High-pressure behavior of Fe₂O₃

High pressure behavior of Fe₂O₃ has been a long-standing subject of research due to its high importance for understanding Earth's interiors. At pressures from 40 to 60 GPa it undergoes a series of transformations, such as structural changes with a large volume discontinuity (~10 %), a drop of the resistivity, a spin crossover of Fe³⁺, and a disappearance of the ordered magnetic state. The crystal structure of the phase(s) observed on compression at ambient temperature above 50 GPa is still under question since only powder X-ray diffraction (XRD) data were available so far. Mössbauer and Raman spectroscopy studies cannot provide definitive structural information. Applying laser heating to Fe₂O₃, compressed up to 70 GPa and above, results in a distinct reconstructive phase transition to the CaIrO₃-type structure, according to powder XRD. Poverty of the available structural data encouraged us to perform a series of high-pressure and high-temperature XRD experiments on single crystals of Fe₂O₃ in diamond anvil cells. We have studied the behavior of Fe₂O₃ at pressures up to 100 GPa and temperatures up to 2500 K. Here we report crystal structures of two novel high-pressure Fe₂O₃ polymorphs, as well as the relations between a spin state of iron atoms and the crystal chemistry of the iron compound. In our compression experiments initially hematite-structured Fe₂O₃ transformed to a new phase at ~54 GPa with 10 % of the volume reduction. This phase has a triclinic distorted perovskite-type structure. The second reconstructive transition occurred at 66-70 GPa with 3 % of the volume discontinuity and resulted in formation of an orthorhombic phase. Laser heating to ~2100¹100 K at pressures above 70 GPa promoted a transition to a Cmcm CaIrO₃-type phase, whose crystal structure was refined by means of single crystal XRD to R₁ ~ 9.7 %. Decompression experiments showed that the Cmcm phase transforms back to hematite at pressures between ~25 and 15 GPa.

Pressure-induced normal-incommensurate-commensurate phase transitions in TiPO₄

The complex interplay between spin, charge, orbital, and lattice degrees of freedom has made low-dimensional quantum spin magnets with strong antiferromagnetic (AF) spin-exchange coupling prime candidates for studying unusual magnetic phenomena. A progressive spin-lattice dimerization in one-dimensional AF Heisenberg chains, which occurs below a critical temperature and induces a singlet ground state with a magnetic gap, is commonly referred to as spin-Peierls (SP) transition. Recently, the compounds TiOX (X = Cl, Br) and TiPO₄ have been intensively investigated due to their unconventional behavior [1,2]. Unlike standard SP systems, TiOX and TiPO₄ undergo a sequence of normal-incommensurate-commensurate phase transitions on cooling at remarkably high transition temperatures. The transition temperatures are related to the direct exchange interactions between Ti ions, which increases strongly with decreasing the distance between the Ti ions, and therefore is very sensitive to the applied hydrostatic pressure. We have performed pressure-dependent single-crystal X-ray diffraction of TiPO₄ using synchrotron radiation. TiPO₄ undergoes a pressure-induced pahse transiton towards an incommensurate phase already below 10 GPa. This transformation is followed by the lock-in phase transition to the dimerized SP phase. Both structures are analogous to those at low temperatures, but reveal significantly larger modulation amplitudes. In this contribution we will present the detailed discussion of the high-pressure structures of TiPO₄ and their behavior on compression. Furthermore, similarities and differences of high-pressure phase diagrams of TiOCl and TiPO₄ and discrepancies between predicted and observed structures will be considered.

The impact of pressure on β-Cyclodextrin·acetaminophen inclusion complexes

Cyclodextrins (CDs) have attracted considerable interest as model systems in supramolecular host-guest chemistry. They are described as hollow truncated cones with a hydrophilic outer surface and a nonpolar inner cavity suitable for small molecules' encapsulation.[1] By virtue of their character, CDs are used as excipients to improve the aqueous solubility of active pharmaceutical ingredients (APIs). High-pressure crystallisation techniques have been established as a suitable tool for exploring the phenomenon of polymorphism and solvate formation of pharmaceutical compounds throughout numerous examples reported in the literature.[2] Thus, exploring the inclusion-complex formation and the polymorphic behaviour of CDs with APIs at high pressure would be an interesting extension of the technique. The present work describes the attempt of an in-situ crystallisation of β-CD·acetaminophen inclusion complex and compression studies of the known β-CD·acetaminophen complex[3] in different crystallisation media at pressures up to 1.0 GPa. A new high-pressure crystal form observed at 0.8 GPa as well as unexpected results are presented herein. The crystals have been characterised by means of polarised optical microscopy, Raman spectroscopy and single-crystal X-ray diffraction using both home and synchrotron sources.

Real time study of wet-chemical reaction dynamics at beamline P02.1 @ PETRA III

The high brilliance synchrotron light source PETRA III in Hamburg, Germany, provides a dedicated X-ray powder diffraction beamline called P02.1 [1]. It is a side station to the hard X-ray diffraction beamline and runs at a fixed photon energy of 60 keV. Its dispersive monochromator produces a highly collimated photon beam of very narrow energy bandwidth and high intensity. These excellent beam characteristics turn P02.1 into an ideal instrument for many different kinds of experiments, ranging from high resolution powder diffraction of polycrystalline materials for structure solution and refinement or microstructure analysis, to the study of nanocrystalline and disordered materials to determine their local structure. In particular, it is the scope of P02.1 to study dynamic processes such as chemical and crystallographic transitions under non-ambient conditions in real time. For this purpose, the beamline is equipped with a large and fast area detector which enables sub-second time-resolution. The accessible range in reciprocal space is beyond Q = 30 Å^-1. Hence, P02.1 is a powerful tool for total scattering experiments as it provides high resolution in real and reciprocal space which are determined by the max. Q and the instrumental resolution, respectively. This presentation describes some recent experiments carried out at P02.1 that relate to pair distribution function (PDF) and total scattering analysis. The focus will be on the investigation of structural changes on the atomic scale during the wet-chemical synthesis of nanoparticles, e.g. in the system ZrO₂. By means of evaluating the changes of bond distances and atomic coordination on a time scale of seconds, it is possible to describe the molecular structure of intermediates and, thus, to deduce the underlying reaction mechanism. On the basis of this information, synthesis processes may be optimised with respect to tuning the properties of the product and to maximize its yield.