The title compound Hf 1.5+ δ Nb 1.5− δ As was characterized by means of single crystal X-ray diffraction. It represents a new structure type of intermetallic compounds (space group Pnma; lattice parameters a=7.142(2) Å, b=3.583(2) Å, c=11.640(2) Å) and shows a small homogeneity range corresponding to (0.1< δ<0.25) at 1400 °C. The crystal structure may be visualized by a combination of As-centred trigonal prisms of the metal atoms and bcc-like fragments formed by metal atoms. Structural relations with various binary arsenides are discussed. The structure of Hf 1.5+ δ Nb 1.5− δ As shows significant preferred site occupation of Hf and Nb at the three independent metal positions (differential fractional site occupancy). Structure–composition relations in the section Hf 3As–Nb 3As which also contains the new phase Hf 2+ δ Nb 1− δ As with Ti 3P-type structure (space group P4 2/ n) are discussed. Ground state energies of various ordered compounds with Hf 1.5+ δ Nb 1.5− δ As-, Ti 3P- and Ta 3As-type structures were calculated from ab initio density functional theory. These energies were used for thermodynamic calculations employing the compound energy formalism (CEF) with the aim to model the experimentally observed site fraction data for both ternary compounds as well as Gibbs energies at the temperature of equilibration (1400 °C).

Goethite surface reactivity: A macroscopic investigation unifying proton, chromate, carbonate, and lead(II) adsorption

The goethite surface structure has been extensively studied, but no convincing quantitative description of its highly variable surface reactivity as inversely related to its specific surface area (SSA) has been found. The present study adds experimental evidence and provides a unified macroscopic explanation to this anomalous behavior from differences in average adsorption capacities, and not in average adsorption affinities. We investigated the chromate anion and lead(II) cation adsorption behavior onto three different goethites with SSA varying from 50 to 94 m 2/g, and analyzed an extensive set of published anion adsorption and proton charging data for variable SSA goethites. Maximum chromate adsorption was found to occupy on average from 3.1 to 9.7 sites/nm 2, inversely related to SSA. Congruency of oxyanion and Pb(II) adsorption behavior based on fractional site occupancy using these values, and a site density analysis suggest that: (i) ion binding occurs to singly and doubly coordinated sites, (ii) proton binding occurs to singly and triply coordinated sites (ranging from 6.2 to 8 total sites/nm 2, in most cases), and (iii) a predominance of (210) and/or (010) faces explains the high reactivity of low SSA goethites. The results imply that the macroscopic goethite adsorption behavior may be predicted without a need to investigate extensive structural details of each specific goethite of interest.

Symmetry-adapted configurational modelling of fractional site occupancy in solids

A methodology is presented, which reduces the number of site-occupancy configurations tobe calculated when modelling site disorder in solids, by taking advantage of the crystalsymmetry of the lattice. Within this approach, two configurations are considered equivalentwhen they are related by an isometric operation; a trial list of possible isometrictransformations is provided by the group of symmetry operators in the parent structure,which is used to generate all configurations via atomic substitutions. We haveadapted the equations for configurational statistics to operate in the reducedconfigurational space of the independent configurations. Each configuration in this space ischaracterized by its reduced energy, which includes not only its energy but also acontribution from its degeneracy in the complete configurational space, via an entropicterm. The new computer program SOD (site-occupancy disorder) is presented,which performs this analysis in systems with arbitrary symmetry and any size ofsupercell. As a case study we use the distribution of cations in iron antimony oxideFeSbO4, where we also introduce some general considerations for the modelling of site-occupancydisorder in paramagnetic systems.

