Single-crystal structures, optical absorptions, and electronic distributions of thorium oxychalcogenides ThOQ (Q = S, Se, Te).

The compounds ThOS, ThOSe, and ThOTe have been synthesized, and their structures have been determined by means of single-crystal X-ray diffraction methods. All three compounds adopt the PbFCl structure type in the tetragonal space group D(4h)(7) - P4/nmm. More precise crystallographic data have been obtained for ThOS and ThOSe, which had previously only been known from X-ray powder diffraction data. ThOS, ThOSe, and ThOTe are yellow-, orange-, and black-colored, respectively. From single-crystal optical absorption measurements the band gaps are 2.22, 1.65, and 1.45 eV, respectively. Optical band gaps, ionic charges, and densities of states were calculated for the three compounds with the use of Density Functional methods.

Syntheses and characterization of six quaternary uranium chalcogenides A2M4U6Q17 (A = Rb or Cs; M = Pd or Pt; Q = S or Se).

The A(2)M(4)U(6)Q(17) compounds Rb(2)Pd(4)U(6)S(17), Rb(2)Pd(4)U(6)Se(17), Rb(2)Pt(4)U(6)Se(17), Cs(2)Pd(4)U(6)S(17), Cs(2)Pd(4)U(6)Se(17), and Cs(2)Pt(4)U(6)Se(17) were synthesized by the high-temperature solid-state reactions of U, M, and Q in a flux of ACl or Rb(2)S(3). These isostructural compounds crystallize in a new structure type, with two formula units in the tetragonal space group P4/mnc. This structure consists of a network of square-planar MQ(4), monocapped trigonal-prismatic UQ(7), and square-antiprismatic UQ(8) polyhedra with A atoms in the voids. Rb(2)Pd(4)U(6)S(17) is a typical semiconductor, as deduced from electrical resistivity measurements. Magnetic susceptibility and specific heat measurements on single crystals of Rb(2)Pd(4)U(6)S(17) show a phase transition at 13 K, the result either of antiferromagnetic ordering or of a structural phase transition. Periodic spin-polarized band structure calculations were performed on Rb(2)Pd(4)U(6)S(17) with the use of the first principles DFT program VASP. Magnetic calculations included spin-orbit coupling. With U f-f correlations taken into account within the GGA+U formalism in calculating partial densities of states, the compound is predicted to be a narrow-band semiconductor with the smallest indirect and direct band gaps being 0.79 and 0.91 eV, respectively.

Oxidation state of uranium in A6Cu12U2S15 (A = K, Rb, Cs) compounds.

Black single crystals of A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) have been synthesized by the reactive flux method. These isostructural compounds crystallize in the cubic space group Ia ̅3d at room temperature. The structure comprises a three-dimensional framework built from US(6) octahedra and CuS(3) trigonal planar units with A cations residing in the cavities. There are no S-S bonds in the structure. To elucidate the oxidation state of U in these compounds, various physical property measurements and characterization methods were carried out. Temperature-dependent electrical resistivity measurement on a single crystal of K(6)Cu(12)U(2)S(15) showed it to be a semiconductor. These three A(6)Cu(12)U(2)S(15) (A = K, Rb, Cs) compounds all exhibit small effective magnetic moments, < 0.58 µ(B)/U and band gaps of about 0.55(2) eV in their optical absorption spectra. From X-ray absorption near edge spectroscopy (XANES), the absorption edge of A(6)Cu(12)U(2)S(15) is very close to that of UO(3). Electronic band structure calculations at the density functional theory (DFT) level indicate a strong degree of covalency between U and S atoms, but theory was not conclusive about the formal oxidation state of U. All experimental data suggest that the A(6)Cu(12)U(2)S(15) family is best described as an intermediate U(5+)/U(6+) sulfide system of (A(+))(6)(Cu(+))(12)(U(5+))(2)(S(2-))(13)(S(-))(2) and (A(+))(6)(Cu(+))(12)(U(6+))(2)(S(2-))(15).

