Dynamic correlations and possible diffusion pathway in the superionic conductor Cu2-xSe.

The superionic conductor Cu2-xSe has regained interest as a thermoelectric material owing to its low thermal conductivity, suggested to arise from a liquid-like Cu substructure, and the material has been coined a phonon-liquid electron-crystal. Using high-quality three-dimensional X-ray scattering data measured up to large scattering vectors, accurate analysis of both the average crystal structure as well as the local correlations is carried out to shed light on the Cu movements. The Cu ions show large vibrations with extreme anharmonicity and mainly move within a tetrahedron-shaped volume in the structure. From the analysis of weak features in the observed electron density, the possible diffusion pathway of Cu is identified, and it is clear from its low density that jumps between sites are infrequent compared with the time the Cu ions spend vibrating around each site. These findings support the conclusions drawn from recent quasi-elastic neutron scattering data, casting doubt on the phonon-liquid picture. Although there is diffusion of Cu ions in the structure, making it a superionic conductor, the jumps are infrequent and probably not the origin of the low thermal conductivity. From three-dimensional difference pair distribution function analysis of the diffuse scattering data, strongly correlated movements are identified, showing atomic motions which conserve interatomic distances at the cost of large changes in angles.

Electron density and thermal motion of diamond at elevated temperatures.

The electron density and thermal motion of diamond are determined at nine temperatures between 100 K and 1000 K via synchrotron powder X-ray diffraction (PXRD) data collected on a high-accuracy detector system. Decoupling of the thermal motion from the thermally smeared electron density is performed via an iterative Wilson-Hansen-Coppens-Rietveld procedure using theoretical static structure factors from density functional theory (DFT) calculations. The thermal motion is found to be harmonic and isotropic in the explored temperature range, and excellent agreement is observed between experimental atomic displacement parameters (ADPs) and those obtained via theoretical harmonic phonon calculations (HPC), even at 1000 K. The Debye temperature of diamond is determined experimentally to be ΘD = 1883 (35) K. A topological analysis of the electron density explores the temperature dependency of the electron density at the bond critical point. The properties are found to be constant throughout the temperature range. The robustness of the electron density confirms the validity of the crystallographic convolution approximation for diamond in the explored temperature range.

Temperature-dependent crystal structure investigation of 4f hybridized thermoelectric clathrate Ba8-xCexAuySi46-y.

Thermoelectric materials allow for conversion of waste heat into electrical energy, and they represent a green solution for improving our energy efficiency. Inclusion of 4f electrons near the Fermi level may boost the Seebeck coefficient, which is essential for high thermoelectric performance. In this study, Ce was successfully substituted for Ba on the guest atom sites in the type-I clathrate Ba8-xCexAuySi46-y and the material was characterized using high-resolution synchrotron powder X-ray diffraction data measured from 100 K to 1000 K to investigate potential structural implications of the inclusion of a 4f element. The thermal expansion and bonding of the host structure are not affected by the presence of Ce, as seen from the linear coefficient of unit-cell thermal expansion of 7.30 (8) × 10-6 K-1 and the average host Debye temperature of 404 (7) K determined from the multi-temperature atomic displacement parameters, both of which are similar to values obtained for pure Ba8AuySi46-y. The anisotropic atomic displacement parameters on the guest atom site in the large clathrate cage populated by Ba surprisingly reveals isotropic behavior, which is different from all other clathrates reported in literature, and thus represents a unique host-guest bonding situation.

Effects of Voigt diffraction peak profiles on the pair distribution function.

Powder diffraction and pair distribution function (PDF) analysis are well established techniques for investigation of atomic configurations in crystalline materials, and the two are related by a Fourier transformation. In diffraction experiments, structural information, such as crystallite size and microstrain, is contained within the peak profile function of the diffraction peaks. However, the effects of the PXRD (powder X-ray diffraction) peak profile function on the PDF are not fully understood. Here, all the effects from a Voigt diffraction peak profile are solved analytically, and verified experimentally through a high-quality X-ray total scattering measurement on Ni powder. The Lorentzian contribution to the microstrain broadening is found to result in Voigt-shaped PDF peaks. Furthermore, it is demonstrated that an improper description of the Voigt shape during model refinement leads to overestimation of the atomic displacement parameter.

