Low-Temperature Crystal Structure of S-camphor Solved from Powder Synchrotron X-ray Diffraction Data by Simulated Annealing

The ordered, low-temperature crystal structure of the pure enantiomer of camphor (C₁₀H₁₆O) has been solved from high-resolution powder synchrotron X-ray diffraction data. The structure is orthorhombic, space group P2₁2₁2₁, Z=8, with a=8.9277(2) Å, b=27.0359(5) Å, and c=7.3814(1) Å at 100 K. The structure was solved by autoindexing of the pattern, space group determination, and then optimization of the positions and orientations of the two independent molecules in the unit cell by simulated annealing. The molecular structure obtained from the restrained Rietveld refinement shows reasonable agreement with that optimized from ab initio molecular orbital calculations. In the crystal structure, the molecules are aligned antiferroelectrically and weak C-H…O hydrogen bonds link together the independent molecules.     

Rietveld Refinement of Tetragonal V-ZrO2 Solid Solutions Obtained from Gels by X-ray Powder Diffraction

The crystal structure of three tetragonal V_xZr_1−xO₂ solid solutions, with x=0.025, 0.05, and 0.075, prepared by heating dried gel precursors at 450°C in air atmosphere, have been determined by Rietveld refinement on the basis of powder X-ray powder diffractometer data. They contain V⁴⁺ cations surrounded by eight oxygens, four at a distance between 2.079 and 2.093 Å and another four at longer distances between 2.369 and 2.348 Å. The estimation of the crystal average oxygen position from the X-ray lattice parameter of V_xZr_1−xO₂ conform with the relationship proposed by Howard et al. (J. Am. Ceram. Soc. 81, 241 (1998)).     

粉末衍射技术解析青蒿素晶体结构方法学比对研究

以粉末X射线衍射技术(PXRD)表征有机物晶体结构为目的,选取我国第1个全新药物青蒿素(Artemisinin)验证粉晶解析有机物晶体结构方法的合理性。粉晶解析结果为正交晶系,P212121空间群,a=23.98223±0.01624,b=9.42480±0.00645,c=6.34589±0.00439,α=β=γ=90°,Z=4,V=1434.693;单晶解析结果为正交晶系,P212121空间群,a=23.9564(9),b=9.3224(5),c=6.3205(3),α=β=γ=90°,P212121,Z=4,V=1411.55(17)3;两者所确定分子非氢结构键长、键角、二面角的相关系数分别为0.9921、0.9833和0.9997,晶胞参数基本吻合,分子构型相似。结果表明,粉晶X射线衍射技术可以求得较为准确的青蒿素晶胞参数及晶胞内分子构型。     

New Approaches for Solving Crystal Structures from Powder Diffraction Data

The determination of crystal structures from single crystal diffraction data can generally be carried out routinely and straightforwardly. However, many crystalline solids can be obtained only as microcrystalline powders and are not suitable for investigation by conventional single crystal diffraction methods. In the past, this problem has limited the ability to elucidate the structural properties of such materials. For the wide range of materials in this category, there is clearly a pressing need to develop and exploit techniques that allow crystal structures to be solved from powder diffraction data. Although traditional techniques for structure solution from powder diffraction data have been applied successfully in several cases, these techniques have certain intrinsic limitations, and for the case of organic molecular crystals the challenges that must be overcome are particularly severe. For these reasons, our recent research has focused on the development and implementation of new methodologies for structure solution from powder diffraction data, leading to new “direct-space” techniques for structure solution in which a hypersurface based on the profile R-factor is searched using Monte Carlo or Genetic Algorithm techniques. This paper presents a brief overview of the problems and challenges associated with structure solution from powder diffraction data. The foundations of the techniques that we have developed are described, and illustrative examples (from the field of organic molecular crystals) are given to highlight the application of these techniques.     

Absolute Configuration of Pharmaceutical Research Compounds Determined by X‐ray Powder Diffraction

The absolute configuration of active pharmaceutical ingredients (APIs) was determined by generating salts of the active pharmaceutical ingredient (API) with counterions of known chirality, and determining the crystal structures by X‐ray powder diffraction. This approach avoids the (often tedious) growth of single crystals, and is successful with very limited quantities of material (less than 1 mg). The feasibility of the method is demonstrated on five examples, and its limitations are discussed as well.     

