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.

Experimental core electron density of cubic boron nitride

The resent progress in powder diffraction provides data of quality beyond multipolar modeling of the valence density. As was recently shown in a benchmark study of diamond by Bindzus et al.[1] The next step is to investigate more complicated chemical bonding motives, to determine the effect of bonding on the core density. Cubic boron nitride lends itself as a perfect candidate because of its many similarities with diamond: bonding pattern in the extended network structure, hardness, and the quality of the crystallites.[2] However, some degree ionic interaction is a part of the bonding in boron nitride, which is not present in diamond. By investigating the core density in boron nitride we may obtain a deeper understanding of the effect of bonding on the total density. We report here a thorough investigation of the charge density of cubic boron nitride with a detailed modelling of the inner atom charge density. By combining high resolution powder X-ray diffraction data and an extended multipolar model an experimental modeling of the core density is possible.[3] The thermal motion is a problem since it is strongly correlated to the changes of the core density, but by combining the average displacement from a Wilson plot and a constrained refinement, a reasonable result has been obtained. The displacement parameters reported here are significantly lower than those previously reported, stressing the importance of an adequate description of the core density. The charge transfer from boron to nitrogen clearly affects the inner electron density, which is evident from theoretical as well as experimental result. The redistribution of electron density will, if not accounted for, result in increased thermal parameters. It is estimated that 1.7-2 electrons is transferred from boron to nitrogen.

Expanding the Chemical Versatility of Thiostannate Anions.

Soluble tin(IV) chalcogenide complexes play a major role in solution processing synthesis of macroelectronic tin(IV) chalcogenide based devices, e.g. thin film transistors (TFTs) and the technological interesting photovoltaic material, Cu2ZnSnS4 (CZTS). The synthesis and study of new soluble thiostannate(IV) complexes without electronic impurity atoms and with low decomposition temperature are of key importance for the further development of tin(IV) chalcogenide based devices. We have from the same aqueous ammonium tin(IV) sulfide solution, synthesized and characterized four new crystal structures with different sized thiostannate(IV) complexes (i.e. monomeric [SnS4]4-, dimeric [Sn2S6]4-, pyramids of [Sn3S9]6- and the linear chain [SnS3]2-). Hirshfeld surface analysis for the anionic dimeric [Sn2S6]4- complex in (NH4)4Sn2S6·3H2O shows that water bound hydrogens interact equally well as the ammonium bound hydrogens with the anionic complex. The elongation of the terminal Sn-S bond depends only on the number of hydrogen atoms which interact with the sulfur atom (regardless of the hydrogen atom is bound in water molecules or in ammonium cations). We present the results for the application of the as-synthesized thiostannate(IV) crystals in solution processing of SnS2 thin films. Crystallographic and electron microscopic methods have established that all films are highly textured with the high mobility ab-plane parallel to the substrate surface. This is ideal for e.g. TFT devices where high mobility is required parallel to the substrate surface.

Exploring Host-Guest Interactions: a Single Crystal High-Pressure study

Single crystal X-ray diffraction data from several Hydroquinone clathrate systems, with various small guest molecules (e.g. HCOOH, MeOH), have been obtained up to a pressure of 10 GPa, using a diamond anvil cell (DAC). Hydroquinone clathrates are key examples of supramolecular aggregates, having a diverse structural chemistry controlled, to a large extent, by the detailed intermolecular interactions between the host and the guest molecules. Although supramolecular chemistry is the foundation for the design and development of advanced materials (e.g. for catalysis, targeted drug delivery, chemical separation and nonlinear optics) the basic understanding leading to such complex systems are often lacking. High pressure (HP) crystallography is an excellent method of systematically increasing host-guest interactions by forcing the molecules closer together, often leading to interesting and unexpected results. At ambient pressure smaller guest molecules are often disordered inside the clathrate cavities. As the external pressure increases the cavities shrink, and it seems likely that guest molecules will order inside the cavity breaking the host symmetry. Guest ordering transitions are also found upon cooling. In this work, results from HP studies of the hydroquinone - formic acid system reveal that the structure is stable up to 10 GPa, at which pressure the guest cavity volume is reduced by more than 50 % without ordering of the guest atoms. Earlier studies have shown that the empty Hydroquinone clathrate undergoes a phase transition into a nonporous structure already at 0.4 GPa. [1] This indicates that formic acid stabilizes the host framework through strong intermolecular host-guest interactions, but without lowering the host symmetry.

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].

