High-resolution X-ray Powder Research Articles

Structural variations across the nepheline (NaAlSiO4)–kalsilite (KAlSiO4) series

Abstract The crystal structures of 19 samples from the nepheline (NaAlSiO4; Ne)–kalsilite (KAlSiO4; Ks) series, previously prepared via ion-exchange, were examined using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Parent materials for the three series include a natural Monte Somma nepheline (series-1), synthetic Na nepheline (series-2), and high-Si synthetic nepheline (series-3), having excess Si mole percentages of 5.2, 1.7, and 12.5%, respectively. Three different structure-types were found to occur among the samples examined: nepheline (P63), tetrakalsilite (P63), and kalsilite (both P63 and P31c intergrowth). Trikalsilite was not observed in this study. Vacancies (☐) at the K site as well as Ca and K atoms at the Na1 site play an important role in the crystal-chemical behavior of nepheline solid solutions. Vacancies cause an elongation in the average <K-O>[9] distance in nepheline. When K atoms enter the Na1 site in nepheline, the average <(Na,K)-O>[7] distance increases linearly and is parallel to the average <(Na,K)-O>[9] distance in kalsilite and the grand mean of such distances in trikalsilite and tetrakalsilite. Before K atoms enter the Na1 site, the average <(Na,K)-O>[7] distance is constant because of the full occupancy of the Na1 site with Na atoms. Ca atoms at the Na1 site in the Monte Somma sample-1 cause a contraction in the <(Na,K)-O>[7] distance. In Na-rich nepheline samples, Na atoms in the large channels occupy a Na(K) site that is off the 63 axis and close to the usual K site. In natural nepheline samples, the K site in most cases contains K atoms and ☐, and the Na1 site is filled mainly with Na, minor Ca, and K atoms in K-rich samples. Nepheline from Monte Somma (sample-1) contains weak satellite reflections that are also present in some other kalsilite samples. Average <T-O> distances indicate a high degree of Al-Si disorder in nepheline but increasing Al-Si order in tetrakalsilite and kalsilite. Increasing the amount of K atoms beyond the ideal composition of K0.25Na0.75[AlSiO4] causes expansion in multiple structural parameters because of the larger size of K+ relative to Na+.

Controlling structural and magnetic properties of SrFe12O19 nanoplatelets by synthesis route and calcination time

Nanocrystalline platelets of Sr hexaferrite, SrFe12O19, were prepared by four techniques (two hydrothermal and two sol–gel techniques) and calcined at 1000 °C for 1, 2, 4, 8 and 16 h. The microstructure of these samples was analyzed using Rietveld refinements of high-resolution synchrotron powder x-ray diffraction data, and the obtained results were correlated with the magnetic properties obtained from vibrating sample magnetometer measurements. The calcination treatment causes the crystallites to preferentially grow along the c-axis, leading to more isotropic crystallites. Moreover, the microstructural changes induced by calcination alter the magnetic properties, yielding a higher saturation magnetization in all samples. The attained coercivity is correlated with the crystallite size along the width of the platelets. Despite the pronounced changes of the microstructure and the magnetic properties after calcination, the calcination duration has a minor effect on the properties, i.e. in most cases the steady state is obtained after 1 h. The starting material has a profound impact on the microstructural change during calcination, despite the high calcination temperature.

Bis-3,5-Diamino-1,2,4-Triazolyl-1,2,4,5-Tetrazine: From Insensitive High Energy Density Materials to Small Molecule Organic Semiconductors

Besides the well-known pyrotechnic applications of insensitive high energy compounds such as s-tetrazine, they are promising materials for relevant applications (e.g., electrochemical energy storage, photocatalysis). Bis-3,5-diamino-1,2,4-triazolyl-1,2,4,5-tetrazine (BDTT) is a nitrogen-rich conjugated heterocyclic compound (nitrogen ≈ 71 wt %), inert to ignition and mechanical stimuli and thermally stable up to 370 °C. The solvent-free crystalline structure of this high energy density compound (named IEF-12 for IMDEA energy framework) is reported here for the first time using the high resolution powder X-ray diffraction (PXRD) and 1H, 13C, and 15N solid state nuclear magnetic resonance (NMR) spectroscopy. We demonstrate that IEF-12 has features of a small molecule organic semiconductor, namely, lamellar π-stacked crystal packing in the solid state, a low energy optical bandgap Egopt = 2.27 eV, and a relatively high value of dielectric constant εr = 4.86. Overall, we demonstrated on IEF-12 that insensitive high energy density materials (HEDM) may open an avenue for nonclassically structured organic semiconductors, paving the way for optoelectronic and photovoltaic applications.

