Mauro Gemmi

Precession Electron Diffraction Assisted Orientation Mapping in the Transmission Electron Microscope

Precession electron diffraction (PED) is a new promising technique for electron diffraction pattern collection under quasi-kinematical conditions (as in X-ray Diffraction), which enables “ab-initio” solving of crystalline structures of nanocrystals. The PED technique may be used in TEM instruments of voltages 100 to 400 kV and is an effective upgrade of the TEM instrument to a true electron diffractometer. The PED technique, when combined with fast electron diffraction acquisition and pattern matching software techniques, may also be used for the high magnification ultra-fast mapping of variable crystal orientations and phases, similarly to what is achieved with the Electron Backscattered Diffraction (EBSD) technique in Scanning Electron Microscopes (SEM) at lower magnifications and longer acquisition times.

Structure solution of the new titanate Li4Ti8Ni3O21 using precession electron diffraction

A sample having stoichiometry Li[Ti(1.5)Ni(0.5)]O(4) has been synthesized to obtain a spinel structure. The resulting crystalline powder revealed a multiphase nature with spinel as the minor phase. The main phase is a new trigonal phase having a = 5.05910 (1), c = 32.5371 (1) A. The structure has been solved by direct methods working on a three-dimensional set of intensities obtained from a precession electron-diffraction experiment, and refined on synchrotron powder diffraction data in the space group P3c1. The model consists of hexagonal layers of edge-sharing octahedra occupied either by the heavy cations Ti and Ni, or preferentially by Li. On the basis of cation-site occupancies the stoichiometry becomes Li(4)Ti(8)Ni(3)O(21), which is compatible with the microanalysis results.

Fe3+ spin transition in CaFe2O4 at high pressure

Abstract Single-crystal diffraction data collected for CaFe2O4 at high pressure reveal above 50 GPa an isosymmetric phase transition (i.e., no change in symmetry) marked by a volume decrease of 8.4%. X-ray emission spectroscopic data at ambient and high pressure confirm that the nature of the phase transition is related to the Fe3+ high-spin/low-spin transition. The bulk modulus K0 calculated with a Birch Murnaghan EoS (K′ = 4) is remarkably different [K0 = 159(2) GPa for CaFe2O4 “high spin” and K0 = 235(10) GPa for CaFe2O4 “low spin”]. Crystal structure refinements reveal a decrease of 12% of the Fe3+ crystallographic site volume. The geometrical features of the low-spin Fe3+ crystallographic site at high pressure (bond lengths, volume) indicate a relevant decrease of Fe3+-O bond lengths, and the results are in agreement with tabulated values for crystal radii of Fe3+ in high- and low-spin state. The reduced crystal size of Fe3+ in the low-spin state suggest that in lower mantle assemblages, Fe3+ partitioning in crystallographic sites should be strongly affected by the iron spin state.

Structure of the new mineral sarrabusite, Pb5CuCl4(SeO3)4, solved by manual electron-diffraction tomography

The new mineral sarrabusite Pb(5)CuCl(4)(SeO(3))(4) has been discovered in the Sardinian mine of Baccu Locci, near Villaputzu. It occurs as small lemon-yellow spherical aggregates of tabular crystals (< 10 µm) of less than 100 µm in diameter. The crystal structure has been solved from and refined against electron diffraction of a microcrystal. Data sets have been measured by both a manual and an automated version of the new electron-diffraction tomography technique combined with the precession of the electron beam. The sarrabusite structure is monoclinic and consists of (010) layers of straight chains formed by alternating edge-sharing CuO(4)Cl(2) and PbO(8) polyhedra parallel to the c axis, which share corners laterally with two zigzag corner-sharing chains of PbO(6)Cl(2) and PbO(4)Cl(4) bicapped trigonal prisms. These blocks are linked together by SeO(3)(2-) flat-pyramidal groups.