Effect of pressure on a mixed valence FeⁱⁱFeⁱⁱⁱ₂ complex

Intramolecular electron transfer (ET) in mixed valence (MV) oxo-centered [FeⁱⁱFeⁱⁱⁱ₂O(carboxylate)₆(ligand)₃]·solvent complexes is highly dependent on temperature, on the nature of the ligands, and on the presence of crystal solvent molecules [1]. Whereas the effects of temperature, crystal solvent, and ligand variation on the details of the ET have been explored thoroughly, the effect of pressure is less well described [2]. The effect of pressure on the ET in MV Fe₃O(cyanoacetate)₆(water)₃ has been investigated with single crystal X-ray diffraction and Mössbauer spectroscopy. Previous multi-temperature studies have shown that at room temperature the ET between the three Fe sites is fast and the observed structure of the Fe₃ core is a perfectly equilateral triangle [3]. Cooling the complex below 130 K induces a phase transition as the ET slows down. Below 120 K the Fe₃ core is distorted due to the localization of the itinerant electron on one of the three Fe sites in the triangle (the complex is then in the valence trapped state). The valence trapping is complete within a temperature interval of just 10 K. The abruptness of the transition has been attributed to the extended hydrogen bond network involving water ligands and cyano groups, promoting intermolecular cooperative effects. The high-pressure X-ray diffraction data show that there is a 90⁰ flip of half the cyano groups at 3.5 GPa, which dramatically changes the hydrogen bond network. At a slightly higher pressure, a phase transition is found to occur. The five single crystals investigated all broke into minor fragments at the transition; however triclinic unit cells, similar to the low temperature unit cell, could be indexed from selected spots. Additional evidence that the complex is valence trapped comes from high pressure Mössbauer spectra measured above the phase transition (4 GPa). The relationship between valence trapping and the structural changes will in this work be highlighted using void space and Hirshfeld surface analysis.

Watching nanocrystals form

Reactions in steel containers under solvo/hydrothermal conditions are widely used to produce crystalline nanoparticles. The solvo/hydrothermal approach often provides excellent control over nanoparticle characteristics such as size, size distribution, morphology and crystallinity. However, most progress in the solvothermal field is empirical in nature. Recent development of in situ X-ray scattering techniques now allow real time monitoring of the formation of nanoparticles under high pressure, high temperature conditions, and this opens up the possibility for synthesizing nanoparticles by design. We have developed unique in situ reactors for studies of reactions in sub- and supercritical fluids [1]. By means of Small Angle X-ray Scattering (SAXS), Wide Angle X-ray Scattering (WAXS), Total scattering and EXAFS we have obtained knowledge on the formation and growth of a range of important nanoparticles all the way from the precursor structures to the final crystalline product. In the talk recent examples will be discussed. [1] (a) Jensen et al., Angew. Chem. 2007, 46, 1113; (b) Bremholm et al., Angew Chem. 2009, 48, 4788; (c) Bremholm et al., Adv. Mater. 2009, 21, 3572; (d) Lock et al, Angew Chem. 2011, 50, 7045; (e) Jensen et al., J. Am. Chem. Soc. 2012, 134, 6785; (f) Tyrsted et al, Angew. Chem. 2012, 51, 9030; (g) Nørby et al., RSC Adv. 2013, 3, 15368; (h) Eltzholtz et al., Nanoscale 2013, 5, 2372

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.

Synchrotron charge density studies of chemical bonding in the polymorphs of FeS2

The experimental charge density (CD) distributions in both polymorphs of the photovoltaic compound iron-disulphide (FeS2; cubic pyrite and orthorhombic marcasite) will be described.[1] The CDs are determined by multipole modelling using synchrotron X-ray diffraction data collected at 10 K on extremely small single crystals (<10 mu) thus minimizing the influence of systematic errors such as absorption, extinction and TDS, and exploiting experiences gained from our recent synchrotron studies of CoSb3.[2] The analysis of the charge density in both polymorphs of FeS2 provides an opportunity to see how the different geometries affect local atomic properties, such as 2-center chemical bonding, atomic charges and d-orbital populations. In particular, the data and the resulting multipole models enable us to link the atomic-centered view that emerges from the multipole analysis with the band structure approach. This is carried out by combination with results from periodic calculations on the compounds in the experimental geometries using WIEN2k, thereby providing unambiguous answers to a number of unsolved issues regarding the nature of the bonding in FeS2. The chemical bonding will be characterized by topological analyses showing that the Fe-S bonds are polar covalent bonds, with only minor charge accumulation but significantly negative energy densities at the bond critical points. Using the IAM as reference, density is found to accumulate in-between the atoms, supporting a partial covalent bonding description. The homopolar covalent S-S interaction is seemingly stronger in pyrite than in marcasite, determined not only from the shorter distance but also from all topological indicators. Integrated atomic (Bader) charges show significantly smaller values than those estimation based on crystal-field theory of Fe2+, S-1. In connection with this, the experimentally derived d-orbital populations on Fe are found to deviate from the commonly assumed full t2g set, empty eg set, and they fit very well with the theoretical individual atomic orbitals projected density of states showing a higher dxy participation in the valence band in marcasite compared with pyrite. Thus, the differences between the two polymorphic compounds are directly reflected in their valence density distributions and d-orbital populations.

