Alistair R. Lennie

Synthesis and Rietveld crystal structures refinement of mackinawaite, tetragonal FeS

Abstract Mackinawite, tetragonal FeS, has been synthesised by reacting iron with Na2S solutions. A Rietveld structure refinement of X-ray powder diffraction data, recorded using X-rays monochromated from synchrotron radiation with a wavelength of 0.6023 A, has been performed. The structure has been refined in the tetragonal space group, P4/nmm, and has the following cell parameters: a = 3.6735(4), c = 5.0328(7) Å, V = 67.914(24) A3. Our refinement shows that the FeS4 tetrahedron in mackinawite is almost perfectly regular, with a much smaller distortion than has been previously reported. An improved X-ray diffraction data set is provided.

Transformation of mackinawite to greigite; an in situ X-ray powder diffraction and transmission electron microscope study

Abstract Synthetic mackinawite (tetragonal FeS) has been found to transform rapidly to greigite (Fe3S4) above ∼373 K during heating experiments, as observed by in situ X-ray diffraction. Using monochromatic synchrotron radiation (λ = 0.60233 Å), we measured the unit-cell parameters of both synthetic mackinawite between 293 and 453 K and of greigite formed from this mackinawite between 293 and 593 K. The coefficients of thermal expansion for mackinawite are α1 = α2 = (1.36 ± 6 0.11) × 10-5, α3 = (2.98 ± 0.12) × 10-5, and αvol = (5.67 ± 0.19) × 10-5 between 293 and 453 K. The coefficients of thermal expansion for greigite are α1 = α2 = α3 = (1.63 ± 0.15) × 10-5, and αvol = (4.86 ± 0.25) x 10-5 between 293 and 593 K. On further heating in situ, we observed the reaction greigite → pyrrhotite + magnetite. Partial transformation of mackinawite to greigite was also observed using transmission electron microscopy (TEM) following in situ heating. Electron diffraction patterns show that (001) of mackinawite is parallel to (001) of greigite, and [110] of mackinawite is parallel to [100] of greigite. This orientation relationship confirms that the cubic closepacked S array in mackinawite is retained in greigite and implies that oxidation of some Fe2+ in mackinawite drives rearrangement of Fe to form the new phase. Small regions of the crystallites show Moiré fringes resulting from the lattice mismatch between mackinawite and greigite. Electron diffraction patterns of mackinawite subjected to prolonged exposure to the atmosphere also show faint spots corresponding to greigite. We propose that in these experiments surplus Fe is accommodated by reaction with either adsorbed O2 or H2O to form amorphous nanophase Fe-O(H). Because greigite is so easily formed by oxidation from mackinawite, greigite should be an important precursor for pyrite nucleation, although any orientation relationship between greigite and pyrite remains to be determined.

Synthesis-dependant structural variations in amorphous calcium carbonate

Amorphous calcium carbonate (ACC) was synthesised in the presence of the additives magnesium and poly(aspartic acid) (pAsp) and the structure and crystallisation of these ACC samples was investigated using a range of techniques including X-Ray Absorption Spectroscopy (XAS), X-Ray Diffraction (XRD) and Infra-Red Spectroscopy (IR). The experiments demonstrated that synthetic ACC can be produced with different short-range structures, according to the solution additives used. While the first Mg–ACC precipitates showed short-range structures most similar to aragonite, with monohydrocalcite short-range structures developing with incubation in solution, the initial pAsp–ACC precipitates possessed short-range structures resembling vaterite. The results show that the influence of these additives on the crystallisation of calcium carbonate is apparent even in the precipitation of the first amorphous precursor phase, and that the first stages of recrystallisation involve the expulsion of water from the structure rather than significant changes in the short-range structure around the calcium ions.

Fast X-ray powder diffraction on I11 at Diamond.

The commissioning and performance characterization of a position-sensitive detector designed for fast X-ray powder diffraction experiments on beamline I11 at Diamond Light Source are described. The detecting elements comprise 18 detector-readout modules of MYTHEN-II silicon strip technology tiled to provide 90° coverage in 2θ. The modules are located in a rigid housing custom designed at Diamond with control of the device fully integrated into the beamline data acquisition environment. The detector is mounted on the I11 three-circle powder diffractometer to provide an intrinsic resolution of Δ2θ approximately equal to 0.004°. The results of commissioning and performance measurements using reference samples (Si and AgI) are presented, along with new results from scientific experiments selected to demonstrate the suitability of this facility for powder diffraction experiments where conventional angle scanning is too slow to capture rapid structural changes. The real-time dehydrogenation of MgH(2), a potential hydrogen storage compound, is investigated along with ultrafast high-throughput measurements to determine the crystallite quality of different samples of the metastable carbonate phase vaterite (CaCO(3)) precipitated and stabilized in the presence of amino acid molecules in a biomimetic synthesis process.

