Christina Drathen

Perovskite to postperovskite transition in NaFeF3.

The GdFeO3-type perovskite NaFeF3 transforms to CaIrO3-type postperovskite at pressures as low as 9 GPa at room temperature. The details of such a transition were investigated by in situ synchrotron powder diffraction in a multianvil press. Fit of the p-V data showed that the perovskite phase is more compressible than related chemistries with a strongly anisotropic response of the lattice metrics to increasing pressure. The reduction in volume is accommodated by a rapid increase of the octahedral tilting angle, which reaches a critical value of 26° at the transition boundary. The postperovskite form, which is fully recoverable at ambient conditions, shows a regular geometry of the edge-sharing octahedra and its structural properties are comparable to those found in CaIrO3-type MgSiO3 at high pressure and temperature. Theoretical studies using density functional theory at the GGA + U level were also performed and describe a scenario where both perovskite and postperovskite phases can be considered Mott-Hubbard insulators with collinear magnetic G- and C-type antiferromagnetic structures, respectively. Magnetic measurements are in line with the theoretical predictions with both forms showing the typical behavior of canted antiferromagnets.

Distribution of Al atoms in the clathrate-I phase Ba8AlxSi46−x at x = 6.9

The clathrate-I phase Ba8AlxSi46-x has been structurally characterized at the composition x = 6.9 (space group Pm3[combining macron]n, no. 223, a = 10.4645(2) Å). A crystal structure model comprising the distribution of aluminium and silicon atoms in the clathrate framework was established: 5.7 Al atoms and 0.3 Si atoms occupy the crystallographic site 6c, while 1.2 Al atoms and 22.8 Si atoms occupy site 24k. The atomic distribution was established based on a combination of (27)Al and (29)Si NMR experiments, X-ray single-crystal diffraction and wavelength-dispersive X-ray spectroscopy.

Structure and magnetic property of potassium intercalated pentacene: observation of superconducting phase in KxC22H14

We report the results from systematic investigations on the structure and magnetic properties of potassium intercalated pentacene as a function of potassium content, K x C22H14 (1  ⩽  x  ⩽  3). Synchrotron radiation powder x-ray diffraction technique revealed that there are two different stable phases can be obtained via potassium intercalation, namely, K1C22H14 phase and K3C22H14 phase. Structural phase transition was induced when the potassium content was increased to the nominal value x  =  3. This phase transition is accompanied by drastic change in their magnetic property, where those samples with compositions K1C22H14 shows ferromagnetic behavior and those with near K3C22H14 lead to observation of superconductivity with transition temperature, T c, of 4.5 K. It is first time that superconductivity was observed in linear oligoacenes. Both magnetization study and synchrotron radiation powder x-ray diffraction clearly indicates that the superconducting phase belong to K3C22H14 as a result of phase transition from triclinic to monoclinic structure induced by chemical doping.

Fast PDF screening of amorphous pharmaceuticals with a Bruker D8-ADVANCE

X-ray diffraction (XRD) is a powerful tool used in the pharmaceutical industry to characterize solid pharmaceutical active compounds during drug discovery, development and production. To increase the binding specificity to protein targets APIs are becoming more complex. This complexity decreases the solubility of the API, reducing their capability of entering into cells and actually reaching their targets. Increasing the solubility would enhance the availability of the API and consequently reduce the dose required. One route to increased solubility is polymorph screening: different molecular packings may have different dissolution properties. A different approach is to reduce the crystallite size of the API, possibly even to an amorphous state. In those cases, the long-range crystal structure breaks down, resulting in broad features in the diffraction pattern that are not easily interpretable. Fortunately, a diffraction pattern of an amorphous compound still contains valuable information in the diffuse scattering. This information can be accessed thru the Fourier transform of corrected and normalized diffraction data, which gives the pair distribution function (PDF). Experimentally, the PDF requires good counting statistics to a high Q-range, which generally necessitates the use of hard radiation (Mo, Ag) and long measurement times on laboratory instruments, making this technique less suitable for screening. To considerably reduce the measurement time, we have used a PILATUS3 R 100K-A 2-dimensional detector in combination with a focusing Goebel mirror on a Bruker D8 ADVANCE diffractometer. Data suitable for extracting the PDF was collected at different measurement times ranging from 30 minutes to a few hours on a series of weakly diffracting pharmaceutical compounds. The obtained PDF data was modelled with DIFFRAC.TOPAS (v6). Here we will discuss the feasibility of using fast laboratory-PDF data for the characterization of APIs, both in crystalline and amorphous forms. In addition to the fingerprinting of pure substances, we also discuss the usefulness of this approach for quantification of phase mixtures via pattern-scaling.

