High-resolution Fe K-edge XANES spectra of a series of crystalline Fe 2+ - and Fe 3+ -bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe 2+ , including 5-coordinated trigonal bipyramidal Fe 2+ , and from 4 to 6 O atoms for Fe 3+ . Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe 2+ and Fe 3+ is 1.4 ± 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre-edge position for mixed Fe 2+ -Fe 3+ compounds occurring between positions for Fe 2+ and Fe 3+ . The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe 2+ and Fe 3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs. pre-edge intensity can be observed, depending on the coordination environment of Fe 2+ and Fe 3+ , with an accuracy in redox determination of ±10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe 3+ /Fe 2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe 2+ /Fe 3+ .

http://www.minsocam.org/msa/ammin/toc/Abstracts/2001_Abstracts/MJ01_Abstracts/Wilke_p714_01.pdf

Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study

The effects of the hydrostatic and shear-inducing pressure on the physicochemical properties of solids are analysed. The mechanochemical reactions and mechanical activation are illustrated with various types of chemical transformations ranging from polymorph transitions to solid-state reactions. The prospects for the use of mechanochemistry for the solution of applied problems are discussed.

https://iopscience.iop.org/article/10.1070/RC2006v075n03ABEH001205/pdf

Mechanochemistry and mechanical activation of solids

Topics in current chemistry

Iron(II) complexes of 2,6-di(pyrazol-1-yl)pyridines—A versatile system for spin-crossover research

The effects of high pressure on pharmaceutical compounds and energetic materials can have important implications for both the properties and performance of these important classes of material. Pharmaceutical compounds are frequently subjected to pressure during processing and formulation, causing interconversion between solid forms that may affect properties such as solubility and bio-availability. Energetic materials experience extremes of both pressure and temperature under conditions of detonation and deflagration, causing changes in properties such as sensitivity to shock and chemical reactivity. This tutorial review outlines the various methods used to study these materials at high pressure, describes how pressure can be used to explore polymorphism, and provides examples of compounds that have been studied at high pressure.

High-pressure studies of pharmaceutical compounds and energetic materials

The results of a high-resolution powder dif- fraction study of the effect of high hydrostatic pressure up to 8 GPa on the pure g-polymorph of glycine (P31) are discussed. A phase transition with a jumpwise change of cell volume and cell parameters was observed. The transi- tion starts at about 2.73 GPa and is still not complete even at 7.85 GPa. The crystal structure of the previously un- known high-pressure polymorph of glycine (d-polymorph) could be solved and refined in the space group Pn. In this structure, glycine zwitter-ions are linked via NH ... O hy- drogen bonds into layers, which form double-layered bands via additional NH ... O hydrogen bonds. The struc- ture of the individual layers in the high-pressure poly- morph is similar to that in the previously known a -( P21/n) and b -( P21) forms, but the packing of the layers is essen- tially different. The pressure-induced polymorphic trans- formation in the g-glycine can be compared with a change in the secondary structure of a peptide, when a helix is transformed into a sheet.

/pdf/effect-of-hydrostatic-pressure-on-the-g-polymorph-of-glycine-48rlw0ssvq.pdf

Effect of hydrostatic pressure on the γ-polymorph of glycine 1. A polymorphic transition into a new δ-form

The anisotropy of lattice strain in the crystals of L-serine (P212121, at ambient conditions a ¼ 5.615(1) �, b ¼ 8.589(2) �, c ¼ 9.346(2) �) on cooling down to 100 K and with increasing hydrostatic pressure up to 4.4 GPa was compared with each other and also with the results previously obtained for the polymorphs of glycine. On cooling, the structure expanded slightly along the crystallographic a-direction, compression along the crystallographic b- and c-directions (normal to the chains of the serine zwitter-ions) was very similar. With increas- ing pressure, the same structure compressed in all the crystallographic directions, linear strain along c-axis was the largest, linear strain along a-axis - - the smallest, linear compression along the b-axis with increasing pressure was slightly larger than that along the a-axis. The different ani- sotropy of lattice strain of the same structure on cooling and under pressure could be correlated with different response of intermolecular hydrogen bonds to these two scalar actions.

A comparative study of the anisotropy of lattice strain induced in the crystals of L-serine by cooling down to 100 K or by increasing pressure up to 4.4 GPa

The effect of hydrostatic pressure up to 4 GPa on the two polymorphs of glycine (α - s,g. P21/n, and γ - s.g. P3 1 (P3 2 )) was studied by X-ray powder diffraction using a synchrotron radiation source (λ = 0.7 A, a MAR345 image plate detector). No polymorphic transformations were detected. Relative volume changes and the anisotropy of structural distortion of the two polymorphs were compared. The directions of maximum and minimum lattice strain were related to the directions of weak and strong hydrogen bonds in the structures.

A comparative study of pressure-induced lattice strain of α- and γ-polymorphs of glycine

Bragg diffraction data were collected on single crystals of the spin-crossover complex [Fe(phen)2(NCS)2] in its low-spin and light-induced metastable high-spin states. Experimental variables included the temperature (32 and 15 K), the X-ray source (sealed tube and synchrotron), and the time interval between laser light excitation of the sample (λ = 647 nm). From a comparison of the structural parameters refined, it is shown that photo-crystallographic measurements suffer significantly and systematically from bias if the probed sample contains residual ground-state species, resulting from an incomplete photo-conversion or a significant metastable- to ground-state relaxation. It follows that a 4% population of species in a different spin state affects the Fe—N bond lengths by more than three standard deviations, and the FeN6 polyhedron volume by as much as seven standard deviations, while the mean atomic position misfit exceeds 0.005 A.

https://hal.archives-ouvertes.fr/hal-01007149/document

On the precision and accuracy of structural analysis of light-induced metastable states

The distribution of iron over the tetrahedral and octahedral sites in natural and oxidized ("kenotetrahedral") magnetites has been studied using the Mössbauer effect. The ratio of the 5 7Fe recoil-free fractions at the two types of sites is 1.00 ± 0.04 at 298 ° K . From the intensity ratios of the two magnetic hyperfine patterns it is concluded that the vacancies in the "kenotetrahedral" samples are statistically distributed over the tetrahedral and octahedral sites. The Verwey transition which occurs at 119 °K in pure magnetite decreases with increasing vacancy concentration. Its spread over a range of at least 20° Κ in the kenotetrahedral magnetites indicates that these are not homogeneous phases. The vacancies appear to be predominantly concentrated at rims of the crystals. * Dedicated to Professor F . Laves on the occasion of his 65 th birthday. * cf. also abstract (with G. KULLERUD) in Trans. Amer. Geophysical U. 51 (1970) 447. * * Chicago, Illinois 60637. 328 HANS-PETER W E B E R and STEFAN S. HAFNER

Hans-Peter Weber

Papers

Effect of hydrostatic pressure on the γ-polymorph of glycine 1. A polymorphic transition into a new δ-form

A comparative study of the anisotropy of lattice strain induced in the crystals of L-serine by cooling down to 100 K or by increasing pressure up to 4.4 GPa

A comparative study of pressure-induced lattice strain of α- and γ-polymorphs of glycine

On the precision and accuracy of structural analysis of light-induced metastable states

Vacancy distribution in nonstoichiometric magnetites