Francesco Mezzadri

Influence of Anions in Silver Supramolecular Frameworks: Structural Characteristics and Sorption Properties.

The complexation of a preorganized thioether-functionalized bis(pyrazolyl)methane ligand (L) with silver precursors produces supramolecular structures organized at two hierarchical levels: [AgL](6)(X)(6) metal-organic cyclic hexamers and their organization in 3D architectures. The cyclic toroidal hexamers of 22-26 Å external diameter are found to be stable already in solution before self-assembly into the crystalline state. In the 3D lattice, the hexameric building block are arranged in different highly symmetric space groups as a function of a variety of anions (cubic Fd3 with PF(6)(-) or BF(4)(-) and rhombohedral R3 with CF(3)SO(3)(-) or NO(3)(-)) and form cavities with the geometrical shapes of Platonic solids (tetrahedron and octahedron) that can be occupied by a variety of solvent molecules. Upon evacuation, cubic crystals can produce stable frameworks with permanent porosity, which can absorb reversibly several vapors, CO(2) and CH(4).

Porous Molecular Crystals by Macrocyclic Coordination Supramolecules

In this study, we show how the combination of metal ions, counter-anions and opportunely functionalized and preorganized ligands gives rise to two distinct supramolecular isomers, coordination polymeric chains and hexameric macrocycles. The hexamers then aggregate to form a cubic structure exhibiting permanent microporosity. The supramolecular assemblies are formed with Ag(+), thioether functionalized bis(pirazolyl)methane ligands and CF3SO3(-)/PF6(-) as the counter-anions. Five different ligands were prepared by modifying the peripheral thioether moiety with naphthyl, methoxy, m-Me, p-Me and F groups (L(SNf), L(SPhOMe), L(SPhm-Me), L(SPhp-Me), and L(SPhF)). Helicoidal coordination polymeric chains are formed with CF3SO3(-) (general formula [Ag(L)]n(CF3SO3)n), whereas macrocyclic hexamers are formed with PF6(-) (general formula [Ag(L)]6(PF6)6). The macrocycles self-assemble into ordered capsules with the shape of a tetrahedron, and the overall framework is sustained by Ag(+)···(PF6(-))···Ag(+) contacts. The capsules generate a highly symmetric structural arrangement, which is characterized by permanent microporosity arising from two distinct types of microporous chambers in the structure. The gas absorption isotherms show that the materials can selectively adsorb CO2 and N2O over CH4 and N2. The modulation of the microporosity of the materials is achieved by the different thioether functionalization of the ligands L(SNf), L(SPhOMe), L(SPhm-Me), and L(SPhF). The diffusion and localization of the gas molecules within the cavities were investigated by 2D (1)H-(13)C solid state NMR on samples loaded with enriched (13)CO2, showing that both types of cavities are accessible to guest molecules from the gas phase.

The structure of (Ca,Co)CoSi2O6 pyroxenes and the Ca-M2+ substitution in (Ca,M2+)M2+Si2O6 pyroxenes (M2+ = Co, Fe, Mg)

Abstract The crystal structure of three C2/c clinopyroxenes with composition (Ca0.8Co0.2)CoSi2O6, (Ca0.6Co0.4) CoSi2O6 and (Ca0.4Co0.6)CoSi2O6 was refined down to R4σ = 2.6% by single-crystal X-ray diffraction. The crystals were synthesized at P = 3 GPa by cooling from 1500 to 1200 °C in a piston-cylinder apparatus. At the end of the refinement cycles, electron density residuals (up to 2.1 e-) were observed close to the M2 site and related to the site splitting of Ca and Co in the M2 polyhedron in the two subsites M2 and M2′. Split refinement significantly improved the agreement factor and decreased the uncertainty in the atomic coordinates. Similar features were found in (Ca,Mg)MgSi2O6 and (Ca,Fe) FeSi2O6 intermediate pyroxenes. The average structural changes related to the cation substitution at the M2 site in (Ca,Co)CoSi2O6, (Ca,Mg)MgSi2O6, and (Ca,Fe)FeSi2O6 pyroxenes are similar: the T tetrahedron becomes more regular, the difference between M1-O bond lengths increases, and the M2-O3 bond lengths with the furthermost O3 oxygen atoms become longer. The changes in the M2-O bond distances are, however, not linear, and they are higher for more increased substitution. The largest structural deformation occurs on the (010) plane, with higher deformation at about 60° from the c axis for any composition. The orientation of the deformation ellipsoid is most related to a shift in tetrahedral chains. The scalar deformation for the cation substitution, εs, is linearly related to the cation radius of the average M2 site (IRM2), i.e., the deformation is higher as the cation size decreases, following the equation: εs = -0.0072(12)IRM2 + 0.0082(13), R2 = 0.75. Increasing deformation with cation substitution is supported as the major limiting factor for solid solution. The displacement parameters for unsplit M2, O2, and O3 atoms increase up to the intermediate composition, indicating a local configuration for the M2 polyhedron centered by Ca and Co. However no significant change in Ueq of the O3 atom is observed up to 20% substitution of the smaller cation in the M2 site. Comparison with Raman spectral data suggests that local chain structural configurations occur only for the substitution of the smaller cation in the M2 site higher than 20%, and that the substitution mechanism is different for C2/c clinopyroxenes with lower and higher Ca content

Tunable luminescence of Bi3+-doped YPxV1 − xO4 (0 ≤ x ≤1)

A systematic investigation of the luminescence spectroscopy of Y(P,V)O4:Bi(3+) is presented. The emission spectra and the decay curves are measured as a function of the host morphology, composition, temperature, excitation wavelength, and doping concentration. On this basis, the nature of the excited states and the radiative and non-radiative relaxation processes are discussed. Colour coordinates and quantum yield measurements are also carried out to provide information about the potential applications of the studied materials.