Stoichiometric oxygen content inNaxCoO2

The crystal structure and oxygen stoichiometry in two-layer Na0.74CoO2 and Na0.38CoO2 at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, lambda=1.1968 Angstroms and lambda=1.5403 Angstroms, are analyzed simultaneously by the Rietveld method, allowing for the independent refinement of all structural parameters. The fractional oxygen site occupancies are found to be 1.01(1) for Na0.74CoO2 and 0.99(2) for Na0.38CoO2 respectively. These results indicate that the oxygen content of these phases is stoichiometric to a precision of 1 to 2%, and therefore the formal cobalt oxidation state is determined solely by the sodium content. The analysis also reveals that both types of sodium ions in the structure are in off-center distorted trigonal prismatic geometry.

Fractional Site Occupation in Ternary Metal Compounds: Structure, Bonding, and Thermodynamics

Ternary compounds of the type (M,M′) x A y where M and M′ are early transition metals of the groups 4–6 and A is a main group element of the groups 14–16 are showing interesting substitution mechanisms among the metal atoms ranging from classical and partially ordered solid solution phases to ternary compounds showing differential fractional site occupation. In these compounds the transition metals show mixed site occupation at the metal positions in combination with pronounced site preferences leading to varying metal mixtures at crystallographically independent sites. The connection between partial ordering and the differences in the local coordination of the respective lattice sites is discussed. Chemical bonding arguments obtained from electronic calculations using the extended Huckel approach are used to understand the observed distribution of the metals over the respective lattice sites and allow a qualitative prediction of site preferences. A thermodynamic model was applied in order to investigate the observed substitution mechanism and Gibbs energies for the occupation of the lattice sites with different metal atoms could be obtained by adjusting the model parameters to the experimentally observed site fractions.

The co-crystal tetraaquabis(2-oxido-3,5-dinitrobenzoato)bis(2-oxido-3,5-dinitrobenzoic acid)tricopper–diaquabis(2-hydroxy-3,5-dinitrobenzoato)(2-oxido-3,5-dinitrobenzoic acid)copper–water (0.54/0.92/4): a reformulation?

In the title compound, [Cu3(C7H3N2O7)2(C7H2N2O7)2(H2O)4]0.54[Cu(C7H3N2O7)2(H2O)2]0.92·4H2O, the CuII ion located on an inversion centre has a fractional site occupancy of 0.54. When this CuII ion is present, a trinuclear complex also involving two adjacent CuII ions arises; when it is not, then the result is two discrete mononuclear complexes. A very similar structure was reported previously by Valigura, Melnık, Koman, Martiŝka, Korabik, Mroziński & Glowiak [Inorg. Chem. Commun. (2004), 7, 548–552]. However, in their material, arising from a different synthesis, the CuII ion with \overline{1} site symmetry is fully occupied, and thus only a trinuclear complex is formed. Here, the mononuclear CuII complex assumes a square-pyramidal coordination geometry, formed by two water mol­ecules and two 3,5-dinitro­salicylate anions, one of which is monodentate and the other bidentate.

Zr7Sb4: A New Binary Zirconium‐Rich Antimonide.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Zr(7)Sb(4): a new binary zirconium-rich antimonide.

Zr(7)Sb(4) has been prepared by arc-melting of the elemental components and annealing at 1000-1150 degrees C. Its crystal structure was determined by X-ray diffraction (Pearson symbol mP44, monoclinic, space group P2(1)/c, Z = 4, a = 8.4905(6) A, b = 11.1557(8) A, c = 11.1217(8) A, beta = 111.443(2) degrees at 295 K). Zr(7)Sb(4) is isotypic to Hf(6)TiSb(4), a compound stabilized by differential fractional site occupancy. It is the first binary group-4 antimonide with this metal-to-antimony ratio, but it differs from the corresponding phosphides and arsenides M(7)Pn(4) (M = Ti, Zr, Hf; Pn = P, As), which adopt the Nb(7)P(4)-type structure. Zr(7)Sb(4) is built up from layers excised from the tetragonal W(5)Si(3)-type structure; these layers are displaced relative to each other to maximize interlayer Zr-Zr and Zr-Sb bonding, as confirmed by band structure calculations.