Facile synthesis of Pd@graphene nanocomposites with enhanced catalytic activity towards Suzuki coupling reaction

A facile and chemical specific method to synthesize highly reduced graphene oxide (HRG) and Pd (HRG@Pd) nanocomposite is presented. The HRG surfaces are tailored with amine groups using 1-aminopyrene (1-AP) as functionalizing molecules. The aromatic rings of 1-AP sit on the basal planes of HRG through π–π interactions, leaving amino groups outwards (similar like self-assembled monolayer on 2D substrates). The amino groups provide the chemically specific binding sites to the Pd nucleation which subsequently grow into nanoparticles. HRG@Pd nanocomposite demonstrated both uniform distribution of Pd nanoparticles on HRG surface as well as excellent physical stability and dispersibility. The surface functionalization was confirmed using, ultraviolet–visible (UV–Vis), Fourier transform infra-red and Raman spectroscopy. The size and distribution of Pd nanoparticles on the HRG and crystallinity were confirmed using high-resolution transmission electron microscopy and powder X-ray diffraction and X-ray photoelectron spectroscopy. The catalytic efficiency of highly reduced graphene oxide-pyrene-palladium nanocomposite (HRG-Py-Pd) is tested towards the Suzuki coupling reactions of various aryl halides. The kinetics of the catalytic reaction (Suzuki coupling) using HRG-Py-Pd nanocomposite was monitored using gas chromatography (GC).

Variation of cation distribution with temperature and its consequences on thermal expansion for Ca3Eu2(BO3)4.

The structure of calcium europium orthoborate, Ca3Eu2(BO3)4, was determined using high-resolution powder X-ray diffraction data collected at the ID22 beamline (ESRF) under ambient conditions, as well as at high temperature. Rietveld refinement allowed determination of the lattice constants and structural details, including the Ca/Eu ratios at the three cationic sites and their evolution with temperature. Clear thermal expansion anisotropy was found, and slope changes of lattice-constant dependencies on temperature were observed at 923 K. Above this temperature the changes in occupation of the Ca/Eu sites occur, exhibiting a tendency towards a more uniform Eu distribution over the three Ca/Eu sites. Possible structural origins of the observed thermal expansion anisotropy are discussed.

NaMoO2: a Layered Oxide with Molybdenum Clusters

NaMoO2 was synthesized as a layered oxide from the reaction between the layered oxide Na2/3MoO2 and metal sodium. Its structure was determined from high-resolution powder X-ray diffraction, and it can be described as an α-NaFeO2 distorted structure in which sodium ions and molybdenum atoms occupy octahedral interstitial sites. Chains of "diamond-like" clusters of molybdenum were evidenced in the [MoO2] layers resulting from the Peierls distortion expected in a two-dimensional triangular lattice formed by transition metal atoms with a d3 electronic configuration. Molybdenum-molybdenum distances as short as 2.58 Å were found in these clusters. The magnetic moment recorded at low temperatures and at room temperature showed that NaMoO2 presents a very low magnetic susceptibility compatible with the localization of the 4d electrons in the Mo-Mo bonds. This localization was confirmed by DFT calculation that showed the NaMoO2 was diamagnetic at 0 K. A sodium battery was built using NaMoO2 as the positive electrode material, and we found that sodium ions can be reversibly deintercalated and intercalated in NaMoO2, indicating that this compound is one of the many phases existing in the NaxMoO2 system.

III-V Clathrate Semiconductors with Outstanding Hole Mobility: Cs8In27Sb19 and A8Ga27Sb19 (A = Cs, Rb).

Three novel unconventional clathrates with unprecedented III-V semiconducting frameworks have been synthesized: Cs8In27Sb19, Cs8Ga27Sb19, and Rb8Ga27Sb19. These clathrates represent the first examples of tetrel-free clathrates that are completely composed of main group elements. All title compounds crystallize in an ordered superstructure of clathrate-I in the Ia3̅ space group (No. 206; Z = 8). In the clathrate framework, a full ordering of {Ga or In} and Sb is observed by a combination of high-resolution synchrotron single-crystal and powder X-ray diffraction techniques. Density functional theory (DFT) calculations show that all three clathrates are energetically stable with relaxed lattice constants matching the experimental data. Due to the complexity of the crystal structure composed of heavy elements, the reported clathrates exhibit ultralow thermal conductivities of less than 1 W·m-1·K-1 at room temperature. All compounds are predicted and experimentally confirmed to be narrow-bandgap p-type semiconductors with high Seebeck thermopower values, up to 250 μV·K-1 at 300 K for Cs8In27Sb19. The latter compound shows carrier concentrations and mobilities, 1.42 × 1015 cm-3 and 880 cm2 ·V-1·s-1, which are on par with the values for parent binary InSb, one of the best electronic semiconductors. The high hole carrier mobility is uncommon for complex bulk materials and a highly desirable trait, opening ways to design semiconducting materials based on tunable III-V clathrates.

