SIR97 is the integration of two programs, SIR92 and CAOS, the ®rst devoted to the solution of crystal structures by direct methods, the second to re®nement via least-squares±Fourier procedures. Several new features have been introduced in SIR97 with respect to the previous version, SIR92: greater automatization, increased ef®ciency of the direct methods section, and a powerful graphics interface. The program also provides publication tables and CIF ®les.

The category Computer Program Abstracts provides a rapid means of communicating up-to-date infor Appl. CrysL (1985). 18,[189][190].J. Appl. CrysL (1994) The crystallographic problem: Our recent developments in direct-methods theory and in crystal-structure refinement persuaded us to develop a fully automated structure-determination program that is able to solve the phase problem and to refine atomic parameters in a single computer run. Method of solution:The method for automatic structure determination is based on the representation theory (Giacovazzo, 1977(Giacovazzo, , 1980. One-phase structure seminvariants and three-phase structure invariants are estimated via their second representation, two-phase seminvariants and four-phase invariants via their first representation. SIR92 is the heir of SIR88 (Burla, Camalli, Cascarano, Giacovazzo, Polidori, Spagna & Viterbo, 1989), and it retains most of the original features (Le. treatment of pseudotranslational symmetry, use of prior information, tangent weighting schemes etc.). In SIR88, one-and two-phase seminvariants and positive estimated three-phase invariants could be actively used in the phasing process. Negative estimated triplet and quartet invariants were used as a figure of merit (FOM) together with psi-zero triplets. In SIR92, negative estimated triplet and quartet invariants and psizero triplets are actively used in the © 1994 International Union of Crystallography Printed in Great Britain -all rights reserved phasing process and proved to be much more robust and efficient. The various trial solutions can be obtained either by magic-integer permutation (Main, 1978) or by random phases (Baggio, Woolfson, Declercq & Germain, 1978). The best trial solution, selected by powerful FOM's, is automatically processed through a cyclic procedure combining structure-factor calculation, least squares and 2Fo-Fc Fourier synthesis. The final outcome is a set of refined atomic parameters (x, y, z and isotropic B values) associated with suitable atomic species. The residual R value is calculated for user usefulness (final value usually between 0.08 and 0.15). The program runs in default mode when supplied with a minimum amount of information (space-group symbol, cell parameters, unit-cell chemical content and reflections) but documentation has been produced to allow the user to change default values. If a graphic device is available, the user can follow the structure solution and refinement on the screen. In the final stage, a menudriven interface is available in order to study molecular geometry and restart the refinement.Software environment: The program is written in standard Fortran77. A module written in C is supplied for interface with X-window or DEC-Window terminals for graphics. Besides a C compiler, an Xlib library is also needed. Two ASCII files are associated with the scattering program: the first contains coefficients for calculating scattering factors, the second, necessary for graphics, contains on-line help.Hardware environment: The program runs on Unix and...

An automatic procedure for recovering a complete crystal structure after a direct phasing process is described. The procedure consists mainly of a Fourier recycling method that can be implemented in any direct-methods package. The residual R value attained at the end of the process provides an estimate of the degree of success of the structure determination. The procedure can also be applied using a small molecular fragment as prior information. The procedure has been implemented into SIR92, the successor to SIR88. Symbols and abbreviationsm: number of symmetry operators of the point group. N: number of non-H atoms in the unit cell. t: number of non-H atoms in the asymmetric unit. NLAR: number of large normalized structure factors selected for active use in a direct procedure. NEXP: number of large normalized structure factors immediately following the NLAR reflections, to which phase expansion procedures can safely be applied.

As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures. Although activated carbons, zeolites, and metal-organic frameworks have been investigated extensively for CH4 storage, there are practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we use a reversible phase transition in a metal-organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp(2-) = 1,4-benzenedipyrazolate) are shown to undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp 'step'. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.

