The crystal structures of two very close, but distinct complex minerals of the lead sulfosalt group have been solved: sterryite, Cu(Ag,Cu)(3)Pb(19)(Sb,As)(22)(As-As)S(56), and parasterryite, Ag(4)Pb(20)(Sb,As)(24)S(58). They are analyzed and compared according to modular analysis. The fundamental building block is a complex column centred on a Pb(6)S(12) triangular prismatic core, with two additional long and short arms. The main chemical and topological differences relate to the short arm, which induces a relative a/4 shift (~2 Å along the elongation parameter) of the constitutive rod layers, as illustrated by distinct cell settings within the same space group (P2(1)/n and P2(1)/c, respectively). Selection of the shortest (i.e. strongest) (Sb,As)-S bonds permitted to enhance the polymeric organization of (Sb,As) atoms with triangular pyramidal coordination. These two quasi-homeotypic structures are expanded derivatives of owyheeite, Ag(3)Pb(10)Sb(11)S(28). The hierarchy of organization levels from zero- to three-dimensional entities is subordinated to building operators, which appear as the driving force for the construction of such complex structures. Minor cations (Ag, Cu) or the As-As pair in sterryite secure the final locking, which favours the formation of one or the other compound.
Five isomorphous d(0) transition metal oxofluoride compounds A(3)[M(2)O(x)F(11-x)]·(AF)(0.333) (A = K, Rb, NH(4); M = Nb, Mo, W; x = 2, 4) have been synthesized from acid fluoride solutions, and their crystal structures have been determined by single-crystal X-ray diffraction. The basic structural building units are dinuclear M(2)X(11) (dimers) formed from NbOF(5) or Mo(W)O(2)F(4) octahedra connected by the fluorine bridging atom. In the Nb(2)O(2)F(9) dimer, the O atoms occupy apical corners. In the M(2)O(4)F(7) (M = Mo, W) dimers two O atoms are also apically placed, whereas the other two O atoms are statistically disordered in equatorial planes. The arrangement of dimers is so that the hexagonal tunnels containing `free' fluoride ions are formed. During the irradiation process the orthorhombic structure of K(3)Nb(2)O(2)F(9)·(KF)(0.333) transforms into a pseudo-trigonal one with a = 23.15 Å, which is the [101] diagonal of the orthorhombic unit cell. The other four trigonal crystals are merohedral twins.
During a systematic investigation of the crystallization behaviour of 9,9'-[1,3,4-thiadiazole-2,5-diylbis(2,3-thiophendiyl-4,1-phenylene)]bis[9H-carbazole] (I), six single crystalline solvates were obtained and characterized by X-ray diffraction at 100 K. The structure of the hemi-2-butanone (MEK) solvate contains two crystallographically independent molecules of (I) related by pseudo-inversion symmetry. The structure is polytypic and composed of non-polar (I) layers and polar solvent layers. It can be described according to an extended order-disorder (OD) theory with relaxed vicinity condition. The observed polytype is of a maximum degree of order (MDO). Layer triples of the second MDO polytype are shown by twinning by inversion. The mono-benzene and mono-toluene solvates are isostructural. Whereas the (I) layers are isostructural to those of the idealized description of the hemi-MEK solvate, the solvent layers are non-polar, resulting in a fully ordered structure. The toluene molecule is ordered, the benzene molecule features disorder. The (I) layers in the sesqui-dioxane and sesqui-benzene solvates are isostructural and unrelated to those in the hemi-MEK, mono-benzene and mono-toluene solvates. The solvent layers are isopointal in both sesqui-solvates, but the stacking differs significantly. The hemi-dideuterodichloromethane (DCM-d(2)) solvate is made up of two kinds of (I) rods, spaced by DCM-d(2) molecules. Rods of one kind are similar to analogous rods in the sesqui-dioxane and the sesqui-benzene solvates, whereas rods of the other kind are only remotely related to rods in the hemi-MEK solvate.