Statistical Inference of Distribution of Crystal Size by X-Ray Powder Diffraction

We describe a method to calculate the distribution of sizes of fine crystals from pure powder-diffraction profile using a method of maximum entropy (MAXENT). We apply a Monte-Carlo technique of simulated annealing to seek a global minimum of the error surface in fitting this diffraction profile. We consider pure diffraction profile (instrument de-convoluted) of a powder specimen without lattice imperfection to a significant extent. Under these circumstances, the distribution of the pure diffraction profile can be attributed to the distribution of crystallite size. We applied this method to three cases of crystal sizes having a highly inhomogeneous distribution with certain noise-tolerance. The results agree well with synthetic data of diffraction.     

Synthesis and Crystal Structure of SnPd3D0.138(7) by Neutron Powder Diffraction

Palladium-rich intermetallic compounds have attracted attention for their use in heterogeneous catalysis and their interesting hydrogenation properties. SnPd₃ was synthesized from the elements using selenium as a mineralizing agent. The stoichiometric composition was confirmed by energy-dispersive X-ray spectra (Sn_0.98(5)Pd_3.02(5)). X-ray diffraction showed an ordered AuCu₃ type structure [Pm3 m, a = 397.799(1) pm]. SnPd₃ takes up hydrogen at 703 K under 5.0 MPa, resulting in a volume expansion of 0.4 %. The crystal structure was determined from neutron powder diffraction data of the deuteride. SnPd₃D_0.138(7) belongs to the cubic anti-perovskite type [Pm3 m, a = 398.338(11) pm]. Deuterium occupies [Pd₆] octahedral sites [d(Pd–D) = 199.169(6) pm] in a statistical manner.     

Evolving Opportunities in Structure Solution from Powder Diffraction Data—Crystal Structure Determination of a Molecular System with Twelve Variable Torsion Angles

The genetic algorithm approach , in which a population of trial structures is allowed to evolve subject to well-defined procedures for mating, mutation, and natural selection, was employed to solve the complex molecular crystal structure of Ph₂P(O)(CH₂)₇P(O)Ph₂ directly from powder diffraction data. The structure solution reveals an interesting (perhaps unexpected) molecular conformation (see picture), which emphasizes the importance of allowing complete conformational flexibility of the molecule in the structure solution calculation.     

Crystal Structure of Cesium Phenylacetylide,CsC2C6H5, Solved and Refined from Synchrotron Powder Diffraction Data

The crystal structure of cesium phenylacetylide, CsC₂C₆H₅, was solved and refined from synchrotron powder diffraction data (Pbca, Z = 8). Each Cs⁺ cation is coordinated by five ligands: four acetylide groups coordinate side‐on and one end‐on. A similar arrangement is found in the crystal structure of NaC₂H (P4/nmm, Z = 2). There is a group‐subgroup relationship between both structures. Most importantly, the crystal structure of CsC₂C₆H₅ could only be solved with the help of synchrotron data, as the very good peak:noise ratio allowed the assignment of several very weak reflections, which finally led to the correct space group, in which a structural solution was possible using direct space methods.     

Crystal Structure of Calcium-Deficient Carbonated Hydroxyapatite. Thermal Decomposition

Full Rietveld refinement of the crystal structure of the synthetic calcium-deficient carbonated apatite Ca1_3.40[Ca2_5.90 (NH₄)_0.10][(PO₄)_4.95(CO₃)_1.05(H₂O)_0.30][(OH)_1.65(H₂O)_0.45] (space group P6₃/m; a=9.437(1), c=6.888(1) Å; Z=1; R_wp=5.23%) was carried out using X-ray powder diffraction data. The use of the model with the split position of O3 atom made it possible to find two orientations of CO₃ triangles sharing one of their edges. They occupy randomly the adjacent faces of a PO₄ tetrahedron that are parallel to the c axis. O3c atoms coordinating carbon atoms are shifted by 0.37 Å from O3p atoms belonging to PO₄ tetrahedra. The charge unbalance occurring when [CO₃]²⁻ ions replace [PO₄]³⁻ groups is primarily compensated by vacancies in Ca1 sites. The studies of the sample thermal decomposition performed by simultaneous thermal analysis and by X-ray diffraction helped to analyze the localization and the amount of lattice water that enhanced the reliability of the structural model.     