Synchrotron total-scattering data applicable to dual-space structural analysis.

Synchrotron powder X-ray diffraction (PXRD) is a well established technique for investigating the atomic arrangement of crystalline materials. At modern beamlines, X-ray scattering data can be collected in a total-scattering setting, which additionally opens up the opportunity for direct-space structural analysis through the atomic pair distribution function (PDF). Modelling of PXRD and PDF data is typically carried out separately, but employing a concurrent structural model to both direct- and reciprocal-space data has the possibility to enhance total-scattering data analysis. However, total-scattering measurements applicable to such dual-space analyses are technically demanding. Recently, the technical demands have been fulfilled by a MYTHEN microstrip detector system (OHGI), which meets the stringent requirements for both techniques with respect to Q range, Q resolution and dynamic range. In the present study, we evaluate the quality of total-scattering data obtained with OHGI by separate direct- and reciprocal-space analysis of Si. Excellent agreement between structural parameters in both spaces is found, demonstrating that the total-scattering data from OHGI can be utilized in dual-space structural analysis e.g. for in situ and operando measurements.

Chemical Bonding in Colossal Thermopower FeSb2.

FeSb2 exhibits a colossal Seebeck coefficient ( S ) and a record-breaking high thermoelectric power factor. It also has an atypical shift from diamagnetism to paramagnetism with increasing temperature, and the fine details of its electron correlation effects have been widely discussed. The extraordinary physical properties must be rooted in the nature of the chemical bonding, and indeed, the chemical bonding in this archetypical marcasite structure has been heavily debated on a theoretical basis since the 1960s. The two prevalent models for describing the bonding interactions in FeSb2 are based on either ligand-field stabilization of Fe or a network structure of Sb hosting Fe ions. However, neither model can account for the observed properties of FeSb2 . Herein, an experimental electron density study is reported, which is based on analysis of synchrotron X-ray diffraction data measured at 15 K on a minute single crystal to limit systematic errors. The analysis is supplemented with density functional theory calculations in the experimental geometry. The experimental data are at variance with both the additional single-electron Sb-Sb bond implied by the covalent model, and the large formal charge and expected d-orbital splitting advocated by the ionic model. The structure is best described as an extended covalent network in agreement with expectations based on electronegativity differences.

Operando X-ray scattering study of thermoelectric ß-Zn4Sb3.

The application of thermoelectrics for energy harvesting depends strongly on operational reliability and it is therefore desirable to investigate the structural integrity of materials under operating conditions. We have developed an operando setup capable of simultaneously measuring X-ray scattering data and electrical resistance on pellets subjected to electrical current. Here, operando investigations of ß-Zn4Sb3 are reported at current densities of 0.5, 1.14 and 2.3 A mm-2. At 0.5 A mm-2 no sample decomposition is observed, but Rietveld refinements reveal increased zinc occupancy from the anode to the cathode demonstrating zinc migration under applied current. At 1.14 A mm-2 ß-Zn4Sb3 decomposes into ZnSb, but pair distribution function analysis shows that Zn2Sb2 units are preserved during the decomposition. This identifies the mobile zinc in ß-Zn4Sb3 as the linkers between the Zn2Sb2 units. At 2.3 A mm-2 severe Joule heating triggers transition into the γ-Zn4Sb3 phase, which eventually decomposes into ZnSb, demonstrating Zn ion mobility also in γ-Zn4Sb3 under electrical current.

Group†13 Precursor Structures and Their Effect on Oxide Nanocrystal Formation.

Precursor structures (PSs) in solution are expected to influence both nanocrystal formation mechanisms, as well as crystallization of specific polymorphs. Herein, Group 13 PS structures determined by pair distribution function and extended X-ray absorption fine structure analysis are reported. Corner-sharing octahedral dimers form from the metal nitrates dissolved in either water, isopropanol, or ethanol at room temperature contradicting previous studies that suggested monomers or larger Keggin clusters. Because all crystalline indium oxides have octahedral coordination, crystals can easily nucleate from the observed PSs. Similarly, MOOH (M=Al and Ga) with octahedral M coordination is expected to form readily from the PSs, whereas formation of γ-M2 O3 requires a partial conversion to tetrahedral M coordination. This explains the long-standing observation of initial AlOOH formation as a bottleneck for γ-Al2 O3 synthesis. Different indium polymorphs crystallize from the various solvents, and thus there is no obvious link between the PSs and observed polymorphism.