Synthesis of ipomoea staphylina plant extract mediated silver nanoparticles

Metallic nanoparticles are being consumed in every phase of science along with engineering including medical fields and are still charming the scientists to explore new dimensions for their respective worth which is generally attributed to their corresponding small sizes. The upcoming researchers have proven their significance of nonlinear optical properties. Among the several noble metal nanoparticles, silver nanoparticles have gotten a special attention. Traditionally silver nanoparticles are produced by chemical method using chemicals as reducing agents then it’s become accountable for various biological risks due to their general toxicity. In this manner, to fathom the goal; natural methodologies are coming up to fill the void; for example green amalgamations utilizing organic atoms got from plant sources as concentrates showing prevalence over compound as well as natural techniques These plant based biological molecules undergo highly controlled assembly for making them suitable for the metal nanoparticle syntheses. The current audit investigates the colossal plant decent variety to be used towards quick and single step convention preliminary technique with green standards over the regular ones and depicts the good stability properties. The Ipomoea staphylina Plant mediated silver nanoparticles is characterized using UV–Visible spectroscopy (UV–Vis.), Fourier transform infra-red spectroscopy (FTIR), High Resolution Transmission Electron Microscope (HRTEM) and Powder X-ray diffraction (XRD). The average particle size of synthesized silver nanoparticles is found by using Scherrer’s formula as 20 nm, which is approximately similar as the size obtained in HRTEM Analysis 22 nm.

Exploring new hydrated delta type vanadium oxides for lithium intercalation.

Three hydrated double layered vanadium oxides, namely Na0.35V2O5·0.8(H2O), K0.36(H3O)0.15V2O5 and (NH4)0.37V2O5·0.15(H2O), were obtained by using mild hydrothermal conditions. Their delta type structural frameworks were solved by high-resolution synchrotron X-ray powder diffraction and the interlayer spacings were interpreted from difference Fourier maps. The inter-slab distances are modulated by the water content and the special arrangements of the alkali and ammonium cations. The XPS measurements denote mixed valence systems with high contents of V4+ ions up to 40%. The monitoring of the V4+ EPR signal over time suggests a reduction of the electronic delocalization on account of the partial oxidation to V5+. The electrochemical performance of the active phases is strongly conditioned by the vacuum-drying process of the electrodes, showing better capacity retention when vacuum is not applied. In situ X-ray diffraction shows a structural mechanism of contraction/expansion of the bilayers upon lithium insertion/extraction where the alkali ions behave as structural stabilizers. Galvanostatic cycling at very low current density implies migration of the alkali "pillars" triggering the collapse of the structure.

Ce(OH)2Cl and lanthanide-substituted variants as precursors to redox-active CeO2 materials.

The cerium(iii) hydroxide chloride Ce(OH)2Cl crystallises directly as a polycrystalline powder from a solution of CeCl3·7H2O in poly(ethylene) glycol (Mn = 400) heated at 240 °C and is found to be isostructural with La(OH)2Cl, as determined from high-resolution synchrotron powder X-ray diffraction (P21/m, a = 6.2868(2) Å, b = 3.94950(3) Å, c = 6.8740(3) Å, β = 113.5120(5)°). Replacement of a proportion of the cerium chloride in synthesis by a second lanthanide chloride yields a set of materials Ce1-xLnx(OH)2Cl for Ln = La, Pr, Gd, Tb. For La the maximum value of x is 0.2, with an isotropic expansion of the unit cell, but for the other lanthanides a wider composition range is possible, and the lattice parameters show an isotropic contraction with increasing x. Thermal decomposition of the hydroxide chlorides at 700 °C yields mixed-oxides Ce1-xLnxO2-δ that all have cubic fluorite structures with either expanded (Ln = La, Gd) or contracted (Ln = Pr, Tb) unit cells compared to CeO2. Scanning electron microscopy shows a shape memory effect in crystal morphology upon decomposition, with clusters of anisotropic sub-micron crystallites being seen in the precursor and oxide products. The Pr- and Tb-substituted oxides contain the substituent in a mixture of +3 and +4 oxidation states, as seen by X-ray absorption near edge structure spectroscopy at the lanthanide LIII edges. The mixed oxide materials are examined using temperature programmed reduction in 10%H2 in N2, which reveals redox properties suitable for heterogeneous catalysis, with the Pr-substituted materials showing the greatest reducibility at lower temperature.

