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One of the main phases formed at the beginning of the carbonation reaction of cementitious building materials is the calcium hemicarboaluminate (abbreviated as Hc). This AFm (shorthand for hydrated calcium aluminate phases structurally related to hydrocalumite) phase was synthesized, crystallized and then studied by synchrotron X-ray powder diffraction and micro-Raman spectroscopy. At room temperature and standard experimental conditions two major cementitious phases were detected, the Hc phase (as a major phase) and carbonated calcium hemicarboaluminate (abbreviated as cHc). By increasing the temperature the Hc form transforms into cHc. The crystal structures of these important AFm phases were successfully solved and refined in the space group of the trigonal crystal system. Hc has the unit-cell parameters a = 5.7757 (1) and c = 48.812 (2) Å, and cHc the unit-cell parameters a = 5.7534 (1) and c = 46.389 (1) Å. The two crystal structures are composed of positively charged main layers, [Ca4Al2(OH)12]2+, and negatively charged interlayers, [OH2 n(CO3)1 − n·4H2O]2−. The structure of the main layers is typical of the AFm family. Conversely, the interlayer region has a characteristic structure built up from water molecules and statistically distributed anions. In the interlayer, the Hc carbonate and hydroxyl anions are distributed in a 0.25:0.5 ratio, whereas the ratio of the anions in the cHc interlayers is 0.4:0.2. [ABSTRACT FROM AUTHOR]

The [4π + 4π] photodimerization process of the 9-substituted anthracene derivative crystalline 9-methylanthracene (9-MA) was investigated using time-resolved X-ray powder diffraction. The study was carried out in-situ using a CCD area detector. Sequential and parametric Rietveld refinement was applied for quantitative phase analysis. The results of traditional sequential Rietveld refinement showed that the evolution of the dimerization process can be described using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The parameters of the JMAK equation were obtained successfully by parametric Rietveld refinement and suggest that the reaction follows heterogeneous nucleation and one-dimensional growth with a decreasing nucleation rate. [ABSTRACT FROM AUTHOR]

2,4-Dioxo-1,3-diazetidine-1,3-bis(methyl- m-phenylene) diisocyanate (dimerized toluene-2,4-diisocyanate, TDI) is one of the most widely used aromatic diisocyanates in the polymer industry, and it crystallizes in at least two polymorphic forms (form A and form B) depending on reaction conditions. The crystal structures of the two forms were determined from high-resolution laboratory X-ray powder diffraction data using simulated annealing and Rietveld refinement. In spite of a marked structural similarity between them, significant discrepancies in the physical properties of the two forms prompted analysis of their partitioned energy terms in an effort to better our understanding of the driving force behind such differences in behaviour. [ABSTRACT FROM AUTHOR]

A quenchable new high-pressure-high-temperature modification of antimony(III) oxide, γ-Sb2O3, has been obtained at hydrostatic pressures of 9-11 GPa and temperatures of 573-773 K. Its crystal structure has been determined from high-resolution synchrotron powder diffraction data. γ-Sb2O3 consists of three-dimensionally cross-linked infinite chains of SbO3 E units ( E = lone pair) with the chains forming tetragonal rod-packing. The underlying topology of γ-Sb2O3 (3,3T8) is found very rarely in inorganic structures; it is realised only for the polyanion [Si4O4N6]10− that occurs in the Ce4(Si4O4N6)O structure type. The structural relation to the two previously known polymorphs of Sb2O3 at ambient pressure, valentinite and senarmontite is discussed. [ABSTRACT FROM AUTHOR]

The article presents a study on the structure and characteristics of calcium selenate subhydrate compound in bassanite structure. The study involved hydrothermal conversion in the preparation of the subhydrate and characterized with thermal analysis and Raman spectrocopy. It mentions that compund crystals are metastable at ambient conditions and transform in air into dihydrate. It also notes that hyrdate water content is higher than expected according to the stoichiometry of bassanite.

