Nuclear Enhanced MEM Used to Analyze Local Distortions in Lead Chalcogenides

We introduce a novel method for reconstructing nuclear density distributions (NDDs): Nuclear Enhanced X-ray Maximum Entropy Method (NEXMEM). NEXMEM offers an alternative route to experimental NDDs, exploiting the superior quality of synchrotron X-ray data compared to neutron data. The method was conceived to analyse local distortions in the thermoelectric lead chalcogenides, PbX (X = S, Se, Te). Thermoelectric materials are functional materials with the unique ability to interconvert heat and electricity, holding much promise for green energy solutions such as waste heat recovery. The extraordinary thermoelectric performance of binary lead chalcogenides has caused huge research activity, but the mechanisms governing their unexpected low thermal conductivity still remain a controversial topic. It has been proposed to result from giant anharmonic phonon scattering or from local fluctuating dipoles on the Pb site.[1,2] No macroscopic symmetry change are associated with these effects, rendering them invisible to conventional crystallographic techniques. For this reason PbX was until recently believed to adopt the ideal, undistorted rock-salt structure. In the present study, we investigate PbX using multi-temperature synchrotron powder X-ray diffraction data in combination with the maximum entropy method (MEM) and NEXMEM. In addition NEXMEM has been validated by testing against simulated powder diffraction data of PbTe with known displacements of Pb. The increased resolution of NEXMEM proved essential for resolving Pb-displacement of 0.2 Å in simulated data. The figure below shows Pb in the (100) plane for MEM, NEXMEM and the actual NDD of the test structure. Our findings outline the extent of disorder in lead chalcogenides, promoting our understanding of this class of high-performance thermoelectric materials. Furthermore we introduce NEXMEM which can be used for widespread characterization of subtle atomic features in crystals with unusual properties.

Characterization of Local Distortions in Thermoelectric Lead Chalcogenides

Thermoelectric materials are functional materials with the unique ability to interconvert heat and electricity, holding much promise for green energy solutions such as efficient waste heat recovery. The extraordinary thermoelectric performance of binary lead chalcogenides has caused huge research activity, but the mechanisms governing their unexpected low thermal conductivity still remain a controversial topic. It has been proposed to result from giant anharmonic phonon scattering or from local fluctuating dipoles on the Pb site, emerging with temperature on the Pb site.[1,2] No macroscopic symmetry change are associated with these effects, rendering them invisible to conventional crystallographic techniques. For this reason lead chalcogenides were until recently believed to adopt the ideal, undistorted rock-salt structure. In the present study, we probe the peculiar structural features in PbX (X = S, Se, Te) using multi-temperature synchrotron powder X-ray diffraction data in combination with the maximum entropy method. Distorted atoms are detected and quantified by refinement of anharmonic probability density functions. The charge density analysis is complemented by nuclear density distributions (NDDs) reconstructed from neutron diffraction data and by a novel method: Nuclear Enhanced X-ray Maximum Entropy Method (NEXMEM). NEXMEM offers an alternative route to experimental NDDs, exploiting the superior quality of synchrotron X-ray data compared to neutron diffraction data. The increased atomic resolution introduced by NEXMEM proved essential for resolving atomic distortions, see figure below showing Pb in the (100) plane. Our findings outline the extent of disorder and anharmonicity in binary lead chalcogenides, promoting our fundamental understanding of this class of high-performance thermoelectric materials. The applied approach can be used in general, opening up for widespread characterization of subtle features in crystals with unusual properties.

In situ growth of SrFe₁₂O₁₉

SrFe12O19 is a highly anisotropic ferrimagnetic compound with relatively high remanence and high coercivity, which is used in permanent magnets. Permanent magnets are everywhere in our daily life and they are responsible for the interconversion between motion and electricity in electrical components ranging from headphones to wind turbines. Three key parameters, important for making permanent magnets, are an anisotropic structure, size of the nanocrystallites and the microstructure. In situ X-ray powder diffraction has been used to follow the growth kinetics of SrFe12O19 under hydrothermal conditions. Synthesis of SrFe12O19 (Sr-Hexaferrite) nanocrystals by hydrothermal methods have the advantage of allowing exhaustive control of the reaction parameters. We have studied the growth and kinetics of SrFe12O19 by carring out time resolved synchrotron experiments at MAX-lab, Sweden. The experiments were carried out at elevated pressure (250 bar) and in temperatures ranging from 250 to 400 oC. The diffraction data allow us to follow the evolution of the crystallite size as function of temperature, time and composition. By controlling the composition of the precursor we can tailor the size of the nanocrystallites. The obtained data have shown that the synthesis takes place through a conversion of tiny hexagonal shaped FeOOH nanocrystallites into the SrFe12O19. Several ex situ studies under comparable conditions have been carried out to compare the magnetic properties and the obtained nanocrystallites have been investigated using high resolution laboratory powder diffraction data.

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.