Gas hydrate formation by allyl alcohol and CH4: Spectroscopic and thermodynamic analysis

We discovered a new structure II (sII) hydrate forming agent, allyl alcohol (AA), in the presence of methane (CH4) for the first time, and characterized the crystal structure, guest distribution, and phase equilibria of the (AA+CH4) hydrate. Using solid-state 13C NMR and Raman spectroscopy, the crystal structure of the (AA+CH4) hydrate was confirmed to be a sII hydrate, and the CH4 molecule was found to be encapsulated in both the large and small cages of the sII hydrate. In addition, AA was found to be included in the large cages of the sII hydrate in the Gauche-Gauche form based on the measured- and calculated-NMR spectra. Notably, we investigated the free OH signal of AA in the Raman spectra to determine whether hydrogen bonding occurred between host and guest molecules; however, we could not determine whether the existence of the free OH signal was consistent with this host-guest interaction. To clearly identify the crystal structure and possible host-guest interactions, a high-resolution powder X-ray diffraction (HRPD) pattern of our (AA+CH4) hydrate sample was analyzed using Rietveld analysis with the direct space method. The crystal structure of the (AA+CH4) hydrate was assigned as the cubic Fd3m structure with a lattice constant of 17.1455 A. In particular, the shortest distance between the AA molecule in the hydrate cages and an oxygen atom in the host water was estimated to be 2.55 A; thus, we concluded that the hydroxyl group of the AA molecule was hydrogen-bonded to the host water framework. Finally, we measured the phase equilibrium conditions of the binary (AA+CH4) hydrate and found that the equilibrium pressure conditions of the binary (AA+CH4) hydrate were slightly higher than those of the pure CH4 hydrate.

3D Electron Diffraction Unravels the New Zeolite ECNU-23 from the "Pure" Powder Sample of ECNU-21.

A novel crystalline high-silica zeolite with 12×8-membered ring (R) channel system is prepared with the aid of the 3D electron diffraction (3D ED) technique. A crystal with the same topology as one of the predicted daughter structures of CIT-13 germanosilicate, named ECNU-23 (East China Normal University 23) was coincidentally detected by the 3D ED investigation during the structure characterization of the "pure" powder sample of existing one-dimension (1D) 10-R ECNU-21. By controlling the alkaline-assisted hydrolysis under moderate conditions, we purified the phase of ECNU-23 by selectively breaking and removing the chemically weak Ge(Si)-O-Ge and metastable Si-O-Si bonds. Its structure was determined based on the 3D ED data, and confirmed by high-resolution TEM images and powder X-ray diffraction (PXRD) data. The aluminosilicate Al-ECNU-23 shows unique catalytic properties in the isomerization/ disproportionation of m-xylene as solid-acid catalyst.

Metal-organic frameworks-derived 3D yolk shell-like structure Ni@carbon as a recyclable catalyst for Cr(VI) reduction

Metal-organic framework (MOFs)-derived Ni@carbon composites with three-dimensional (3D) yolk shell-like structure have been successfully synthesized by a self-template strategy. After continuous carbonization treatment, the metallic Ni was uniformly embedded in the carbon matrix that could effectively protect Ni from corrosion and subsequent leaching. Carbon layers in Ni@carbon composites mainly existed in the form of graphene structure according to high-resolution transmission electron microscopy and powder X-ray diffraction pattern. As-prepared materials inherited superior properties of Ni-MOFs, such as large specific surface area, layered 3D yolk shell-like structure and abundant Ni content. As a consequence, among all synthetic materials, the Ni@carbon450 exhibited the most excellent catalytic activity towards Cr(VI) reduction in the presence of HCOOH. Impressively, highly toxic Cr(VI) in aqueous solution can be completely reduced to hypotoxic Cr(III) by Ni@carbon450 within 30 min. The catalytic efficiency was much higher than that of other Ni-based and some precious metal catalysts. In addition, the reusability experiment showed that the Ni@carbon450 had good stability and high catalytic activity after 10 cycles of utilization. Ni also gave Ni@carbon catalyst ferromagnetism, and composites obtained were easily separated under the external magnetic field. The catalytic reduction mechanism of Ni@carbon450 for Cr(VI) was also described and discussed in detail. More importantly, this study is expected to provide a new idea for the synthesis of non-noble metal catalysts from MOFs by self-template carbonization.