Direct space structure solution from precession electron diffraction data: Resolving heavy and light scatterers in Pb13Mn9O25

The crystal structure of a novel compound Pb(13)Mn(9)O(25) has been determined through a direct space structure solution with a Monte-Carlo-based global optimization using precession electron diffraction data (a=14.177(3)A, c=3.9320(7)A, SG P4/m, R(F)=0.239) and compositional information obtained from energy dispersive X-ray analysis and electron energy loss spectroscopy. This allowed to obtain a reliable structural model even despite the simultaneous presence of both heavy (Pb) and light (O) scattering elements and to validate the accuracy of the electron diffraction-based structure refinement. This provides an important benchmark for further studies of complex structural problems with electron diffraction techniques. Pb(13)Mn(9)O(25) has an anion- and cation-deficient perovskite-based structure with the A-positions filled by the Pb atoms and 9/13 of the B positions filled by the Mn atoms in an ordered manner. MnO(6) octahedra and MnO(5) tetragonal pyramids form a network by sharing common corners. Tunnels are formed in the network due to an ordered arrangement of vacancies at the B-sublattice. These tunnels provide sufficient space for localization of the lone 6s(2) electron pairs of the Pb(2+) cations, suggested as the driving force for the structural difference between Pb(13)Mn(9)O(25) and the manganites of alkali-earth elements with similar compositions.

Phase transformations and reaction kinetics during the temperature-induced oxidation of natural olivine

Abstract This work describes the sequence of transformations and their reaction kinetics during the oxidation of olivine in air at high temperature. A natural olivine sample from the layered series of the Ivrea-Verbano igneous complex (Western Alps, Italy) was heated in the temperature range 25-1300 °C in air and investigated by in situ, real time powder X-ray diffraction (PXRD). The evolution of the peaks (measured integrated intensities) was followed in non-isothermal conditions using variable heating rates (b = 20, 22, 25, 27, and 30 °C/min). The total time of the experiments ranged from about 256 (b = 30 °C/min) to 277 (b = 20 °C/min) min including the time for the data collections. An additional isothermal run was performed at 800 °C. The analysis of the kinetic data was attempted with the use of different equations including the classical Avrami theory for solid-state reactions. The kinetic results were confirmed by independent experimental data from electron microscopy (SEM, TEM). In the transformation sequence, hematite appears at about 600 °C producing amorphous segregations. of silica that later recombine with forsterite to form pyroxene Hematite is stable up to 1130 °C where it is transformed into magnetite. The rate limiting step for the formation of hematite is a twodimensional diffusion with constant or decelerating nucleation rate and apparent activation energy of 15 kcal/mol. The concentration of Fe3+ in Fe-rich regions favors the heterogeneous nucleation of hematite, which may take place on existing defects or at the grain boundaries with impurity phases such as serpentine. At 1130 °C, magnetite is formed at the expenses of hematite, with a contracting volume interface-controlled reaction in two or three dimensions with an apparent activation energy of 30-31 kcal/mol. The hematite to magnetite transformation is direct, without a metastable amorphous intermediate. It is described by the “shrinking core model,” with the formation of a magnetite outer layer at the surface of the hematite particles that proceeds toward the core of the reacting hematite by diffusion of the oxygen throughout the newly formed magnetite layer. Its rate is limited by the advancement of the reaction front. The rate-limiting step for the formation of pyroxene is two-dimensional diffusion with decelerating nucleation rate with an apparent activation energy of 29 kcal/mol.

Non-ideality and defectivity of the åkermanite-gehlenite solid solution: An X-ray diffraction and TEM study

Abstract This paper reports a structure analysis of the åkermanite-gehlenite solid solution. This solution is non-ideal with a negative excess volume in the entire compositional range. X-ray diffraction shows anomalous behavior of the cell parameters close to the gehlenite end-member (åk08ge92, åk35ge65). This behavior is correlated with an excess of Si and deficiency of Al with respect to the Mg content, which implies a defective, non-stoichiometric structure with Ca vacancies. Electron microscopy images have confirmed an increase in the defectivity on the atomic scale for Al-rich compositions, and single-crystal structure refinements show a correlated decrease of the tetrahedral volume preferentially occupied by Si. The incommensurate modulation, characteristic of åkermanite, has been observed also in åk95ge05, and it is still visible as diffuse scattering in åk75ge25. (210) twinning has been observed in the entire compositional range.