Structure, properties, and theoretical electronic structure of UCuOP and NpCuOP.

The compounds UCuOP and NpCuOP have been synthesized and their crystal structures were determined from low-temperature single-crystal X-ray data. These isostructural compounds crystallize with two formula units in space group P4/nmm of the tetragonal system. Each An atom (An = U or Np) is coordinated to four O and four P atoms in a distorted square antiprism; each Cu atom is coordinated to four P atoms in a distorted tetrahedron. Magnetic susceptibility measurements on crushed single crystals indicate that UCuOP orders antiferromagnetically at 224(2) K. Neutron diffraction experiments at 100 and 228 K show the magnetic structure of UCuOP to be type AFI (+ - + -) where ferromagnetically aligned sheets of U atoms in the (001) plane order antiferromagnetically along [001]. The electrical conductivity of UCuOP exhibits metallic character. Its electrical resistivity measured in the ordered region with the current flowing within the tetragonal plane is governed by the scattering of the conduction electrons on antiferromagnetic spin-wave excitations. The electrical resistivity of single-crystalline NpCuOP shows semimetallic character. It is dominated by a pronounced hump at low temperatures, which likely arises owing to long-range magnetic ordering below about 90 K. Density of state analyses using the local spin-density approximation show covalent overlap between AnO and CuP layers of the structure and dominant contributions from 5f-actinide orbitals at the Fermi level. Calculations on a 2 × 2 × 2 supercell of NpCuOP show ferromagnetic ordering within the Np sheets and complex coupling between these planes. Comparisons of the physical properties of these AnCuOP compounds are made with those of the family of related tetragonal uranium phosphide compounds.

First-principles investigations of Ti-substituted hydroxyapatite electronic structure.

The electronic structure of Ti-substituted hydroxyapatite is investigated using density functional theory within a periodic slab model. Two sorption mechanisms have been considered: i.e., Ti(4+) and Ti(OH)(2)(2+) as the likely species to exchange with Ca(2+). Ti(4+) has a small ionic radius compared to Ca(2+) and can dope into both distinct sites, showing no site preference; however, when two H were removed from the OH channel to obtain charge compensation, preferential site II substitution appears, accompanied with a large O shift forming a strong Ti-O bond. The species Ti(OH)(2)(2+) displays a strong site preference: substitution by Ti(OH)(2)(2+) on the hydroxyl channel (site II) is exothermic and favored strongly over the Ca column (site I). Ti(OH)(2)(2+) substitution for Ca(2+) induces a large geometry relaxation and distortion, especially within the OH channel and Ca(2+) column, with a considerable shift of Ti compared to the Ca sites in pure HA. These results are consistent with the experimental observation that material synthesis with high Ti doping (atomic ratio > 0.1) shows irregular particles formation with reduced crystallinity. The calculated cell shape and volume relaxations indicate that the volume and cell parameters both expand in all the substituted HA models. The site preference and volume expansion differences found are attributed to the metal ion shift caused in meeting the requirement of strong Ti-O coordination in site I and site II polyhedra.

Separation and molecular-level segregation of complex alkane mixtures in metal-organic frameworks.

In this computational work we explore metal-organic frameworks (MOFs) for separating alkanes according to the degree of branching. We show that the structure MOF-1 shows an adsorption hierarchy for a 13-component light naphtha mixture precisely as desired for increasing the research octane number of gasoline. In addition we report an unusual molecular-level segregation of molecules based on their degree of branching.

Initial stages of hydration and Zn substitution/occupation on hydroxyapatite (0001) surfaces.