Colloidal nanocrystals (NCs) of APbX3-type lead halide perovskites [A = Cs+, CH3NH3+ (methylammonium or MA+) or CH(NH2)2+ (formamidinium or FA+); X = Cl–, Br–, I–] have recently emerged as highly versatile photonic sources for applications ranging from simple photoluminescence down-conversion (e.g., for display backlighting) to light-emitting diodes. From the perspective of spectral coverage, a formidable challenge facing the use of these materials is how to obtain stable emissions in the red and infrared spectral regions covered by the iodide-based compositions. So far, red-emissive CsPbI3 NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI3 exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI3 and FA-doped CsPbI3 NCs that are uniform in size (10–15 nm) and nearly cubic in shape and exhibit drastically higher robustness than their MA- or Cs-only cousins with similar sizes and morphologies. Detailed structural analysis indicated that the FAPbI3 NCs had a cubic crystal structure, while the FA0.1Cs0.9PbI3 NCs had a 3D orthorhombic structure that was isostructural to the structure of CsPbBr3 NCs. Bright photoluminescence (PL) with high quantum yield (QY > 70%) spanning red (690 nm, FA0.1Cs0.9PbI3 NCs) and near-infrared (near-IR, ca. 780 nm, FAPbI3 NCs) regions was sustained for several months or more in both the colloidal state and in films. The peak PL wavelengths can be fine-tuned by using postsynthetic cation- and anion-exchange reactions. Amplified spontaneous emissions with low thresholds of 28 and 7.5 μJ cm–2 were obtained from the films deposited from FA0.1Cs0.9PbI3 and FAPbI3 NCs, respectively. Furthermore, light-emitting diodes with a high external quantum efficiency of 2.3% were obtained by using FAPbI3 NCs.

We report here on a detailed study on PbS colloidal quantum dots. A characterization via X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) allowed us to reliably determine the diameter and the shape of the nanocrystals. These data, together with second-derivative analysis of the absorption spectra, allowed us to determine the size dependence of seven transitions in the absorption spectrum; some of these transitions were identified on the basis of their normalized confinement energy. The size dependence of the first excitonic transition was best modeled by a four-band envelope approach which considers the anisotropy of the band edges (Andreev, A. D.; Lipovskii, A. A. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 15402-15404). The extinction coefficients were calculated using concentrations obtained from inductively coupled plasma atomic emission spectrometry (ICP-AES), and their size dependence was found to follow a power law with exponent equal to approximately 2.5. In contrast with what was expected from the effective mass approximation, the per particle absorption cross section of the lowest transition was found to be strongly dependent on the particle size.

Bright green emitters with adjustable photoluminescence (PL) maxima in the range of 530–535 nm and full-width at half-maxima (fwhm) of <25 nm are particularly desirable for applications in television displays and related technologies. Toward this goal, we have developed a facile synthesis of highly monodisperse, cubic-shaped formamidinium lead bromide nanocrystals (FAPbBr3 NCs) with perovskite crystal structure, tunable PL in the range of 470–540 nm by adjusting the nanocrystal size (5–12 nm), high quantum yield (QY) of up to 85% and PL fwhm of <22 nm. High QYs are also retained in films of FAPbBr3 NCs. In addition, these films exhibit low thresholds of 14 ± 2 μJ cm–2 for amplified spontaneous emission.

Crystal defects in highy luminescent colloidal nanocrystals (NCs) of CsPbX3 perovskites (X = Cl, Br, I) are investigated. Here, using X-ray total scattering techniques and the Debye scattering equation (DSE), we provide evidence that the local structure of these NCs always exhibits orthorhombic tilting of PbX6 octahedra within locally ordered subdomains. These subdomains are hinged through a two-/three-dimensional (2D/3D) network of twin boundaries through which the coherent arrangement of the Pb ions throughout the whole NC is preserved. The density of these twin boundaries determines the size of the subdomains and results in an apparent higher-symmetry structure on average in the high-temperature modification. Dynamic cooperative rotations of PbX6 octahedra are likely at work at the twin boundaries, causing the rearrangement of the 2D or 3D network, particularly effective in the pseudocubic phases. An orthorhombic, 3D γ-phase, isostructural to that of CsPbBr3 is found here in as-synthesized CsPbI3 NCs.