In situ total X-ray scattering study of WO₃ nanoparticle formation

In situ total scattering in combination with pair distribution function (PDF) and powder X-ray diffraction (PXRD) methods have been used to unravel the mechanism of WO₃ nanoparticle formation from aqueous precursor solution of ammonium metatungstate [(NH₄)₆H₂W₁₂O₄₀.xH₂O (AMT)] under hydrothermal condition. Total scattering studies can extract precise atomic scale structural information from solutions, amorphous solids, nanosized structures as well as from crystals [1]. The reaction mechanism was followed in an in situ reactor at synchrotron [2]. The study reveals that a complex precursor structure exists in the solution. It consists of edge and corner sharing WO₆ octahedra. While heating the solution, the precursor structure undergoes a reorientation with time converting the edge sharing octahedra to corner sharing octahedra before forming the nanoparticles. While the octahedra locally become reoriented there is no evidence of long range order. After 10 min. of heating, the nuclei in the solution abruptly cluster together and form crystalline particles. The sudden formation of nano crystals is also confirmed by in situ PXRD measurement. Further PDF analysis also reveals that local structure in hexagonal WO₃ is different than the average structure and it also rationalizes the formation of two different hexagonal phase of WO₃ in two different syhtesis procedure [3].

Formation of γ-Fe2O3 in hydrothermal synthesis: In situ total scattering studies

The properties of metal oxide nanoparticles are highly dependent on particle characteristics such as size, crystallinity, and structural defects. To obtain particles with tailormade properties, it is crucial to understand the mechanisms that govern these characteristics during material synthesis. For this purpose, in situ studies of particle synthesis have proven powerful.[1] Here, in situ Total Scattering (TS) combined with in situ PXRD studies of the hydrothermal synthesis of γ-Fe2O3 (maghemite) from ammonium iron citrate will be presented. In situ TS with Pair Distribution Function (PDF) analysis has recently shown to be an efficient tool for understanding the fundamental chemical processes in particle crystallization.[2,3] The full γ-Fe2O3 crystallization process from ionic complexes over nanoclusters to crystalline particles is followed and material formation mechanisms are suggested. The study shows that the local atomic structure of the precursor solution is similar to that of the crystalline coordination polymer [Fe(H2citrate)(H2O)]n where corner sharing [FeO6] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge sharing [FeO6] units form. Tetrahedrally coordinated iron subsequently appears in the structure and as the synthesis continues, the clusters slowly assemble into nanocrystalline maghemite. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the large clusters along corner sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles below 6 nm are highly disordered. Whole Powder Pattern Modelling of the PXRD data shows that the final crystallite size (<10 nm) is dependent on synthesis temperature and that the size distribution of the particles broadens with synthesis time.

In- and ex situ PXRD studies of ZnO nanoparticle growth in sub-critical water

Zinc oxide (ZnO) is a material of great scientific and industrial relevance and is used widely in a variety of applications. Synthesis of ZnO nanoparticles can be performed by a wide range of methods resulting in a tremendous variety of sizes and shapes. Different in situ characterization methods have been used to investigate the ZnO formation under various synthesis conditions; these include numerous spectroscopic methods and small angle scattering. Common for these studies is that the primary focus has been to extract information on particle size and shape of ZnO, while a more rigorous microstructural and structural analysis has been lacking. Furthermore, the aforementioned studies have primarily been focused on soft chemical synthesis methods, at low temperatures and in non-aqueous media, thus omitting the widely used environmentally benign and versatile hydrothermal method. In the present work the formation of ZnO during hydrothermal synthesis has been followed using in situ powder X-ray diffraction (PXRD) combined with Rietveld refinement, thus enabling the extraction of crystallographic as well as microstructural information during the formation and growth of ZnO. Supporting ex situ syntheses and characterization by electron microcopy, high resolution PXRD and other techniques have been used to corroborate the findings from the in situ experiments. Mapping out a vast parameter space has led to a deeper understanding of the intricate mechanisms governing the nucleation and growth of ZnO nanoparticles during hydrothermal synthesis. Among the parameters studied were the influence of temperature, type of base used and the influence of different ionic salts as synthesis directing agents. The various synthesis parameters were found to influence the following structural and microstructural features: crystallite shape, morphology and size as well as the twin-fault concentration, degree of doping and crystallinity.

HEIMDAL: A novel materials science neutron powder diffractometer at ESS

Developing new materials is the most challenging task for the future demands due to limitations in energy, resources but also environmental damage. Improvements in material performances are reached for example by the incorporation of advanced ceramics and polymers into heterogeneous systems. Their performances usually depend on the interplay between properties defined by the atomic, nano/mesoscopic and microscopic structure. In-situ and in-operandi investigations will be in the focus of such investigations. The instrument HEIMDAL proposed for the European spallation neutron source ESS will offer here perfect prospects, as the instrumental resolution of this powder diffractometer can widely be adapted taking either full advantage of the broad pulse of ESS (2.86ms) offering highest intensity, or using a fraction of the pulse for highest resolution. A thermal and a cold guides pointing to the same virtual source extend the spatial window of the instrumental from an atomic scale (0.3Å-1≤Q≤50Å-1) to a nano/meso scale, 0.002Å-1≤Q≤0.1Å-1 by adding a narrow-band SANS instrument behind. Our chopper system allows switching the different operation modes electronically. Traditionally such structural information has been collected in separated experiments such as powder diffraction (PD), wide angle diffraction scale, small angle diffraction and direct space imaging techniques (sub-micronic to millimeter scale), whereas HEIMDAL can offer these options in its final stage at the same time at therefore for absolutely identical experimental conditions. The top-loading geometry foreseen not only accepts auxiliary from the ESS pool (cryogenics, pressure cells, magnets) but also allows implementing bulky brought-in user equipment. It can be pretested off-line at the instrument, but already fully connected to the HEIMDAL electronics. Figure 1: a) Operation modes of the HEIMDAL b) Layout of the instrument HEIMDAL