The initial stages of the hydration process have been simulated on a single-Ca(I) terminated hydroxyapatite (0001) surface in step-by-step fashion using periodic slab density functional theory (DFT). Adsorption configurations and energetic properties have been described at different H(2)O coverage. At low H(2)O coverage, oxygen prefers to form CaO bonds with surface Ca cations, but as coverage increases, H(2)O tends to loosely float on the already-formed water layer. The height of the first layer H(2)O relative to the surface is found to be 1.6A. The hydration process does not cause the decomposition of surface phosphate groups and hydroxyl channel, but does affect the energetics of subsequent Zn substitution and occupation on Ca(I) and Ca(II) sites. The Ca(II) vacancy site is found to be energetically more favorable for occupation due to less spatial constraint. This suggested mechanism of preferential occupation is different from previous attempts to explain the cation substitution site preference in bulk by ionic radius and electronegativity differences.

Syntheses, structures, physical properties, and electronic properties of some AMUQ3 compounds (A = alkali metal, M = Cu or Ag, Q = S or Se).

The seven new isostructural quaternary uranium chalcogenides KCuUS 3, RbCuUS 3, RbAgUS 3, CsCuUS 3, CsAgUS 3, RbAgUSe 3, and CsAgUSe 3 were prepared from solid-state reactions. These isostructural materials crystallize in the layered KZrCuS 3 structure type in the orthorhombic space group Cmcm. The structure is composed of UQ 6 octahedra and MQ 4 tetrahedra that share edges to form (2) infinity[UMQ 3 (-)] layers. These layers stack perpendicular to [010] and are separated by layers of face- and edge-sharing AQ 8 bicapped trigonal prisms. There are no Q-Q bonds in the structure, so the formal oxidation states of A/U/M/Q may be assigned as 1+/4+/1+/2-, respectively. CsCuUS 3 shows semiconducting behavior with thermal activation energy E a = 0.14 eV and sigma 298 = 0.3 S/cm. From single-crystal absorption measurements in the near IR range, the optical band gaps of these compounds are smaller than 0.73 eV. The more diffuse 5f electrons play a much more dominant role in the optical properties of the AMUQ 3 compounds than do the 4f electrons in the AMLnQ 3 compounds (Ln = rare earth). Periodic DFT spin band-structure calculations on CsCuUS 3 and CsAgUS 3 establish two energetically similar antiferromagnetic spin structures and show magnetic interactions within and between the layers of the structure. Density-of-states analysis shows M-Q orbital overlap in the valence band and U-Q orbital overlap in the conduction band.

Structural, Electrical, Magnetic, and Spectroscopic Properties of Ring-Oxidized Molecular Metals Produced by Iodination of Metal-Free and Nickel Tetrabenzporphyrins.

Detailed studies of the structure, conductivitity, magnetoresistance, optical spectra, and magnetic properties (susceptibility, EPR) for the new molecular metal tetrabenzporphyrin iodide (H(2)(tbp)I) and the electrical, spectral, and magnetic properties of Ni(tbp)I are reported. Paramagnetic transition-ion impurities were carefully excluded during the synthesis of H(2)(tbp)I and Ni(tbp)I, and both materials show much higher, metal-like conductivites than previously seen for less-pure Ni(tbp)I. Comparison of the specular reflectance data for Ni(tbp)I and H(2)(tbp)I allows a distinction between purely ring pi-transitions and metal-involved charge-transfer transitions, and the spectra fix the energy levels of the pi orbitals involved in conduction. Transport, magnetic, and optical properties show that both H(2)(tbp)I and Ni(tbp)I are ring-based conductors that have metal-like conductivities, varying as approximately 1/T, down to ca. 30-40 K. However, the remaining level of defects is higher in the tbp conductors than in H(2)(pc)I, and whereas the latter is metallic down to the mK temperature range, the defects in the (tbp) compounds localize the conduction electrons at approximately 10 K (Ni(tbp)I) and approximately 30 K (H(2)(tbp)I), leading to transport through one-dimensional variable-range hopping. EPR g-values for H(2)(tbp)I and Ni(tbp)I are close to that for the free electron and are nearly temperature-independent. The line widths for both samples are extremely narrow and also are nearly temperature-independent. These results show that Ni(tbp)I does not display doubly-mixed valence, as thought earlier: Paramagnetic impurities significantly altered the EPR signals of the prior samples. H(2)(tbp)I crystallizes in the space group P4/mcc with cell constants of a = 14.173(10) Å and c = 6.463(4) Å. Full-matrix least-squares refinement of 63 variables gave an R index of 0.061 on F(o)(2).

