Parametric Rietveld refinement

In this paper the method of parametric Rietveld refinement is described, in which an ensemble of diffraction data collected as a function of time, temperature, pressure or any other variable are fitted to a single evolving structural model. Parametric refinement offers a number of potential benefits over independent or sequential analysis. It can lead to higher precision of refined parameters, offers the possibility of applying physically realistic models during data analysis, allows the refinement of `non-crystallographic' quantities such as temperature or rate constants directly from diffraction data, and can help avoid false minima

Averaging the intensity of many-layered structures for accurate stacking-fault analysis using Rietveld refinement

Many technologically important synthetic and natural materials display stacking faults which lead to complex peak broadenings, asymmetries and shifts in their powder diffraction patterns. The patterns can be described using an enlarged unit cell (called a supercell) containing an explicit description of the layers. Since the supercell can contain hundreds of thousands of atoms with hundreds of thousands of hkl reflections, a Rietveld approach has been too computationally demanding for all but the simplest systems. This article describes the implementation of the speed-ups necessary to allow Rietveld refinement in the computer program TOPAS Version 6 (Bruker AXS, Karlsruhe, Germany). Techniques implemented include: a peaks buffer that allows hundreds of thousands of hkl-dependent peak shapes to be automatically approximated by a few hundred peaks; an averaging process for hundreds of large supercells with minimum impact on computational time; a smoothing technique that allows for the use of small supercells which approximate supercells ten to 20 times larger; and efficient algorithms for stacking sequence generation. The result is Rietveld refinement of supercells operating at speeds several thousand times faster than traditional Rietveld refinements. This allows quantitative and simultaneous analysis of structure and microstructure in complex stacking-faulted samples

Microstructures of negative and positive azeotropes

Azeotropes famously impose fundamental restrictions on distillation processes, yet their special thermodynamic properties make them highly desirable for a diverse range of industrial and technological applications. Using neutron diffraction, we investigate the structures of two prototypical azeotropes, the negative acetone–chloroform and the positive benzene–methanol azeotrope. C–H⋯O hydrogen bonding is the dominating interaction in the negative azeotrope but C–Cl⋯O halogen bonding contributes as well. Hydrogen-bonded chains of methanol molecules, which are on average longer than in pure methanol, are the defining structural feature of the positive azeotrope illustrating the fundamentally different local mixing in the two kinds of azeotropes. The emerging trend for both azeotropes is that the more volatile components experience the more pronounced structural changes in their local environments as the azeotropes form. The mixing of the acetone–chloroform azeotrope is essentially random above 20 Å, where the running Kirkwood–Buff integrals of our structural model converge closely to the ones expected from thermodynamic data. The benzene–methanol azeotrope on the other hand displays extended methanol-rich regions and consequently the running Kirkwood–Buff integrals oscillate up to at least 60 Å. Our study provides the first experimental insights into the microstructures of azeotropes and a direct link with their thermodynamic properties. Ultimately, this will provide a route for creating tailored molecular environments in azeotropes to improve and fine-tune their performances

Weak spin interactions in Mott insulating La2O2Fe2OSe2

Identifying and characterizing the parent phases of iron-based superconductors is an important step towards understanding the mechanism for their high-temperature superconductivity. We present an investigation into the magnetic interactions in the Mott insulator La2O2Fe2OSe2. This iron oxyselenide adopts a 2-k magnetic structure with low levels of magnetic frustration. This magnetic ground state is found to be dominated by next-nearest-neighbor interactions J2 and J2′ and the magnetocrystalline anisotropy of the Fe2+ site, leading to 2D-Ising-like spin S=2 fluctuations. In contrast to calculations, the values are small and confine the spin excitations below ∼25 meV. This is further corroborated by sum rules of neutron scattering. This indicates that superconductivity in related materials may derive from a weakly coupled and unfrustrated magnetic structure

Beyond classical applications of powder diffraction

This article provides a short tutorial review of how laboratory powder diffraction methods can be used to establish a variety of "non-traditional'' pieces of information about solid state materials-information beyond simple phase identification or structure determination at a specific temperature. The examples show how unique insights into synthetic pathways, reaction mechanisms, reaction kinetics, polymorphism, phase transitions and physical properties can be obtained for a variety of different types of materials

Synthesis and characterisation of a new high pressure polymorph of Cu2WS4

In this communication we report the synthesis and structural characterisation of a new body centred polymorph of Cu2WS4 prepared using hydrothermal methods. I-Cu2WS4 crystallises in space group I (4) over bar 2m with cell parameters a = b = 5.44427(8), c = 10.0687(2) Angstrom and has a new structure type containing layers of edge-sharing CuS4 and WS4 tetrahedra

Synthesis, structure and properties of the oxychalcogenide series A4O4TiSe4 (A=Sm, Gd, Tb, Dy, Ho, Er and Y)

Seven oxyselenide materials have been synthesised with composition A4O4TiSe4 (A=Sm, Gd–Er, Y) via solid state reactions of A2O3, TiSe2 and Se at 900 °C. They are all isostructural with Gd4O4TiSe4. Structures have been refined from powder X-ray diffraction data and have monoclinic C2/m symmetry with unit cell parameters of a≈15.7 Å, b≈3.75 Å, c≈9.65 Å and β≈117.5°. They contain infinite ribbons of edge-sharing A4O and A3TiO tetrahedra 4 units wide, which are linked by chains of TiSe4O2 edge-sharing octahedra. Compositions A=Gd–Ho, Y are semiconductors with conductivities 1–3 Sm−1 at 300 K, with electronic band gaps of between 0.25 and 0.37 eV. Magnetic susceptibility is reported from 1.8 K to 300 K for compositions A=Gd–Ho. Rare earth moments appear to order antiferromagnetically at low temperatures with Gd and Tb showing evidence of ferromagnetism due to spin canting over a narrow temperature range close to TN

The kinetics of low-temperature oxygen migration in ZrWMoO8

ZrWMoO8 has been shown to undergo a phase transition at remarkably low temperatures in which oxygen atoms become dynamically disordered. The kinetics of this process have been studied by a variety of laboratory-based time and temperature dependent powder diffraction experiments. Oxygen mobility from temperatures as low as 205 K has been confirmed and an activation energy of 34 kJ mol(-1) estimated for the migration process

The crystal structure of alpha-La2Mo2O9 and the structural origin of the oxide ion migration pathway

We describe for the first time the full 3D atomic structure of room-temperature alpha-La2Mo2O9. The material, despite its simple chemical formula, has a remarkable 312 crystallographically unique atoms and is thus one of the most complex oxide structures reported to date. Despite this complexity, the structural results offer significant insight into the O2- migration pathway in the anion conducting high-temperature form, beta-La2Mo2O9. The material contains a mixture of 4, 5, and 6 coordinated Mo sites, suggesting that variable Mo coordination number is a key factor in providing a low-energy O2- conduction pathway. We provide quantitative analysis showing that the positions in the ordered array of 216 oxygen sites of alpha-La2Mo2O9 are directly related to the average sites occupied in beta-La2Mo2O9, providing compelling evidence that the high-temperature conducting form of the material has a time-averaged version of the low-temperature structure