The new barium compound Ba4Al7+x: formation and crystal structure

Abstract Combining laboratory X-ray powder diffraction with in-situ high-temperature synchrotron experiments and differential scanning calorimetry, it has been shown that Ba21Al40, Ba3Al5, Ba7Al10 and Ba4Al5 decompose peritectically at 914, 826, 756, and 732°C, respectively. In addition, a new binary compound with the composition Ba4Al7+x (x = 0.17) and the formation temperature of 841°C was found. The initial structural model (space group P63/mmc, a = 6.0807(1), c = 39.2828(8) Å) with four Ba and five Al crystallographic positions was developed. It is based on the intergrowth concept involving the neighboring Ba21Al40 and Ba3Al5 phases and the derived atomic arrangement is subsequently refined using X-ray diffraction data. The crystal structures of all phases in the Ba–Al system, except BaAl4, exhibit Kagomé nets of aluminum atoms resembling those observed for the B atoms in the Laves phases AB2. In the crystal structure of Ba4Al7+x, single Kagomé layers alternate with double slabs (MgZn2 motif) along [001] and are separated by Ba cations. Intergrowth features of Ba4Al7+x are discussed together with the neighboring Ba–Al compounds and Sr5Al9.

Structural modulations in multiferroic tetragonal tungsten bronze KxMnxFe1+xF3

Multiferroic materials showing coupling of the different order parameters (ferroelectric, ferromagnetic, ferroelastic) are interesting not only from a fundamental perspective, but also from a technological point of view, e.g. for to the development of new storage technologies. However, the coexistence of (ferro)magnetism and ferroelectricity is considered a rare phenomenon. Whilst this may be true for perovskite oxides, where empty d shells favor the off-centering of ions but counteract magnetism, this intrinsic limitation can be avoided by moving to different structure types, and/or away from oxides. An example of non-perovskite, non-oxide multiferroic systems are the tetragonal tungsten bronze (TTB) fluorides KxM>2+xM3+1−xF3 (x = 0.4 – 0.6), which show coexistence of electric and magnetic ordering 1. Here we present a detailed structural study on a series of TTB fluorides, KxMnxFe1−xF3 (x = 0.4 – 0.55). KMnFeF6 has been previously described as tetragonal P42bc and orders ferrimagnetically below T = 148 K 2. Additional satellite reflections were found in transmission electron microscopy experiments and attributed to ferroelastic domains arising from tilting of MF6 octahedra, but the reported bulk powder XRD measurements indicated only tetragonal symmetry 3. We used high-resolution powder diffraction techniques to reinvestigate the crystal structure as a function of temperature in comparison with DSC data. Our results reveal a structural distortion to orthorhombic symmetry (Ccc2) at room temperature, which diminished when moving to the end members of the series (x → 0.4 and x → 0.6). Although structurally subtle, this distortion may indicate a ferroelectric state, similar to KxFeF3, where ferroelectricity is observed only in the orthorhombic phase. On heating, an anomaly in the c-axis lattice parameter accompanies a phase transition to centrosymmetric P42/mbc around 320 – 350 K, marking the transition from ferroelectric – paraelectric state.