The structure of strontium-doped hydroxyapatite: an experimental and theoretical study.

First-principles modeling combined with experimental methods were used to study hydroxyapatite in which Sr2+ is substituted for Ca2+. Detailed analyses of cation-oxygen bond distributions, cation-cation distances, and site 1-oxygen polyhedron twist angles were made in order to provide an atomic-scale interpretation of the observed structural modifications. Density functional theory periodic band-structure calculations indicate that the Ca2+ to Sr2+ substitution induces strong local distortion on the hydroxyapatite lattice: the nearest neighbor Sr-O bond structures in both cationic sites are comparable to pure SrHA, while Sr induces more distortion at site 2 than site 1. Infrared vibrational spectroscopy (FTIR) and extended X-ray absorption fine structure (EXAFS) analysis suggest increasing lattice disorder and loss of OH with increasing Sr content. Rietveld refinement of synchrotron X-ray diffraction patterns shows a preference for the Ca1 site at Sr concentrations below 1 at.%. The ideal statistical occupancy ratio Sr2/Sr1=1.5 is achieved for approximately 5 at.%; for higher Sr concentrations occupation of the Ca2 site is progressively preferred.

A structural analysis of lead hydroxyvanadinite.

Hydroxyvanadinite, Pb(10)(VO(4))(6)(OH)(2), was prepared by the co-precipitation method and analyzed by X-ray absorption spectroscopy (XANES, EXAFS), infrared spectroscopy, Raman scattering and X-ray diffraction (XRD). The results showed that the structure is very similar to that of vanadinite, Pb(10)(VO(4))(6)Cl(2), with space group P6(3)/m (176) and cell parameters a = 10.2242(3) A and c = 7.4537(2) A. A Rietveld refinement of the structure was performed using vanadinite as the starting model and fixing the geometry of the vanadate ion as a rigid body. First-principles Density Functional embedded cluster models are developed to analyze electronic structures, bonding, and densities of states. Interaction of Pb with the OH channel anion is examined in detail, as an important structural feature. A periodic band structure approach was used to obtain a further estimate of relaxed atomic coordinates.

Synthesis and structural characterization of some selenoruthenates and telluroruthenates.

The reaction of solid [RuClCp(PPh(3))(2)] with TeSe(3)(2-) or Se(n)(2-) in DMF leads to the formation of [RuCp(PPh(3))(mu(2)-Se(2))](2) (1). In the structure of this compound the two bridging Se(2) groups lead to a six-membered Ru(2)Se(4) ring in a chair conformation. Attached to each Ru center is a PPh(3) ligand in an equatorial position and a Cp ring in an axial position. The compound is diamagnetic. The compound [Ru(2)Cp(2)(mu(3)-Se(2))(mu(3)-Se)](2) (2) is obtained under similar conditions in the presence of air. This structure comprises a centrosymmetric Ru(4)Se(6) dimer formed from the two bridging Se groups and the two bridging Se(2) groups. Each Ru center is pi-bonded to a Cp ring. The reaction of solid [RuClCp(PPh(3))(2)] with a Te(n)(2-) polytelluride solution in DMF leads to the diamagnetic compound [(RuCp(PPh(3)))(2)(mu(2)-(1,4-eta:3,6-eta)Te(6))] (3). Here the Ru centers are bound to a bridging Te(6) chain at the 1, 4, 3, and 6 positions, leading to a bicyclic Ru(2)Te(6) ring. Each Ru atom is bound to a Cp ring and a PPh(3) group. This dimer possesses a center of symmetry. The structure of 3 is the first example of a bicyclic complex where fusion occurs along a Te-Te bond. If the same reaction is carried out in DMF/CH(2)Cl(2), rather than DMF, then [(RuCp(PPh(3)))(2)(mu(2)-(1,4-eta:3,6-eta)Te(6))].CH(2)Cl(2) (4) is obtained. In the solid state it possesses the same Ru(2)Te(6) structural unit as does 3, but the unit lacks a crystallographically imposed center of symmetry. The electronic structures of 3 and 4 have been analyzed with the use of first principles density functional theory. Bond order analysis indicates that the Te-Te bond where fusion occurs has a shared bonding charge of about (2)/(3) of that found for Te-Te single bonds.