Effects of “changing the wheels” on the inclusion properties in metal–organic diols

New metal–organic wheel and axle diols of general formula [Pd(ligand)2X2] (X = Cl, Br, I) are designed and synthesised, with the aim to study their inclusion properties. In particular, the overall shape of the complexes (bulkiness of the terminal groups, length of the axle) is varied by a judicious choice of the ligands and the effects of these changes on the packing are investigated. By means of the X-ray single crystal structure determination of several mono- and bis-solvate complexes, it was possible to hypothesize the structural rearrangements that go with the release of the guest.

Thermal expansion coefficients of β-Ga2O3 single crystals

The lattice parameters of monoclinic β-Ga2O3 were determined by powder diffraction in a wide temperature range. The experiments provide a quantitative evaluation of the thermal expansion of the unit cell. It was observed that the expansion of the monoclinic cell is not isotropic. These data are of great technological interest for the deposition of β-Ga2O3 films on foreign substrates as well as for the deposition of different materials (such as GaN) on Ga2O3 substrates.

Ca-Zn solid solutions in C2/c pyroxenes: Synthesis, crystal structure, and implications for Zn geochemistry

Abstract The effect of Zn substitution on a series of clinopyroxenes along the join CaZnSi2O6-Zn2Si2O6 was studied. The pyroxenes were synthesized at P = 4-5 GPa and T = 1000-1200 °C by a multi-anvil apparatus. SEM-EDS and XRD analysis showed complete solid solution; all of the samples have the C2/c space group. No miscibility gap between clino- and orthopyroxene nor phase transition to the P21/c space group was found. Moreover, the cell volume of Ca-Zn pyroxenes decreases less than expected from the decrease of the average cation size for the substitution of Zn for Ca. The crystal structures of three synthetic pyroxenes of composition (Ca0.5Zn0.5)ZnSi2O6, (Ca0.3Zn0.7) ZnSi2O6, and (Ca0.2Zn0.8)ZnSi2O6 were refined by single-crystal X‑ray diffraction (R4s between 3 and 4.5%). It was observed that the Ca-Zn substitution occurs in the M2 polyhedron, with a sub-site splitting of Zn in a position at approximately 0.7 Å from Ca. In this position, Zn retains a highly distorted fourfold coordination; moreover, the tetrahedral chain configuration is little changed along the series, and the M1 polyhedral size increases with Zn substitution in M2. An implication of the present work is the interpretation of the partitioning of Zn between clinopyroxene and melt. The distribution coefficients of Zn and Co are quite different in rocks of the same composition, despite their very similar ionic radius, and the difference is related to the preference of Zn for the M2 site, where Zn may find a suitable atomic coordination.

The real structure of ε-Ga2O3 and its relation to κ-phase

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called e phase; i.e. they exhibited hexagonal P63mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga2O3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna21 space group symmetry, referred to as κ-Ga2O3. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P63mc space group symmetry usually referred to as e-Ga2O3 in the current literature. The crystal structure of these Ga2O3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna21 space group and previous observations.

Structural and Electric Evidence of Ferrielectric State in Pb2MnWO6 Double Perovskite System

In this paper we describe the new ferri-electric compound Pb2MnWO6 (PMW), a double perovskite that can be considered as a novel structural prototype showing complex nuclear structure and interesting electric properties. According to single-crystal synchrotron data, PMW crystallizes in the noncentrosymmetric polar group Pmc21, in which the two symmetry-independent lead atoms give rise to a ferrielectric arrangement. The accurate crystallographic characterization indicates the presence of a complex distortion of the perovskite lattice driven by the local instability induced by the 6s(2) lone pair of the lead atoms. These peculiar structural features are confirmed by the complete electrical characterization of the system. Dielectric and transport measurements indicate an insulating character of the sample, while pyroelectric measurements point out a ferrielectric state characterized by different contributions. The magnetic transition at 45 K is accompanied by a magnetostrictive effect indicating a probable spin-lattice coupling. The characterizations carried out on PMW, showing the evidence of a coexistence of antiferromagnetism and ferrielectricity at low temperature, could lead to the definition of a new class of multiferroic materials.

Magnetoelectric coupling driven by inverse magnetostriction in multiferroic BiMn3Mn4O12

Inserting both polar A and magnetic B ions in a same crystalline phase, such as A = Bi3+, B = Fe3+ or Mn3+ in simple perovskites ABO(3), has been successful in achieving multiferroic properties with large ferroelectric and magnetic orders. However, modest magnetoelectric couplings have been hitherto reported, thus preventing any application for future electronics. By means of neutron diffraction, we found a large uniform C-type modulation of an E-type antiferromagnetic structure of the Mn3+ ions in the quadruple perovskite BiMn3Mn4O12. A symmetry analysis indicates that this modulation is induced by the internal strain created by the polar Bi3+ ion, which gives evidence of a large magnetoelectric coupling driven by inverse magnetostriction. This modulation is indeed absent in the isomorphic and isovalent compound LaMn3Mn4O12 containing the nonpolar La3+ ion. Our analysis indicates that this coupling mechanism is effective owing to the symmetry-limited structural distortions and inhomogeneities characteristic of the quadruple perovskite structure, thus preventing the release of the strain. We conclude that internal strain is a key control parameter to achieve large magnetoelectric couplings in proper ferroelectrics.