Locating Cu in Zeolite SSZ-13 by Synchrotron PXRD Using Maximum Entropy Method

Efficient elimination of environmentally harmful gaseous NOx compounds from automotive diesel emission remains a challenging task. State-of-the-art zeolites with the chabazite framework containing catalytically active Cu2+ (Cu-SSZ-13) have been commercialized as NOx after-treatment catalysts in diesel-powered vehicles, due to its superior activity, selectivity, and durability.[1] However, to meet current and future legislative demands, continuous improvement is of fundamental interest. Prerequisites for an in depth understanding and further improvements, are detailed complete structural models of the Cu-loaded catalyst. This may be achieved by the use of high resolution synchrotron powder X-ray diffraction (PXRD) and iterative Rietveld analysis and Maximum Entropy Method (MEM). Since the content of Cu2+ is low, a protonated system (H-SSZ-13) and model system with monovalent Ag+ ions (Ag-SSZ-13) are also examined. The protonated and dehydrated H-SSZ-13 shows perfectly empty voids, i.e. no water residue or other non-framework species. The H-SSZ-13 structure is used as the initial model for the MEM calculations. For Ag-SSZ-13 MEM analysis clearly pinpoints the Ag+ ion as being located in the 6-ring shifted into the chabazite cage (Figure 1), consistent with the generally accepted site for Ag+ ions in chabazite and reveals the strength of the iterative Rietveld/MEM analysis. For the more challenging case of Cu-SSZ-13 it was still possible through careful analysis and reasoning to locate two separate positions for the Cu2+ in Cu-SSZ-13 (Figure 1). The B site has been suggested by several other studies, but never confirmed experimentally.[2] This is the most complete structural description of zeolite SSZ-13 with stabilizing and catalytically active Cu2+ ions.[3]

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.

Material Design Inputs from Charge Density Analysis in Organic Semiconductors

In recent years, semiconducting organic materials have attracted a considerable amount of interest to develop all-organic or hybrid organic-inorganic electronic devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), or photovoltaic cells. Rubrene (5,6,11,12-tetraphenyltetracene, RUB) is one of the most explored compound in this area as it has nearly 100% fluorescence quantum efficiency in solution. Additionally, the OFET fabricated by vacuum-deposited using orthorhombic rubrene single crystals show p-type characteristics with high mobility up to 20cm²/Vs (Podzorov et al., 2004). The large charge-carrier mobilities measured have been attributed to the packing motif (Fig a) which provides enough spatial overlap of the π-conjugated tetracene backbone. In the same time, RUB undergoes an oxidation in the presence of light to form rubrene endoperoxide (RUB-OX) (Fumagalli et al., 2011). RUB-OX molecules show electronic and structural properties strikingly different from those of RUB, mainly due to the disruption in the conjugate stacking of tetracene moieties. The significant semiconducting property of RUB is not clear yet. In this context, high resolution single crystal X-ray data of RUB (Fig b) and RUB-OX have been collected at 100K. Owing to the presence of weak aromatic stacking and quadrupolar interactions, the neutron single crystal data is also collected at 100K. The C-H bond distances and scaled anisotropic displacement parameters (ADP) of hydrogens from the neutron experiment are used in the multipolar refinements of electron density. The chemical bonding features (Fig c), the topology of electron density and strength of weak interaction are calculated by the Atoms in Molecules (AIM) theory (Bader, 1990). It is further supported by the source function description and mapping of non-covalent interactions based on the electron density. The detailed comparison of two organic semiconductors, RUB and RUB-OX will be discussed.

Investigating anharmonicity and disorder in cadmium and zinc telluride

CdTe and ZnTe are often referred to as II-VI semiconductors. Due to the structural and photoelectric properties and low-cost manufacturability, CdTe and ZnTe based thin films are used in the photovoltaic technology and in variety of electronic devices such as infrared, X-ray and gamma ray detectors (Eisen at al., 1998). The structure of another telluride, PbTe, has recently been reviewed and the emerging atomic disorder with temperature seems to have an indissoluble liaison with the high thermoelectric figure of merit of such promising material (Bozin et al., 2010). Deviations of the cation from its position in the ideal rock-salt structure have been probed by means of Maximum Entropy Method (MEM) calculations on Synchrotron powder X-ray diffraction data (SPXRD) (Kastbjerg et al., 2013). Motivated by the peculiar structural features in lead telluride, we investigate anharmonicity and disorder of the cations in both the zincblende structures, CdTe and ZnTe. High resolution SPXRD data at 100 K have been collected for both compounds. High energy radiation and minute capillaries have been used with the aim to minimize systematic errors on the data such as absorption and anomalous scattering. Accurate Rietveld refinements have been carried out in order to extract the best dataset of structure factors. Maximum Entropy Method calculations have hence been computed, providing the least-biased information deduction from experimental data. The disorder, anharmonicity and chemical bonding within the crystalline CdTe and ZnTe have been deeply investigated through the MEM densities and comparisons with the cation displacement in the structure of lead telluride have been established.