Understanding the structural changes in lithiated graphite through high-resolution operando powder X-ray diffraction

To develop more efficient and safer batteries, a deeper understanding of lithium ion intercalation and de-intercalation dynamics upon operation in lithium-ion batteries is of great importance. We have performed operando high-resolution powder X-ray diffraction (PXRD) studies of the intercalation and de-intercalation process in a graphite electrode material using custom-made capillary-based lithium-ion battery cells. Using high-resolution PXRD, it was possible to resolve the diffraction peaks from a number of lithiated graphite phases occurring during intercalation/de-intercalation of lithium and obtain information about the transformation processes, both related to the staging process and the in-plane transformation. In the staging related to the intercalation of lithium, two-phase and solid-solution behavior is identified. Similar phase behavior is observed when examining the in-plane parameters. The mechanism of intercalation is proposed to involve charge transfer between the lithium ion and the π orbitals of the graphene layer. Broadening of the hk0 peaks may be related to non-uniform reduction of the graphene layers depending on the staging number and the graphene layer neighboring environment.

Crystal Chemistry of Birefringent Uvarovite Solid Solutions

The crystal chemistry of five optically anisotropic uvarovite samples from different localities (California, Finland, Russia, and Switzerland) were studied with electron-probe microanalysis (EPMA) and the Rietveld method. Monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data were used, and Rietveld refinement was carried out with the cubic space group, I a 3 ¯ d . The general formula for garnet is [8]X3[6]Y2[4]Z3[4]O12. Uvarovite has the ideal formula, Ca3Cr2Si3O12, which may be written as Ca3{Cr,Al,Fe}Σ2[Si3O12] because of solid solutions. HRPXRD traces show multiple cubic garnet phases in each sample that has a heterogeneous chemical composition. The optical and back-scattered electron (BSE) images and elemental maps contain lamellar and concentric zoning as well as patchy intergrowths. With increasing a unit-cell parameter for uvarovite solid solutions, the Z–O distance remains constant, and the average <X–O> distance increases slightly in response to the Cr3+ ⇔ Al3+ cation substitution in the Y site. The Y–O distance increases most because Cr3+ (radius = 0.615 Å) is larger than Al3+ (radius = 0.545 Å) cations. The Fe3+ (radius = 0.645 Å) cation is also involved in this substitution. Structural mismatch between the cubic garnet phases in the samples gives rise to strain-induced optical anisotropy.

Structural Trends and Solid-Solutions Based on the Crystal Chemistry of Two Hausmannite (Mn3O4) Samples from the Kalahari Manganese Field

The crystal chemistry of two hausmannite samples from the Kalahari manganese field (KMF), South Africa, was studied using electron-probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD) for sample-a, and high-resolution powder X-ray diffraction (HRPXRD) for sample-b, and a synthetic Mn3O4 (97% purity) sample-c as a reference point. Hausmannite samples from the KMF were reported to be either magnetic or non-magnetic with a general formula AB2O4. The EPMA composition for sample-a is [Mn2+0.88Mg2+0.11Fe2+0.01]Σ1.00Mn3+2.00O4 compared to Mn2+Mn3+2O4 obtained by refinement. The single-crystal structure refinement in the tetragonal space group I41/amd gave R1 = 0.0215 for 669 independently observed reflections. The unit-cell parameters are a = b = 5.7556(6), c = 9.443(1) Å, and V = 312.80(7) Å3. The Jahn–Teller elongated Mn3+O6 octahedron of the M site consists of M–O × 4 = 1.9272(5), M–O × 2 = 2.2843(7), and an average <M–O>[6] = 2.0462(2) Å, whereas the Mn2+O4 tetrahedron of the T site has T–O × 4 = 2.0367(8) Å. The site occupancy factors (sof) are M(sof) = 1.0 Mn (fixed, thereafter) and T(sof) = 1.0008(2) Mn. The EPMA composition for sample-b is [Mn0.99Mg0.01](Mn1.52Fe0.48)O4. The Rietveld refinement gave R (F2) = 0.0368. The unit-cell parameters are a = b = 5.78144(1), c = 9.38346(3) Å, and V = 313.642(1) Å3. The octahedron has M–O × 4 = 1.9364(3), M–O × 2 = 2.2595(6), and average <M–O>[6] = 2.0441(2) Å, whereas T–O × 4 = 2.0438(5) Å. The refinement gave T(sof) = 0.820(9) Mn2+ + 0.180(9) Fe2+ and M(sof) = 0.940(5) Mn3+ + 0.060(5) Fe3+. Samples-a and -b are normal spinels with different amounts of substitutions at the M and T sites. The Jahn–Teller elongation, Δ(M–O), is smaller in sample-b because atom substitutions relieve strain compared to pure Mn3O4.