The single-crystal diffraction structures of 38 salt forms of the base tyramine (4-hydroxyphenethylamine) are reported for the first time. Together with literature examples, these structures are discussed with respect to cation conformation, cation packing, hydrogen bonding and hydrate formation. It is found that isostructural cation packing can occur even with structurally different anions, with different hydration states and with different hydrogen bonding. Hydrate formation is found to be more likely both (i) when there is an increase in the total number of potential hydrogen bond acceptor and donor atoms; and (ii) when the ratio of potential hydrogen bond donor to acceptor atoms is low.
The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez-Folch et al. (1997). J. Chem. Cryst. 27, 367-369; Marsh (2004). Acta Cryst. B60, 252-253], and as polymorph (II), P2(1)2(1)2 with Z = 2 [Martins et al. (2009). J. Phys. Chem. A, 113, 5998-6003]. We have redetermined both structures at somewhat lower temperatures [(I) at 180 K rather than room temperature; (II) at 100 K rather than 150 K]. For polymorph (I) the space group Fdd2 is confirmed rather than the original choice of Cc. The molecular structures of both polymorphs are essentially identical, with exact crystallographic twofold symmetry, approximate C(2v) symmetry, and a trans orientation of the H-N-C=O moiety. In both polymorphs the molecules associate into chains of rings with graph set C(4)[R(2)(1)(6)] via bifurcated hydrogen-bond systems C(N-H)(2)···O=C. In the polar structure (I) the chains are necessarily all parallel, whereas in (II) equal numbers of parallel and antiparallel chains are present. Further physical investigations [differential scanning calorimetry (DSC), powder investigations, solvent-induced phase conversions] were undertaken: these showed: (i) that the commercially available compound consists predominantly of polymorph (II), which on heating transforms into polymorph (I) by an endothermic reaction, so that both polymorphs are related by enantiotropism; (ii) that polymorph (I) represents the more stable modification at room temperature, where polymorph (II) is metastable, with the thermodynamic transition temperature lying somewhere between 253 K and room temperature. An apparent third polymorph, consisting of fibrous needles, was shown by powder diffraction to consist of a mixture of polymorphs (I) and (II).
A comparative single-crystal X-ray diffraction structure analysis of the family of Al-Cu-Me (Me = Co, Rh and Ir) decagonal quasicrystals is presented. In contrast to decagonal Al-Cu-Co, the other two decagonal phases do not show any structured disorder diffuse scattering indicating a higher degree of order. Furthermore, the atomic sites of Rh and Ir can be clearly identified, while Cu and Co cannot be distinguished because of their too similar atomic scattering factors. The structure models, derived from charge-flipping/low-density elimination results, were refined within the tiling-decoration method but also discussed in the five-dimensional embedding approach. The basic structural building units of the closely related structures are decagonal clusters with 33 Å diameter, which are consistent with the available electron-microscopic images. The refined structure models agree very well with the experimental data.
The crystal structure of a phase-change recording material (the compound Ag(3.4)In(3.7)Sb(76.4)Te(16.5)) enclosed in a vacuum capillary tube was investigated at various temperatures in a heating process using a large Debye-Scherrer camera installed in BL02B2 at SPring-8. The amorphous phase of this material turns into a crystalline phase at around 416 K; this crystalline phase has an A7-type structure with atoms of Ag, In, Sb or Te randomly occupying the 6c site in the space group. This structure was maintained up to around 545 K as a single phase, although thermal expansion of the crystal lattice was observed. However, above this temperature, phase separation into AgInTe(2) and Sb-Te transpired. The first fragment, AgInTe(2), reliably maintained its crystal structure up to the melting temperature. On the other hand, the atomic configuration of the Sb-Te gradually varied with increasing temperature. This gradual structural transformation can be described as a continuous growth of the modulation period γ.
BNC nanotubes and nanofibers have been synthesized in the high isostatic pressure apparatus in Ar at 1923 K and 1.5 MPa in the presence of yttrium aluminium garnet. Some of the nanotubes obtained were filled with Al(2)O(3). Transmission electron microscopy (TEM) studies have shown that the nanotubes and nanofibers have a polygonal cross-section (prismatic shape), and most often they are twisted, which is due to the transversal instability of the nanotubes originating under the growth conditions, including temperature treatment. Twisting also revealed itself in the appearance of the moiré fringes during the TEM observation of some of the nanotubes and nanofibers. Analysis of these fringes has shown that the facets of these nanotubes represent the slightly misoriented hexagonal BN and/or C plates. An Al(2)O(3) filling of the nanotube makes it harder to twist when subjected to torque, which conforms to the tube deformation theory.