In a systematic approach, the ability of the Maximum Entropy Method (MEM) to reconstruct the most probable electron density of highly disordered crystal structures from X-ray powder diffraction data was evaluated. As a case study, the ambient temperature crystal structures of disordered α-Rb2[C2O4] and α-Rb2[CO3] and ordered δ-K2[C2O4] were investigated in detail with the aim of revealing the `true' nature of the apparent disorder. Different combinations of F (based on phased structure factors) and G constraints (based on structure-factor amplitudes) from different sources were applied in MEM calculations. In particular, a new combination of the MEM with the recently developed charge-flipping algorithm with histogram matching for powder diffraction data (pCF) was successfully introduced to avoid the inevitable bias of the phases of the structure-factor amplitudes by the Rietveld model. Completely ab initio electron-density distributions have been obtained with the MEM applied to a combination of structure-factor amplitudes from Le Bail fits with phases derived from pCF. All features of the crystal structures, in particular the disorder of the oxalate and carbonate anions, and the displacements of the cations, are clearly obtained. This approach bears the potential of a fast method of electron-density determination, even for highly disordered materials. All the MEM maps obtained in this work were compared with the MEM map derived from the best Rietveld refined model. In general, the phased observed structure factors obtained from Rietveld refinement (applying F and G constraints) were found to give the closest description of the experimental data and thus lead to the most accurate image of the actual disorder. [ABSTRACT FROM AUTHOR]

For SiBr4 no crystal structures have been reported yet. In this work the crystal structures of SiBr4 were predicted by global lattice-energy minimizations using force-field methods. Over an energy range of 5 kJ mol-1 above the global minimum ten possible structures were found. Two of these structures were experimentally determined from X-ray synchrotron powder diffraction data: The low-temperature β phase crystallizes in P21/c, the high-temperature a phase in Pa3¯. Temperature- dependant X-ray powder diffraction shows that the phase transition occurs at 168 K. [ABSTRACT FROM AUTHOR]

The crystal structure with the idealized formula Mg3(OH)5Cl·4H2O, the so-called F5 phase according to 5Mg(OH)2·MgCl2·8H2O in the system MgCl2–MgO–H2O, has been solved ab initio from high-quality laboratory powder diffraction data at room temperature. The F5 phase is structurally related to 3Mg(OH)2·MgCl2·8H2O (F3 form). The F5 phase consists of infinite triple chains with one Mg(OH)6 and two Mg(OH)4(OH2)2 octahedra as building units intercalated by chlorides, which are partly substituted by disordered hydroxides in the real structure. The F5 phase is of technological importance as the most important binder phase in Sorel cements. Knowledge of the crystal structure enables the full quantitative phase analysis of magnesia cements for the first time, which turns out to be very helpful in the search for possible causes of broken or bleached magnesia floors. Two real-life examples are given. [ABSTRACT FROM AUTHOR]

High-quality in situ synchrotron powder diffraction data have been used to investigate the decomposition products of bischofite in the temperature range 298 ≤ T≤ 873 K. At least eight phases could be identified: MgCl2· nH2O ( n = 1, 2, 4 and 6), MgOHCl· nH2O (0 ≤ n≤ 1.0), MgCl2 and MgO. The crystal structures of three magnesium chloride hydrates MgCl2· nH2O ( n = 1, 2, 4) were determined ab initio, replacing published Rietveld refinements from low-quality powder diffraction data based on similarity criteria. MgCl2·4H2O was found to be disordered and has been correctly determined for the first time. The crystal structures of bischofite and MgCl2·4H2O consist of discrete Mg(H2O)6 and MgCl2(H2O)4 octahedra, respectively. The crystal structure of MgCl2·2H2O is formed by single chains of edge-sharing MgCl2(H2O)4 octahedra, while in the case of MgCl2·H2O double chains of edge-sharing MgCl(H2O)5 octahedra are found. The phases in the system MgCl2–H2O are intermediates in the technologically important process of MgO and subsequently Mg production. The same phases were recently found to be of key importance in the understanding of cracks in certain magnesia concrete floors. [ABSTRACT FROM AUTHOR]