Study of Kinetics of Solid Phase Transition in Tetracosane С24Н50 by High-Resolution Synchrotron X-Ray Powder Diffraction

Using the small-angle synchrotron X-ray diffraction method at the research facility “BELOK” of the Kurchatov synchrotron radiation source at the NRC Kurchatov Institute, it was shown that the first-order solid phase transition in tetracosane C24H50 passes according to a heterogeneous mechanism in a narrow temperature range ΔT ~ 1 K in accordance with the theory of diffuse phase transitions.

A new organic nonlinear optical material: L-asparagine cetrimonium bromide single crystal for photonic applications

Abstract The new organic nonlinear optical material, L-Asparagine Cetrimonium Bromide (LACB) was synthesized by using slow evaporation method at ambient conduction. The crystallographic data was analyzed by single crystal X-ray diffraction (XRD) and crystalline perfection was studied under high resolution powder X-ray diffraction technique. The functional groups were found out by FT-IR spectroscopy and various optical parameters were performed by using UV–Vis NIR spectroscopy. In fluorescence spectrum the sharp emission peak was observed at 529 nm it reveals the green emission. The second harmonic generation efficiency was determined by the Kurtz powder method using Nd-YAG laser. The laser induced damage threshold value was found to be higher than most of the reported organic single crystals. The dielectric permittivity and dielectric loss were analyzed as a function of frequency. The photoconductivity study implies that the grown crystal has positive photoconductive nature. Different mechanical properties were analyzed by Vickers microhardness test.

Fluorescent Zr(IV) Metal-Organic Frameworks Based on an Excited-State Intramolecular Proton Transfer-Type Ligand.

We report here the preparation of a series of Zr(IV) metal-organic frameworks (MOFs) of the MIL-140 structure type incorporating a ligand exhibiting an intense excited-state intramolecular proton transfer (ESIPT) fluorescence. These solids were obtained by systematically varying the substitution rate of 4,4'-biphenyldicarboxylate by 2,2'-bipyridine-3,3'-diol-5,5'-dicarboxylate, and they were thoroughly characterized by complementary techniques, including high-resolution powder X-ray diffraction, solid-state NMR spectroscopy, nitrogen sorption experiments, and time-resolved fluorescence. We show that the incorporation of the ESIPT-type ligand induces an increase of the hydrophilicity, leading ultimately to a higher sensitivity toward hydrolysis, a phenomenon rarely observed in this structure type, which is considered as one of the most stable among the Zr carboxylate MOFs. Eventually, optimization of the amount of fluorescent ligand within the structure allowed combining a decent microposity ( SBET > 750 m2·g-1) and a high stability even in boiling water, together with a high fluorescence quantum yield (>30%).

Reconciling Crystallographic and Physical Property Measurements on Thermoelectric Lead Sulfide.

For many decades the lead chalcogenides PbTe, PbSe, and PbS (and their solid solutions) have been preferred high-performance thermoelectric materials due to their exceptional electronic and thermal properties as well as great stability during operation. However, there is a lack of understanding about the fundamental relation between the reported high-defect crystal structure containing cation disorder and vacancies and the observed transport properties, which follow expectations for an ideal rock salt crystal structure. Here we have studied a series of undoped lead sulfide samples (Pb1- xS) with presumed small chemical variations. Crystallographic refinements of high-resolution synchrotron powder X-ray diffraction data give unphysically low lead occupancies (0.75-0.98), in contradiction with the measured charge carrier concentration, resistivity, mobility, and Seebeck coefficient, which show no signs of lead vacancies. A new Rietveld refinement model including preferred orientation parameters and anisotropic strain gives almost full lead occupancy and improved agreement factors. However, transmission electron microscopy analysis reveals that there is no preferred orientation in this system. Instead it is the diffuse scattering due to directional correlated disorder in the structure that necessitates the additional parameters when modeling Bragg intensities. The present approach is a general method for absorbing effects of direction-dependent correlations in advanced materials.