The structural relation between malachite and the brochantite MDO (maximum degree of order) polytypes is discussed. It is demonstrated that the same building blocks which form the basis of brochantite polytypism also occur in malachite. The different arrangements of these building blocks in the two mineral structures are rationalized as a result of the different coordination geometries required by the respective non-metal atoms acting as linkers. The compound stoichiometries are discussed in light of a common structured formula scheme, in which pairs of H atoms can play a similar role as single non-H atoms. An overview on the occurrence of malachite-like building blocks in several other crystal structures is given.
It is evident from the literature that the 122 iron arsenides, XFe(2)As(2) with X = Ca, Sr, Ba or Eu, undergo one or more phase transitions from a higher-temperature paramagnetic tetragonal structure in grey group I4/mmm1' to an antiferromagnetic structure with magnetic space group C(A)mca. Symmetry analysis is used to enumerate the possibilities for the transition (or transitions) between the higher- and lower-symmetry structures, and the results are used as a basis for comment on published experimental results.
The bisphosphoramidate (C(6)H(5)O)(2)P(O)NH(CH(2))(4)NHP(O)(OC(6)H(5))(2) crystallizes in two polymorphs, one (ndl) with a needle habit from tetrahydrofuran (THF)/ethanol and another (prm) which forms prisms from H(2)O/ethanol. The molecules in the two forms differ from each other in some torsion angles and the orientation of the diaminobutane bridge, although the differences between the similar bond lengths are not significant for the two polymorphs. The geometry optimizations at the B3LYP/6-31+G* level for isolated molecules show that the two conformers which exist in the crystalline state also represent local gas-phase energy minima. The decrease in the N-H distance from the optimized to the crystal structures has been described in terms of the decrease in electron density (ρ) at the bond-critical point (b.c.p.) of the N-H bond path when the molecule participates in hydrogen bonding, comparing the results of atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses for fully optimized structures ndl and prm with their hydrogen-bonded model clusters.
The packing density of various structures is important not only for understanding and the prediction of high-pressure phase transitions, but also because of its reported correlation with thermodynamic stability. Plotting the cube root of formula volume against the cation radii (R) for nine morphotropic series with isolated tetrahedral anions, A(2)MO(4) (M = Si, Ge, S, Se, Cr, Mn, Mo, W) and A(2)BeF(4), permits the comparison of packing densities for 13 structure types (about 80 individual compounds and several solid solutions) stable at (or near) ambient temperature. The spinel type is the densest. The next densest types are those of K(2)MoO(4), Tl(2)CrO(4), β-Ca(2)SiO(4), β-K(2)SO(4), Ag(2)CrO(4) and Sr(2)GeO(4). In three series (M = Ge, Mo, W) the densest type comes with somewhat intermediate values of R, and not the largest, in contrast to the classical homology rule. Another contradiction with traditional views is that some of the densest phases have abnormally low overall binding energies. The correlation between packing density and coordination number (CN) is better when CN of A counts entire MX(4) groups rather than individual X atoms; many, but not all, A(2)MX(4) structures have binary A(2)M analogues (of course, A and M are not necessarily the same in these structure types). The most frequent arrangement of A around M is of the Ni(2)In type: a (distorted) pentacapped trigonal prism.
The structure of low-temperature grown GaAs with equidistant δ-layers of Sb and P was studied by analysis of the X-ray curves, which was supported by optical absorption measurements and transmission electron microscopy. The simultaneous fitting of the X-ray reflectivity curve and diffraction ones for GaAs (004) and GaAs (115) crystallographic planes provided reliable information about the period of δ-layer superlattice, thickness of the Sb and P δ-layers, and amount of excess As. Variation of these parameters was documented when excess As precipitated into As nanoinclusions upon annealing. The Sb and P δ-layers impact differently on the As precipitation processes in low-temperature grown GaAs. The combination of Sb and P δ-layers appears to be an effective tool for spatial patterning of the nanoinclusion array and prevention of the defect formation under annealing.