X-ray powder diffraction study of monoclinic V-ZrO2 solid solutions obtained from gels

Rietveld refinement of six monoclinic V_xZr_1−xO₂ solid solutions, with x=0, 0.01, 0.02, 0.05, 0.075 and 0.1, prepared by heating dried gel precursors at 1300°C in air atmosphere, has been characterized using X-ray powder diffractometer data. The present results confirm that crystal structure of these solid solutions contain V⁴⁺(Zr⁴⁺) cations surrounded by seven oxygens, four at a distance between 2.13 and 2.28 Å (referred as to O(2) in the tetrahedrally coordinated oxygens) and other three at a distance between 2.03 and 2.20 Å (denoted as O(1) in the triangularly coordinated oxygens). The trends in the lattice parameter variation of V_xZr_1−xO₂ solid solutions specimens with the nominal vanadium amount are in accordance with previous results obtained by experiments measured using an internal standard.     

Crystal Structures,Dimorphism and Lithium Mobility of Li7MO6 (M = Bi,Ru, Os)

Li₇MO₆ (M = Bi, Ru, Os) have been synthesized by solid state reaction of Li₂O with Bi₂O₃, or MO₂ (M = Ru, Os) and characterized using powder X‐ray diffraction, differential scanning calorimetry, magnetic susceptibility (for M = Ru, Os), ionic conductivity and ⁶Li solid state NMR (for M = Bi) measurements. All three compounds exhibit a temperature induced triclinic – rhombohedral phase transition. Structures of the new low temperature triclinic phases have been refined by the Rietveld method from powder X‐ray data using atomic parameters of Li₇TaO₆ as a starting model ( Li₇BiO₆ : triclinic, , a = 5.5071(1), b = 6.0425(1), c = 5.5231(1) Å, α = 116.912(1), β = 120.867(1), γ = 62.234(1)°, V = 133.96(1) ų, Z = 1, T = 230 K; Li₇RuO₆ : triclinic, , a = 5.3654(1), b = 5.8584(1), c = 5.3496(1) Å, α = 117.182(1), β = 119.117(1), γ = 62.632(1)°, V = 124.43(1) ų, Z = 1, T = 295 K; Li₇OsO₆ : triclinic, , a = 5.3786(1), b = 5.8725(1), c = 5.3591(1) Å, α = 117.193(1), β = 119.277(1), γ = 62.700(1)°, V = 125.15(1) ų, Z = 1, T = 295 K). Upon cooling, Li₇RuO₆ and Li₇OsO₆ undergo a magnetic transition at 12 and 13 K, respectively, from the paramagnetic to the antiferromagnetic state. The higher ionic conductivity of Li₇BiO₆ at T < 300 °C, as compared to Li₇RuO₆ and Li₇OsO₆, can be ascribed to the undergoing of the triclinic – rhombohedral transition at a much lower temperature. At T > 300 °C, the ionic conductivity of all three compounds increases sharply due to the melting of the lithium sublattice; for Li₇RuO₆ and Li₇OsO₆ the latter effect is superimposed by the phase transitions to the rhombohedral modifications.     

The Crystal Structure of Trimethyl Borate by Neutron and X‐ray Powder Diffraction

The crystal structure of one of the simplest organoboron compounds, trimethyl borate does not appear to have been determined hitherto. The compound is of interest for the study of π‐donor ligands and their interaction with the π‐acceptor behavior of trigonal boron and the consequences of such interactions on molecular structure. We used powder neutron (with isotopically labeled material) and X‐ray diffraction to determine the crystal structure of trimethyl borate at 15 K and 200 K (neutron) and 200 K (X‐ray). The material is hexagonal (Z = 2) with a = b = 6.950(8) Å and c = 6.501(3) Å at 15 K. The unit cell volume is 272.00(1) ų. The space group is P6₃/m (SG 176) at 15 K and 200 K. This is the first crystal structure solved on the Neutron Powder Diffractometer (NPDF) at the Lujan Center.