Early stages of solid state reactions: insights from micro-XRD and XAS

The mechanism of a solid state reaction in its early stages can be explored by investigating the time evolution of a model reactive system made of a thin layer of one reagent deposited onto a single crystal slab of the other reagent. Insights can be retrieved by comparing results at both local and long length scales obtained with films of different thicknesses and deposited onto different crystal orientations. In particular, reaction between ZnO and Al2O3has been chosen, as the spinel-forming reactions have been and still remain a model experimental system for investigating solid state reactions and because in the ZnO/Al2O3phase diagram, spinel is the only stable compound. The reaction initial steps have been investigated by using synchrotron X-ray diffraction, atomic force microscopy and X-ray absorption spectroscopy at the Zn-K edge starting from zincite films deposited onto (110)-, (012)-, (001)-oriented corundum single crystals [1,2]. The reaction eventually yields ZnAl2O4spinel but via a complex mechanism involving side and intermediate non-equilibrium compounds that do not appear in the equilibrium phase diagram of the pseudo-binary system. Spinel, when occurs, is polycrystalline at the end but initially forms with a few preferred orientations. Intermediate phases form before and in parallel with the growth of the spinel. Their number, composition, structure and kinetic role strongly depend on substrate orientation and film thickness. A more detailed understanding of the reactivity can be inferred by comparing EXAFS results to those of grazing incidence diffraction experiments of the films deposited on the (001) face of Al2O3and heat-treated at 10000C for different lengths of time. Information on the structure of the intermediate phases is given and results are discussed by comparing different films thickness to clarify the role of interfacial free energy and crystallographic orientation

Mixing and ordering behaviour in manganocolumbite-ferrocolumbite solid solution: a single-crystal X-ray diffraction study

The structural changes upon cation substitution in natural AB2O6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb2O6- FeNb2O6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Qm ∼ 0.7. A discontinuity at this Qm value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modiÞ cations of the polyhedra by folding along the common edge

Low-alkali metal content in beta-vanadium mixed bronzes: the crystal structures of β-Kx(V,Mo)6O15 (x = 0.23 and 0.32) by single-crystal X-ray diffraction

The vanadium–molybdenum mixed oxide bronzes of composition K0.23(V5.35Mo0.65)O15 and K0.32(V5.48Mo0.52)O15 have a monoclinic structure with s.g. C2/m, Z ¼ 2, and unit-cell dimensions a ¼ 15.436(2), b ¼ 3.6527(5), c ¼ 10.150(1)A ˚ , b ¼ 108.604(3)1 and a ¼ 15.452(2), b ¼ 3.6502(5), c ¼ 10.142(1)A ˚ , b ¼ 109.168(3)1, respectively, as determined by single-crystal X-ray diffraction. These compounds show the b-NaxV6O15 tunnel structure, which is isostructural with bannermanite, natural sodium–potassium vanadate. Structure refinements from diffracted intensities collected in the 2–381y range converged to final R ¼ 5.58% and 7.48% for the two crystals, respectively. The V atoms are distributed on three different crystallographic sites. Partial substitution of V with Mo occurs in only one of these positions. Oxygen atoms involved in vanadyl groups point toward the tunnels. The K ions in the tunnels are coordinated by seven oxygen atoms. The alkali metal content in these crystals is much lower than the solubility limit found for the analogous Na containing compound

08_1641Tarantino.indd

ABSTRACT The structural changes upon cation substitution in natural AB 2 O 6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb 2 O 6 -FeNb 2 O 6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Q m ∼ 0.7. A discontinuity at this Q m value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modiÞ cations of the polyhedra by folding along the common edge

Monomeric versus dimeric structures in ternary complexes of manganese(II) with derivatives of benzoic acid and nitrogenous bases: structural details and spectral properties