New layered rubidium rare-earth selenides: syntheses, structures, physical properties, and electronic structures for RbLnSe(2).

The compounds RbLnSe(2) (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Lu) have been synthesized by means of the reactive flux method at 1173 K. These isostructural compounds, which have the alpha-NaFeO(2) structure type, crystallize with three formula units in space group D(3d)(5)-R(-)3m of the trigonal system in cells at T = 153 K of dimensions (a, c in A) La, 4.4313(4), 23.710(3); Ce, 4.3873(3), 23.656(3); Pr, 4.3524(11), 23.655(7); Nd, 4.3231(5), 23.670(4); Sm, 4.2799(4), 23.647(3); Gd, 4.2473(7), 23.689(5); Tb, 4.2197(4), 23.631(3); Ho, 4.1869(6), 23.652(5); Er, 4.1541(8), 23.576(7); Lu, 4.1294(6), 23.614(5). The structure consists of close-packed Se layers in a pseudocubic structure distorted along [111]. The Rb and Ln atoms occupy distorted octahedral sites in alternating layers. The Rb-centered octahedra share edges with the Ln-centered octahedra between layers. Within a given layer, both the Rb-centered and Ln-centered octahedra share edges with themselves. RbTbSe(2) and RbErSe(2) exhibit Curie-Weiss paramagnetism between 5 and 300 K, and RbCeSe(2) exhibits Curie-Weiss paramagnetism between 100 and 300 K. The optical transitions for RbCeSe(2), RbTbSe(2), and RbErSe(2) are in the 2.0-2.2 eV region of the spectrum, both from diffuse reflectance spectra and from first-principles calculations. These calculations also provide insight into the electronic structures and chemical bonding in RbLnSe(2). A quadratic fit for the lanthanide contraction of the Ln-Se distance is superior to the linear one only if the closed-shell atoms La and Lu are included.

Structural analysis of porphyrin molecular squares using molecular mechanics and density-functional methods.

"Molecular squares" formed from Re(CO)(3)Cl corners and porphyrin sides have potential applications as hosts for catalytic sites and as building blocks for membranes. In these materials, knowledge of the conformations of the squares is important. Molecular-mechanics (MM) and density-functional (DF) calculations have been used iteratively in this work to find the minimum-energy configurations of several porphyrin molecular squares. MM predicts that the steric and torsional interactions at connecting junctures of the square framework determine the overall geometry. Torsional degrees of freedom around these junctures were therefore analyzed using DF methods, giving further insight and helping choose among MM force-field options. Single-point DF calculations on the entire squares showed that the energy and conformation of the entire square could be reliably obtained by performing DF calculations on the critical elements of the square and then piecing them together. This "piecewise" strategy allows for both the major torsional motions and the most important local relaxations of large supramolecular species such as molecular squares.

New quaternary bismuth sulfides: syntheses, structures, and band structures of AMBiS4 (A = Rb, Cs; M = Si, Ge).