Multipole electron densities and atomic displacement parameters in urea from accurate powder X-ray diffraction.

Electron density determination based on structure factors obtained through powder X-ray diffraction has so far been limited to high-symmetry inorganic solids. This limit is challenged by determining high-quality structure factors for crystalline urea using a bespoke vacuum diffractometer with imaging plates. This allows the collection of data of sufficient quality to model the electron density of a molecular system using the multipole method. The structure factors, refined parameters as well as chemical bonding features are compared with results from the high-quality synchrotron single-crystal study by Birkedal et al. [Acta Cryst. (2004), A60, 371-381] demonstrating that powder X-ray diffraction potentially provides a viable alternative for electron density determination in simple molecular crystals where high-quality single crystals are not available.

Three-dimensional morphology of anatase nanocrystals obtained from supercritical flow synthesis with industrial grade TiOSO4 precursor.

Anatase TiO2 (a-TiO2) nanocrystals are vital in catalytic applications both as catalysts (e.g. photodegradation) and as a carrier material (e.g. NOx removal from exhaust). The synthesis of a-TiO2 nanocrystals and their properties have been heavily scrutinized, but there exists a clear gap between the scientific literature, and the scale and price expectation of industrial application. Here it is demonstrated that the industrially most attractive Ti precursor, titanyl sulfate (TiOSO4), can be combined with the green, scalable and fast supercritical flow method to produce phase pure and highly crystalline a-TiO2 nanoparticles with high specific surface area. Control of the nanocrystal morphology is important since it is known that certain facets substantially promote catalytic activity. It is, however, in itself challenging to determine nanocrystal morphology to provide a rational basis for the synthesis control. Here we advocate the use of advanced Rietveld refinement of powder X-ray diffraction data including anisotropic size broadening models in aiding to establish the sample three-dimensional morphology. This relatively quick and robust method assists in overcoming the often encountered ambiguity inherent in two-dimensional to three-dimensional reconstruction of selected particle morphologies with transmission electron microscopy and tomography techniques.

Accessing the rich carbon nitride materials chemistry by heat treatments of ammonium thiocyanate, NH4SCN.

Carbon nitride materials include functional materials, and their chemical diversity and complexity are becoming increasingly appreciated. Heating of NH4SCN leads to a range of new carbon nitride compounds, which have been structurally characterized by single-crystal X-ray diffraction. Heating at ambient pressure to 175°C leads to guanidinium thiocyanate, H6CN3SCN (1), and when maintaining that temperature for about 12 h a water-insoluble carbon nitride product is formed, which is a co-crystal between melamine and melamium thiocyanate, [H6C3N6]·[H10C6N11]+·[SCN]- (2). In situ powder X-ray diffraction measurements of this material reveal a gradual transformation from (2), via two intermediate products, to a final melon-like end product. The first of these forms between 350 and 400°C, and is an adduct of melam and melamium thiocyanate, [H9C6N11]·2[H10C6N11]+·2[SCN]- (3). The second forms between 400 and 480°C, and is identified as melem, 2,5,8-triamino-tri-s-triazine, H6C6N10 (4). On heating of (2) in a sealed ampoule to 600°C, various crystals were obtained and six crystal structures were determined from the batch: 1,3,5-triazine-2,4,6-triamino, H6C3N6 (5), 1,3,5-triazine-2,4-diamino, H5C3N5 (6), 1,1',3,3',5,5'-triazine-2,2',4,4'-tetraamino, H8C6N10 (7), 2[H6C3N6]·[H10C6N11]+·[SCN]- (8) and 2[H6C3N6]·[H7C3N6]+·[SCN]- (9). Finally, a recrystallized decomposition product was found to be [H6C3N6]·[H7C3N6]+·[SCN]-·[H2O] (10).

Crystal structure and phase transition of thermoelectric SnSe.