The violet pigment methylbenzimidazolonodioxazine, C22H12Cl2N6O4 (systematic name: 6,14-dichloro-3,11dimethyl-l,3,9,11-tetrahydro-5,13-dioxa-7,15-diazadiimidazo[4,5-b:4',5'-m]pentacene-2,10-dione), shows an X-ray powder diagram consisting of only ca 12 broad peaks. Indexing was not possible. The structure was solved by global lattice energy minimizations. The program CRYSCA [Schmidt & Kalkhof (1999), CRYSCA. Clariant GmbH, Pigments Research, Frankfurt am Main, Germany] was used to predict the possible crystal structures in different space groups. By comparing simulated and experimental powder diagrams, the correct structure was identified among the predicted structures. Owing to the low quality of the experimental powder diagram the Rietveld refinements gave no distinctive results and it was difficult to prove the correctness of the crystal structure. Finally, the structure was confirmed to be correct by refining the crystal structure of an isostructural mixed crystal having a better X-ray powder diagram. The compound crystallizes in P&macr1;, Z = 1. The crystal structure consists of a very dense packing of molecules, which are connected by hydrogen bridges of the type N-H…O=C. This packing explains the observed insolubility. The work shows that crystal structures of molecular compounds may be solved by lattice energy minimization from diffraction data of limited quality, even when indexing is not possible. [ABSTRACT FROM AUTHOR]

The crystal structures of tert-butyl-tris(trimethylsilyl)silane, Si[C(CH[sub 3])[sub 3]][sub l][Si(CH[sub 3])[sub 3]][sub 3] (Bu1), and di-tert-butyl-bis(trimethylsilyl)silane, Si[C(CH[sub 3])[sub 3]][sub 2][Si(CH[sub 3])[sub 3]][sub 2] (Bu2), at room temperature and at 105 K have been determined by X-ray powder diffraction; the high-pressure behavior for pressures between 0 and 5 GPa is reported. The room-temperature structures have cubic Fm&3macr;m symmetry (Z=4) with a = 13.2645 (2) Å, V = 2333.87 (4) ų for Bul and a = 12.9673 (1) Å, V = 2180.46 (3) ų for Bu2. The molecules are arranged in a cubic close packing (c.c.p.) and exhibit at least 48-fold orientational disorder. Upon cooling both compounds undergo a first-order phase transition at tempera- tures T[sub c] = 230 (5) K (Bu1) and T[sub c] = 250 (5) K (Bu2) into monoclinic structures with space group P2[sub 1]/n. The structures at 105 K have a = 17.317 (1), b = 15.598 (1), c = 16.385 (1) Å, γ = 109.477(4)°, V= 4172.7 (8) ų and Z=8 for Bu1 and a = 17.0089 (9), b = 15.3159 (8), c = 15.9325 (8) Å, γ = 110.343 (3)°, V= 3891.7 (5) ų and Z = 8 for Bu2. The severe disorder of the room-temperature phase is significantly decreased and only a two- or threefold rotational disorder of the molecules remains at 105 K. First-order phase transitions have been observed at pressures of 0.13-0.28 GPa for Bul and 0.20-0.24 GPa for Bu2. The high-pressure structures are isostructural to the low-temperature structures. The pressure dependencies of the unit-cell volumes were fitted with Vinet equations of state and the bulk moduli were obtained. At still higher pressures further anomalies in the pressure dependen- cies of the lattice parameters were observed. These anomalies are explained as additional disorder-order phase transitions. [ABSTRACT FROM AUTHOR]