Four new compounds, RbSiBiS(4), RbGeBiS(4), CsSiBiS(4), and CsGeBiS(4), have been synthesized by means of the reactive flux method. The isostructural compounds RbSiBiS(4), RbGeBiS(4), and CsGeBiS(4) crystallize in space group P2(1)/c of the monoclinic system with four formula units in cells of dimensions at 153 K of a = 6.4714(4) A, b = 6.7999(4) A, c = 17.9058(11) A, and beta = 108.856(1) degrees for RbSiBiS(4), a = 6.5864(4) A, b = 6.8559(4) A, c = 17.9810(12) A, and beta = 109.075(1) degrees for RbGeBiS(4), and a = 6.5474(4) A, b = 6.9282(4) A, c = 18.8875(11) A, and beta = 110.173(1) degrees for CsGeBiS(4). CsSiBiS(4) crystallizes in a different structure type in space group P2(1)/c of the monoclinic system with four formula units in a cell of dimensions at 153 K of a = 9.3351(7) A, b = 6.9313(5) A, c = 12.8115(10) A, and beta = 109.096(1) degrees. The two structure types are closely related and consist of [MBiS(4)(-)] (M = Si, Ge) layers separated by bicapped trigonal-prismatically coordinated alkali-metal atoms. In each, the M atom is coordinated to a tetrahedron of four S atoms and the Bi atom is coordinated to seven S atoms comprising five close S atoms at the corners of a square pyramid with Bi near the center of the basal plane and the sixth and seventh S atoms further away to complete a distorted monocapped trigonal prism. The optical band gaps of 2.23 eV for RbGeBiS(4) and 2.28 eV for CsGeBiS(4) were deduced from their diffuse reflectance spectra. From a band structure calculation, the optical absorption for RbGeBiS(4) originates from the [GeBiS(4)(-)] layer. The Ge 4p orbitals, Bi 6p orbitals, and S 3p orbitals are highly hybridized.

Surface structures of SrTiO3 (001): a TiO2-rich reconstruction with a c(4 x 2) unit cell.

We report the solution of the c(4 x 2) reconstruction of SrTiO(3) (001), obtained through a combination of high-resolution transmission electron microscopy, direct methods analysis, and density functional theory. The structure is characterized by a single overlayer of TiO(2) stoichiometry in which TiO(5) polyhedra are arranged into edge-shared structures, in contrast to the corner-shared TiO(6) polyhedra in bulk. This structural pattern is similar to that reported by us earlier for the (2 x 1) reconstruction of the same crystal face formed at higher temperature. We discuss probable mechanisms of surface stabilization as revealed by these two solutions which are likely to apply to other reconstructions of SrTiO(3) (001) and, possibly, other perovskites in general.

The structure and chemistry of the TiO(2)-rich surface of SrTiO(3) (001).

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of 'dangling bonds' by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO(3)--a standard model for oxides with a perovskite structure--have been studied extensively. Here we report a solution of the 2 x 1 SrTiO(3) (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO(6-x) units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO(3). We suggest that this structural concept can be extended to perovskite surfaces in general.

Syntheses, structure, some band gaps, and electronic structures of CsLnZnTe3 (Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Y).

Eleven new quaternary rare-earth tellurides, CsLnZnTe3 (Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Y), were prepared from solid-state reactions at 1123 K. These isostructural materials crystallize in the layered KZrCuS3 structure type in the orthorhombic space group Cmcm. The structure is composed of LnTe6 octahedra and ZnTe4 tetrahedra that share edges to form [LnZnTe3] layers. These layers stack perpendicular to [010] and are separated by layers of face- and edge-sharing CsTe8 bicapped trigonal prisms. There are no Te-Te bonds in the structure of these CsLnZnTe3 compounds so the formal oxidation states of Cs/Ln/Zn/Te are 1+/3+/2+/2-. Optical band gaps of 2.13 eV for CsGdZnTe3 and 2.12 eV for CsTbZnTe3 were deduced from single-crystal optical absorption measurements. A first-principles calculation of the density of states and the frequency-dependent optical properties was performed on CsGdZnTe3. The calculated band gap of 2.1 eV is in good agreement with the experimental value. A quadratic fit for the lanthanide contraction of the Ln-Te distance is superior to a linear one if the closed-shell atom is included.