Tin selenide-based functional materials are extensively studied in the field of optoelectronic, photovoltaic and thermoelectric devices. Specifically, SnSe has been reported to have an ultrahigh thermoelectric figure of merit of 2.6 ±â€…0.3 in the high-temperature phase. Here we report the evolution of lattice constants, fractional coordinates, site occupancy factors and atomic displacement factors with temperature by means of high-resolution synchrotron powder X-ray diffraction measured from 100 to 855 K. The structure is shown to be cation defective with a Sn content of 0.982 (4). The anisotropy of the thermal parameters of Sn becomes more pronounced approaching the high-temperature phase transition (∼ 810 K). Anharmonic Gram-Charlier parameters have been refined, but data from single-crystal diffraction appear to be needed to firmly quantify anharmonic features. Based on modelling of the atomic displacement parameters the Debye temperature is found to be 175 (4) K. Conflicting reports concerning the different coordinate system settings in the low-temperature and high-temperature phases are discussed. It is also shown that the high-temperature Cmcm phase is not pseudo-tetragonal as commonly assumed.

Anisotropic compressibility of the coordination polymer emim[Mn(btc)].

The effect of pressure on the crystal structure of a coordination polymer, emim[Mn(II)(btc)] (emim = 1-ethyl,3-methyl imidazolium cation, btc = 1,3,5-benzene-tricarboxylate), was investigated with single-crystal X-ray diffraction. At 4.3 GPa the unit-cell volume had decreased by 14% compared with ambient conditions. The unit-cell contraction is highly anisotropic, with the a- and b-axes decreasing by 5.5 and 9.5%, respectively, and the c-axis compressing a mere 0.25% up to 1.7 GPa followed by a 0.2% expansion between 1.7 and 4.3 GPa. The 0.2% increase in length of the c-axis in this interval happens above the quasi-hydrostatic limit of the pressure-transmitting medium and therefore it might be a consequence of strain gradients. Under ambient conditions, two MnO6 units are connected by two carboxylate ligands to form dimeric units. On increasing pressure, a non-bonded O atom from a bridging carboxylate group approaches the Mn atom, with the Mn-O distance decreasing from 2.866 (1) Šat 0.3 GPa to 2.482 (6) Šat 4.3 GPa, increasing the coordination environment of the Mn ion from six- to seven-coordinated.

Carrier concentration dependence of structural disorder in thermoelectric Sn1-x Te.

SnTe is a promising thermoelectric and topological insulator material. Here, the presumably simple rock salt crystal structure of SnTe is studied comprehensively by means of high-resolution synchrotron single-crystal and powder X-ray diffraction from 20 to 800 K. Two samples with different carrier concentrations (sample A = high, sample B = low) have remarkably different atomic displacement parameters, especially at low temperatures. Both samples contain significant numbers of cation vacancies (1-2%) and ordering of Sn vacancies possibly occurs on warming, as corroborated by the appearance of multiple phases and strain above 400 K. The possible presence of disorder and anharmonicity is investigated in view of the low thermal conductivity of SnTe. Refinement of anharmonic Gram-Charlier parameters reveals marginal anharmonicity for sample A, whereas sample B exhibits anharmonic effects even at low temperature. For both samples, no indications are found of a low-temperature rhombohedral phase. Maximum entropy method (MEM) calculations are carried out, including nuclear-weighted X-ray MEM calculations (NXMEM). The atomic electron densities are spherical for sample A, whereas for sample B the Te electron density is elongated along the 〈100〉 direction, with the maximum being displaced from the lattice position at higher temperatures. Overall, the crystal structure of SnTe is found to be defective and sample-dependent, and therefore theoretical calculations of perfect rock salt structures are not expected to predict the properties of real materials.

Nuclear-weighted X-ray maximum entropy method - NXMEM.