The synthesis and characterization of a series of halogen-substituted pseudoterpyridine Zn(II) homoleptic mononuclear complexes, based on ligands L(11)-L(44) [2,6-pyridinedicarboxaldehydebis(p-R-phenylimines), R = F, Cl, Br, I] are reported. Neither of the structures contain relatively strong classical hydrogen bonds (OH···O, NH···O, OH···N, NH···N) and the structure packing is thus determined by a subtle interplay of weaker interactions. Isostructurality of the four halogen analogues is very rare, and in this study -Br, -Cl and -F are found to be isostructural in different degrees, whereas -I is not. Interestingly, although it is closely isostructural to the -Cl and -Br compounds, the F analogue is shown not to form F···O bonds, while the Cl and the Br analogues do form Hal···O bonds. This raises an important question on the role of Hal···O bonds in the structuration of the crystal packing, particularly the stabilization effect. Similarly, while the CH···Hal interaction seems to give one-dimensional cohesion in the -Cl and -Br analogues, this feature is absent in the -F analogue, despite its close isostructurality. CH···O interactions appear to dominate to a first degree the cohesion between the anionic trifluoromethanesulfonate network and the cationic Zn-pyridinedicarboxaldehydebis(p-R-phenylimines) network. The analysis of these interactions is corroborated by reduced density gradient calculations based on promolecular densities.
A detailed analysis of correlations between structural features and cation conductivity is performed for KAlO(2) polymorphs in a wide temperature range of 300-1023 K. To explore the migration maps of K(+) cations we have used neutron diffraction data for low- and high-temperature KAlO(2) polymorphs and applied for the first time a novel algorithm based on the natural tiling concept and implemented it into the program package TOPOS. Five independent elementary channels for the K(+) cation migration have been revealed whose cross-sections were found to be essentially different in the low-temperature form, indicating a high anisotropy of the cation conductivity. During the transition to the cubic high-temperature phase all five channels become equivalent with sharply increased cross-sections that account for the jump-like increase of the cation conductivity and gives rise to its three-dimensional character.
Hydroxycarbonates with the general formula Me(2)(CO(3))(OH)(2) are widely used materials in industrial processes and are widespread in nature. The Cu term, malachite, Cu(2)CO(3)(OH)(2), is monoclinic, P2(1)/a. Substitution of Cu(2+) with other bivalent cations such as Mg, Zn, Fe, Cu or Ni is possible and leads to a different structure type, rosasite, P2(1)/a or P2(1)/b11 in the same cell setting as malachite. Rosasite structure is topologically similar to malachite, but the symmetry elements are oriented differently with respect to structural units. The stability of the malachite-like structure (MS) compared with the rosasite-like structure (RS) has been suggested to be related to the Jahn-Teller effect in CuO(6) coordination polyhedra. For this reason the hypothesis of the phase transition of malachite, Cu(2)CO(3)(OH)(2), to a rosasite structure at high pressure, as a result of the reduced Jahn-Teller effect, has been tested and confirmed by powder and single-crystal diffraction structural studies: above 6 GPa the malachite structure is no longer stable and transforms to a RS structure. RS Cu(2)CO(3)(OH)(2) is 3% more dense than malachite and the bulk modulus is remarkably higher, 80 (2) GPa compared with 48 (4) GPa. The longer apical Cu-O bonds in the distorted Me1 octahedral site are progressively shortened with increasing pressure, revealing a decrease in the Jahn-Teller effect at high pressure. The transition has a first-order character, is reversible with a significant hysteresis, and there is no evidence of any intermediate phase between the two structures. We then have further evidence that in the Me(2)(CO(3))(OH)(2) compounds, the two main structural types, MS and RS, are closely related. The former structure is stabilized only when Cu is the prevalent cation in the octahedral sites, and it can transform directly to the RS as a function of thermodynamic changes.