Adducts formed by [Mn(2,6-dmb)2(H2O)3]n · nH2O, 2,6-dmb=2,6-dimethoxybenzoate(1–), Mn(2,4-dhb)2 · 8H2O, Mn(2,5-dhb)2 · 4H2O or Mn(2,6-dhb)2 · 8H2O, dhb=dihydroxybenzoate(1–), and 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (Me2bpy) or 4,7-dimethyl-1,10-phenanthroline (Me2phen) were isolated in the solid state and characterised by IR, EPR and thermogravimetry. Two of them, [Mn(2,6-dhb)2(bpy)2] (1) and [Mn2(2,6-dmb)4(Me2Phen)2(H2O)2] · 2EtOH (2), were studied by single crystal X-ray diffraction. The adduct 1 is mononuclear and consists of hexa-co-ordinate manganese(II) ions bound to two bipyridine and two 2,6-dihydroxybenzoate ligands in a cis-octahedral arrangement. The complex 2 exhibits a dinuclear structure in which two manganese(II) ions share two carboxylate groups adopting a rather uncommon single-atom bridging mode. The results allow us to conclude that weak, e.g., hydrogen bonding and stacking interactions govern the type of structure, monomeric or dimeric. The spectral features of the complexes are discussed. In particular, the solid-state EPR features of the complexes are interpreted in terms of D, E and Hmax, the high-field resonance. For the monomeric species, the higher is the D value, the higher is Hmax. A series of adducts formed by Mn2+ ion with derivatives of benzoic acid and aromatic nitrogenous bases is described. The formation of monomeric or dimeric complexes is influenced by weak interactions, like hydrogen bonding and stacking interactions

Kinetics of Fe2+-Mg order-disorder in P21/c pigeonite

The kinetics of the Fe-Mg intracrystalline exchange reaction in P21/c pigeonite (Wo10En47Fs43) free of exsolved augite, from the Parand rhyodacite sample BTS308, was studied by single-crystal X-ray diffraction (XRD). Isothermal disordering annealing experiments, with oxygen fugacity controlled at the IW buffer, were performed on two crystals at 650, 700, 750, and 800 °C until the Fe-Mg exchange equilibrium was reached. The XRD data were collected from the two untreated crystals and after each annealing experiment. Structure refinements were carried out taking into account the recently discovered stronger preference of Mn for the M2 site compared to Fe2+. The linear regression of lnkD* vs. 1/T yielded the following equation: lnkD*=-2925(±110)/T(K)+0.574(±0.111); (R2=0.997) The Tc values calculated using this equation were 566 (±6) and 571 (±6) °C for the two crystals. Analysis of the kinetic data was performed according to Mueller's model, which allowed retrieval of the disordering rate constants C0Kdis+ for all four temperatures. The Arrhenius relation: lnKdis+=lnK0-Q/(RT)=20.45(±1.91)-25191(±1900)/T(K); (R2=0.989) yielded an activation energy of 50.03 (±3) kcal/mol for the Fe-Mg exchange process. Cooling time constants, calculated at the QMF buffer conditions of the host rock were, for the two crystals, η= 0.94 x 10-1 K-1 year-1 and η= 1.10 x 10-1 K-1 year-1, and gave cooling rates on the order of 10 °C/h consistent with very fast lava cooling

Synthesis and structure determination of 1,4,7,11,14,17,21,24,27-nonaoxatriacontan-8,10,18,20,28,30-esaone

The title compound crystallizes in the trigonal space group R-3, with unit-cell parameters: a = 23.261(4), c = 9.537(2) angstrom; lambda(MoK alpha) = 0.71073 angstrom, V = 4469(2) angstrom(3), and Z = 6. The structure has been solved by direct methods using X-ray diffraction techniques. The final reliability index for the computed structure is 0.0826 for 929 observed reflections and 121 refined parameters. Crown ether adopts an almost circular shape and macrocycles are piled up in a columnar arrangement forming tubular nanochannels. The channels are filled with guest CDCl3 molecules, characterised by rotational disorder