The compounds tetrakis(trimethylstannyl )germane, Ge(SnMe3)4 (1), and tetrakis(trimethylstannyl)silane, Si(SnMe3)4 (2), have crystal structures with the quasispherical rnolecules in a closed-packed stacking. At room temperature both structures have the space group P&macr1; (Z = 2) with a = 9.94457 (5), b = 14.52927 (8). c = 916021 (5) Å, α = 90.53390 (30), β = 11.L73080 (30), γ = 90.0049 (4)°, and V = 1229.414 (12) ų for (1) and a = 9.92009 (7), b = 14.51029 (11), c = 9.13585 (7) Å, α = 90.4769 (4), β = 111.6724 (4), γ = 89..9877 (6)°, and V = 1222.037 (16) ų for (2). The molecules are found to be ordered as a result of steric interactions between neighboring molecules, as shown by analyzing the distances between the atoms. Upon heating, both compounds undergo a first-order phase transition at temperatures Tc = 348 ± 5 K, as characterized by a relative jump of the lattice parameter of ∼16%. At 353 K, both structures have the space group P&macr1;(Z = 4). with a = 14.2037 (2) Å, and V = 2865.52 (7) &ARing;³ for (1) and a = 14.1346 (2) &ARing;, and V = 2823.90 (7) ų for (2). Rietveld refinements were performed for the low-temperature phases measured at T = 295 K [Rwp = 0.0844 for (1). Rwp = 0.0940 for (2)] and for the high- temperature phases measured at T = 353 K [Rwp = 0.0891 for (I), Rwp = 0.0542 for (2)]. The combination of high-resolution X-ray powder diffraction measurements and variable-temperature magic-angle-spinning 13C, 29Si and 119Sn NMR experiments demonstrates low crystallographic and molecular (C1) symmetries for the low-temperature phases of (1) and (2) at temperatures T < 348 ± 5 K and high crystaftiographic symmetry due to rotational disorder for the high-temperature phases at temperatures T < 348 ± 5 K. [ABSTRACT FROM AUTHOR]

Reports on the determination of the crystal structure of the organometallic compound, pentamethylcyclopentadienylsodium, from high-resolution X-ray powder diffraction. Crystallization of the compound in space group Cmcm; Formation of polymeric multidecker chains along the a axis.

Reports on the crystal structures of the chalcogenide triniobium tetratelluride (Nb[sub 3]Te[sub 4] and In[sub x]Nb[sub 3]Te[sub 4]) for a series of pressures between 0 and 40 GPa. Crystallization of both groups in the space group P6[sub 3]; Measurement of high-pressure X-ray powder diffraction using a diamond anvil cell and synchrotron radiation.

The pressure dependence of the crystal structure of cubic tetrakis(trirnethylsilvl)methane C[Si(CH3)3]4 (TC) (P < 16.0 GPa, T = 298 K) is reported using high-resolution angle-dispersive X-ray powder diffraction. The compound has crystal structures with the molecules in a cubic-close- packed (c.c.p.) arrangement. It shows three phase transitions in the measured pressure range. At ambient conditions, TC has space group Pm&macr3;m (Z = 4) with a = 12.8902 (2) Å. V = 2141.8 (1) ų (phase I). Between 0 and 0.13 GPa TC exhibits a first-order phase transition into a structure with space group Pa&macr3; (phase II).. A second first-order phase transition occurs between 0.2 and 028 GPa into a structure with space group P213 (phase III). Under non-hydrostatic pressure conditions (P > 10 GPa) a transformation is observed into a c.c.p. structure that is different from the face-centred-cubic structure at ambient conditions. A non-linear compression behaviour is observed, which could be described by a Vinet relation in the range 0.28-4.8 GPa. The extrapolated bulk modulus of the high-pressure phase III was determined to be K0 = 7.1 (8) GPa. The crystal structures in phase III are refined against X-ray powder data measured at several pressures between 0.49 and 4.8 GPa, and the molecules are found to be fully ordered. This is interpreted to result from steric interactions between neighbouring molecules, as shown by analysing the pressure dependence of intramolecular angles, torsion angles and intermolecular distances. Except for their cell dimensions, phases I. II and III are found to be isostructural to the corresponding phases at low temperatures. [ABSTRACT FROM AUTHOR]

Reports that the compounds tetrakis(trimethylsilyl)methane (TC) and tetrakis(trimethylsilyl)silane (TSi) have crystal structures with the molecules in a cubic closed-packed stacking. Space group at room temperature of both structures; Description of an X-ray scattering data by a molecule with approximately sixfold orientational disorder, ruling out structure with free rotating molecules.

Examines the crystal structure of pentamethylcyclopentadienyl-lithium, from a high-resolution powder pattern by modeling and the maximum entropy method. Crystallization of the compound in space group R3m; Finding that the pentamethylcyclopentadienyl anions are coplanar with each other and show threefold rotational disorder.