High-pressure behavior of åkermanite and gehlenite and phase stability of the normal structure in melilites

Abstract Åkermanite (Ca2MgSi2O7) and gehlenite (Ca2Al2SiO7) have been studied at high pressure by synchrotron radiation powder and single-crystal diffraction up to 30 GPa. At about 2 GPa, the incommensurately modulated structure (IC) transforms to a normal structure (N). The bulk modulus for the N structure, fitted with a Birch Murnhagan EoS on powder data, is 93.5(5) GPa. The compressibility is anisotropic, and it is greater along the c axis, in the direction perpendicular to the tetrahedral layers of the structure. Above 15 GPa, a phase transition is observed, marked by a discontinuity in the elastic behavior and a small change in intensity and in the full-width at half maximum (FWHM) of the powder diffraction peaks. The diffraction patterns are indexed with respect to tetragonal cell of the Nmelilite structure up to 30 GPa. A hysteresis in the elastic behavior is observed during decompression. In contrast, single-crystal data show a new monoclinic phase appearing above 15 GPa. The unit-cell parameters are a = 8.82(1) Å, b = 7.34(1) Å, c = 9.13(1) Å, β = 115.1(2)°. This unit cell is similar to that of Ca2ZnGe1.25Si0.75O7 reported in the literature. A refinement using the corresponding model in space group P21/n fits the single-crystal data with a reasonable RBragg = 15%, considering that the crystal is twinned and the mosaicity is large. Gehlenite has a higher bulk modulus, 106.1(4) GPa, than does åkermanite. The compressibility is anisotropic, and the behavior is similar to that of åkermanite, but the presence of Al in tetrahedral sites decreases the compressibility parallel to the (001) plane. The structure of gehlenite is stable up to 25 GPa, when a phase transition occurs.

In situ simultaneous synchrotron powder diffraction and mass spectrometry study of methane anaerobic combustion on iron-oxide-based oxygen carrier

Reduction of iron oxide/CeO2 solid due to methane anaerobic combustion has been studied using in situ time-resolved synchrotron X-ray powder diffraction and mass spectrometry measurements. All the chemical reactions involved in the process have been mapped. Rietveld profile fitting analysis has given a quantitative estimation of the different phases at every step of the reaction. The reaction temperature has been estimated using thermal expansion of both Fe2O3 and CeO2. From the structural parameters of CeO2 it has been possible to claim that CeO2 participates in the reduction process by transforming into a reduced CeOx form. The number, x, of O atoms per formula unit has been estimated by measuring the percentage variation of the unit-cell parameters.

Airborne Concentrations of Chrysotile Asbestos in Serpentine Quarries and Stone Processing Facilities in Valmalenco, Italy

Asbestos may be naturally present in rocks and soils. In some cases, there is the possibility of releasing asbestos fibres into the atmosphere from the rock or soil, subsequently exposing workers and the general population, which can lead to an increased risk of developing asbestos-related diseases. In the present study, air contaminated with asbestos fibres released from serpentinites was investigated in occupational settings (quarries and processing factories) and in the environment close to working facilities and at urban sites. The only naturally occurrence of asbestos found in Valmalenco area was chrysotile; amphibole fibres were never detected. An experimental cut-off diameter of 0.25 μm was established for distinguishing between Valmalenco chrysotile and antigorite single fibres using selected area electron diffraction analyses. Air contamination from chrysotile fibres in the examined occupational settings was site-dependent as the degree of asbestos contamination of Valmalenco serpentinites is highly variable from place to place. Block cutting of massive serpentinites with multiple blades or discs and drilling at the quarry sites that had the highest levels of asbestos contamination generated the highest exposures to (i.e. over the occupational exposure limits) asbestos. Conversely, working activities on foliated serpentinites produced airborne chrysotile concentrations comparable with ambient levels. Environmental chrysotile concentrations were always below the Italian limit for life environments (0.002 f ml(-1)), except for one sample collected at a quarry property boundary. The present exposure assessment study should encourage the development of an effective and concordant policy for proper use of asbestos-bearing rocks and soils as well as for the protection of public health.