The concept that equates oxidation and pressure has been successfully utilized in explaining the structural changes observed in the M(2)S subnets of M(2)SO(x) (x = 3, 4) compounds (M = Na, K) when compared with the structures (room- and high-pressure phases) of their parent M(2)S `alloy' [Martínez-Cruz et al. (1994), J. Solid State Chem. 110, 397-398; Vegas (2000), Crystallogr. Rev. 7, 189-286; Vegas et al. (2002), Solid State Sci. 4, 1077-1081]. These structural changes suggest that if M(2)SO(2) would exist, its cation array might well have an anti-CaF(2) structure. On the other hand, in an analysis of the existing thermodynamic data for M(2)S, M(2)SO(3) and M(2)SO(4) we have identified, and report, a series of unique linear relationships between the known Δ(f)H(o) and Δ(f)G(o) values of the alkali metal (M) sulfide (x = 0) and their oxyanion salts M(2)SO(x) (x = 3 and 4), and the similarly between M(2)S(2) disulfide (x = 0) and disulfur oxyanion salts M(2)S(2)O(x) (x = 3, 4, 5, 6 and 7) and the number of O atoms in their anions x. These linear relationships appear to be unique to sulfur compounds and their inherent simplicity permits us to interpolate thermochemical data (Δ(f)H(o)) for as yet unprepared compounds, M(2)SO (x = 1) and M(2)SO(2) (x = 2). The excellent linearity indicates the reliability of the interpolated data. Making use of the volume-based thermodynamics, VBT [Jenkins et al. (1999), Inorg. Chem. 38, 3609-3620], the values of the absolute entropies were estimated and from them, the standard Δ(f)S(o) values, and then the Δ(f)G(o) values of the salts. A tentative proposal is made for the synthesis of Na(2)SO(2) which involves bubbling SO(2) through a solution of sodium in liquid ammonia. For this attractive thermodynamic route, we estimate ΔG(o) to be approximately -500 kJ mol(-1). However, examination of the stability of Na(2)SO(2) raises doubts and Na(2)SeO(2) emerges as a more attractive target material. Its synthesis is likely to be easier and it is stable to disproportionation into Na(2)S and Na(2)SeO(4). Like Na(2)SO(2), this compound is predicted to have an anti-CaF(2) Na(2)Se subnet.
The structures of ternary oxides and chalcogenides of alkali metals are dissected in light of the extended Zintl-Klemm concept. This model, which has been successfully extended to other compounds different to the Zintl phases, assumes that crystal structures can be better understood if the cation substructures are contemplated as Zintl polyanions. This implies the occurrence of charge transfer between cations, even if they are of the same kind. In this article, the charge transfer between cations is even more illustrative because the two alkali atoms have different electronegativity, so that the less electropositive alkali metal and the O/S atom always form skeletons characteristic of the group 14 elements. Thus, partial structures of the zincblende-, wurtzite-, PbO- and SrAl(2)-type are found in the oxides/sulfides. In this work, such an interpretation of the structures remains at a topological level. The analysis also shows that this interpretation is complementary to the model developed by Andersson and Hyde which contemplates the structures as the intergrowth of structural slabs of more simple compounds.
Stacking interactions in the crystal structures of square-planar transition metal complexes from the Cambridge Structural Database with five- and six-membered chelate rings fused with C(6-arom) rings (arom = aromatic) were analyzed. The distribution of distances between the closest C(6-arom)-C(6-arom) and C(6-arom)-chelate contacts shows that in a large fraction of the intermolecular interactions the C(6-arom) ring of one molecule is closer to the chelate than to the C(6-arom) ring of the other molecule. These results indicate a possible preference of the C(6-arom) ring to form stacking contacts with the chelate rings. The preference is ubiquitous and does not depend on the metal type.
2,4-Dioxo-1,3-diazetidine-1,3-bis(methyl-m-phenylene) diisocyanate (dimerized toluene-2,4-diisocyanate, TDI) is one of the most widely used aromatic diisocyanates in the polymer industry, and it crystallizes in at least two polymorphic forms (form A and form B) depending on reaction conditions. The crystal structures of the two forms were determined from high-resolution laboratory X-ray powder diffraction data using simulated annealing and Rietveld refinement. In spite of a marked structural similarity between them, significant discrepancies in the physical properties of the two forms prompted analysis of their partitioned energy terms in an effort to better our understanding of the driving force behind such differences in behaviour.