Subtle structural features such as disorder and anharmonic motion may be accurately characterized from nuclear density distributions (NDDs). As a viable alternative to neutron diffraction, this paper introduces a new approach named the nuclear-weighted X-ray maximum entropy method (NXMEM) for reconstructing pseudo NDDs. It calculates an electron-weighted nuclear density distribution (eNDD), exploiting that X-ray diffraction delivers data of superior quality, requires smaller sample volumes and has higher availability. NXMEM is tested on two widely different systems: PbTe and Ba(8)Ga(16)Sn(30). The first compound, PbTe, possesses a deceptively simple crystal structure on the macroscopic level that is unable to account for its excellent thermoelectric properties. The key mechanism involves local distortions, and the capability of NXMEM to probe this intriguing feature is established with simulated powder diffraction data. In the second compound, Ba(8)Ga(16)Sn(30), disorder among the Ba guest atoms is analysed with both experimental and simulated single-crystal diffraction data. In all cases, NXMEM outperforms the maximum entropy method by substantially enhancing the nuclear resolution. The induced improvements correlate with the amount of available data, rendering NXMEM especially powerful for powder and low-resolution single-crystal diffraction. The NXMEM procedure can be implemented in existing software and facilitates widespread characterization of disorder in functional materials.

Pulsed supercritical synthesis of anatase TiO2 nanoparticles in a water-isopropanol mixture studied by in situ powder X-ray diffraction.

A new step in supercritical nanoparticle synthesis, the pulsed supercritical synthesis reactor, is investigated in situ using synchrotron powder X-ray diffraction (PXRD) to understand the formation of nanoparticles in real time. This eliminates the common problem of transferring information gained during in situ studies to subsequent laboratory reactor conditions. As a proof of principle, anatase titania nanoparticles were synthesized in a 50/50 mixture of water and isopropanol near and above the critical point of water (P = 250 bar, T = 300, 350, 400, 450, 500 and 550 °C). The evolution of the reaction product was followed by sequentially recording PXRD patterns with a time resolution of less than two seconds. The crystallite size of titania is found to depend on both temperature and residence time, and increasing either parameter leads to larger crystallites. A simple adjustment of either temperature or residence time provides a direct method for gram scale production of anatase nanoparticles of average crystallite sizes between 7 and 35 nm, thus giving the option of synthesizing tailor-made nanoparticles. Modeling of the in situ growth curves using an Avrami growth model gave an activation energy of 66(19) kJ mol(-1) for the initial crystallization. The in situ PXRD data also provide direct information about the size dependent macrostrain in the nanoparticles and with decreasing crystallite size the unit cell contracts, especially along the c-direction. This agrees well with previous ex situ results obtained for hydrothermal synthesis of titania nanoparticles.

Understanding the formation and evolution of ceria nanoparticles under hydrothermal conditions.

Supercritical growth: The formation and evolution of ceria nanoparticles during hydrothermal synthesis was investigated by in situ total scattering and powder diffraction. The nucleation of pristine crystalline ceria nanoparticles originated from previously unknown cerium dimer complexes. The nanoparticle growth was highly accelerated under supercritical conditions.

Solvothermal synthesis, multi-temperature crystal structures and physical properties of isostructural coordination polymers, 2C4H12N+[M3(C8H4O4)4]2-.3C5H11NO, M = Co, Zn.

Two isostructural metal organic framework (MOF) structures have been synthesized by solvothermal methods and examined by single-crystal X-ray diffraction. A microcrystal of 2C4H12N+[Co3(C8H4O4)4]2-.3C5H11NO (1) was investigated at T = 120 K using synchrotron radiation. 2C4H12N+[Zn3(C8H4O4)4]2-.3C5H11NO (2) was investigated at multiple temperatures (T = 30, 100, 200 and 300 K) on a conventional diffractometer. The thermal expansion of the structure of (2) is anisotropic and along the a axis, which corresponds to the metal chain direction. The structures contain anionic frameworks with cations and solvent molecules trapped in the voids. The magnetic susceptibility (chi) and heat capacity (C(p)) have been measured from 1.8 to 350 K. Compound (1) orders ferromagnetically with a broad phase transition observed in C(p) at approximately 6 K. The magnetic moment reaches a value of 3 micro(B) per Co at 2 K in a magnetic field of 9 T, and a Curie-Weiss fit to chi(T) gives an effective moment (mu(eff)) of 4.2 mu(B) and a Weiss temperature (theta) of 23 K. The exchange mechanism for the magnetic coupling is suggested to involve the Co-O-Co bridges in the individual three-metal-atom subchains. The three-dimensional magnetism presumably is due to super-exchange through two out of the three unique C8H4O4 linker molecules, which have the carboxylate and benzene pi systems well aligned.