Scanning tunnelling microscopy studies of α-Fe2O3(0001)

Abstract Scanning tunnelling microscopy (STM) images of two different reconstructions of an α-Fe2O3(0001) crystal are presented. Annealing the sample to 1000 K creates a selvedge stabilised by a thin film of Fe3O4, with its (111) plane parallel to the basal plane of the underlying substrate. The STM images confirm that this surface is structurally equivalent to that previously reported for the surface of Fe3O4(111) single crystals, in that two coexisting terminations, denoted A and B, are present separated by alternate steps. Termination A has been identified with 1 4 ML of O atoms capping 3 4 ML of Fe atoms, while termination B consists of 1 2 ML of Fe atoms overlaying a close-packed O layer. Some regions of the sample are disordered but contain small triangular islands of termination A. This structure is attributed to Ar ion induced sputter damage. A different termination, created by annealing the sample at 1100 K in 1 × 10−6 mbar O2, has a distinctive hexagonal LEED pattern, with all the main beams floreted, being surrounded by a hexagon of smaller spots. The STM results show that this surface is stabilized by coexisting α-Fe2O3(0001) and Fe1−xO(111) phases, with each phase existing in atomically well ordered islands of mesoscopic dimensions. The islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between the two types of oxygen sub-lattices.

Fe3O4(111) termination of α-Fe2O3(0001)

Scanning tunnelling microscopy (STM) has been used to investigate the structure formed on an α-Fe2O3(0001) substrate after argon ion bombardment and annealing in 1 × 10−6 mbar of O2 at 1000 K. The STM images recorded at positive sample bias reveal an hexagonal array, with a distance between (Fe) atoms of 6.0 ± 0.1 rA and steps in multiples of 4.8 A. These results are consistent with formation of an Fe3O4(111) epitaxial layer terminating in a 14 monolayer of Fe atoms.

[Mn6] under Pressure: A Combined Crystallographic and Magnetic Study†

Folding under pressure: Crystallographic studies on a Mn6 single-molecule magnet under high pressure conditions show the drastic structural changes of the magnetic core (see picture, Mn purple, O red, N blue), which impact on the magnetic properties of ferromagnetic exchange between the metal atoms will be in booster weaker, and under extremely high pressure, a transition to antiferromagnetic behavior.

Transformation of synthetic mackinawite to hexagonal pyrrhotite: A kinetic study

Abstract The kinetics of the transformation of synthetically prepared mackinawite to hexagonal pyrrhotite have been studied to clarify the relationship between these two phases. Kinetic analysis using the Johnson-Mehl equation is based on measurements of the fraction of hexagonal pyrrhotite obtained by heating mackinawite samples in the temperature range of 530-545 K. This gives isothermal rate constants from which an activation energy of 493 kllmol and a frequency factor of 3.7 × 1045min-1 have been established. The mechanism proposed for this transformation is that of a solid-state diffusion process. Kinetic data derived from this experiment may be used to show that persistence of mackinawite in nature can be explained by kinetic factors alone. Reactions at low temperatures, in which mackinawite is oxidized or further sulfidized, are therefore more important than the kinetically inhibited transformation to hexagonal pyrrhotite. The irreversible mackinawite to hexagonal pyrrhotite relation is compared with the reversible tetragonal FeSe to NiAs-type FeSe phase transformation. This contrast in stability behavior is examined in terms of the thermal stability of the respective tetragonal phases relative to the upper magnetic ordering temperatures of their corresponding hexagonal phases.

Putting pressure on elusive polymorphs and solvates

The reproducible crystallisation of elusive polymorphs and solvates of molecular compounds at high pressure has been demonstrated through studies on maleic acid, malonamide, and paracetamol. These high-pressure methods can be scaled-up to produce ‘bulk’ quantities of metastable forms that can be recovered to ambient pressure for subsequent seeding experiments. This has been demonstrated for paracetamol form II and paracetamol monohydrate. The studies also show that the particular solid form can be tuned by both pressure and concentration.

Pressure-induced Jahn–Teller switching in a Mn12 nanomagnet

Pressure-induced switching of a fast-relaxing single-molecule magnet to a slow-relaxing isomer is observed for the first time by using a combination of high pressure single-crystal X-ray diffraction and high pressure magnetic measurements.