New refinement of the crystal structure of 1,3-diaminopropane dihydrochloride

The structure of C₃H₁₀N₂·2HCl,M _r =147.05,a=8.567(3),b=9.341(2),c=9.444(9) Å,V=755.7 ų,Pbcn,d ₀=1.30,d _c =1.29 Mg m^–3,Z=4,F(000)=312e, which had been previously solved (Hirokawa etal., 1968), has been refined by full matrix to a finalR value of 0.034 as compared to 0.196 in the work cited above.     

An X-ray diffraction refinement of the crystal structure of natural apophyllite

The crystal structure of apophyllite (Andersberg — GDR), with a = 8.966(2), c = = 15.767(1) Å, P4/mnc, z = 2, Q = 2.35 gm · cm⁻³, has been refined (R = 0.035) by least squares with 976 reflections collected with a diffractometer. Apophyllite is a sheet structure with the bringing Si O bond lengths 1.6236 Å, while the non-bridging bond length Si O(3) is 1.5843 Å. The two independent Si O Si angles are 140.09, 140.76 degrees. The mean Si O bond length is 1.6138 Å. As it was not possible to locate unequivocally the H-atoms with the X-ray data, the interpretation of the of water in the structure was based on the charge balance approach of DONNAY and ALLMAN . Assuming that the fluorine ion is bonded to calcium ion (Ca-F = = 2.416 Å) and to H3 atom (F-H3 = 0.9145(1) Å) and water molecule hydrogen bonded to silicate framework. The average bond distances O-H are 0.962(2) Å and angle 105.24(1)°.     

X‐ray diffraction refinement of the crystal structure of anosovite prepared from leucoxene

The crystal structure of anosovite, (Ti_1.69Al_0.26Fe_0.05)³⁺(Ti_0.97Zr_0.03)⁴⁺O₅, prepared by carbothermal reduction of leucoxene under vacuum at 1450°C has been refined by Rietveld method using powder X‐ray diffraction data. It was found that it represents slightly monoclinically deformed pseudobrookite type structure with lattice parameters a = 9.8111 Å, b = 3.7509 Å, c = 9.9468 Å, β = 90.628°. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the crystal structure stability of calcite-type carbonates

The preparation of carbonates of the bivalent metals (Mg, Ca, Ba, Sr, Mn, Fe, Co, Ni, Cu and Zn) by two methods under ordinary conditions has been investigated. The stability of calcite type carbonates has been discussed. The shortest O—O distance (d*_O—O) between two neighbouring octahedra has been proposed as a criterion for the formation of stable calcite type carbonates. The lower stereochemical boundary of the calcite carbonate stability has been found to be d*_O−O ∼ 2.9 Å. The relative stability of carbonates with d_O−O* < 2.9 Å (Zn, Co and Ni) has been studied with respect to their isomorphous incorporation into MnCO₃.     

Crystal structure refinement of nitroguanidine

The crystal and molecular structure of nitroguanidine was refined using X-ray single-crystal diffraction methods. Previous structure determinations achieved with single crystal photographic or neutron powder techniques yielded values for the C—N formal double bond-length that either equaled or exceeded the formal C—N single-bond lengths. Bond lengths were more precisely and accurately determined here than in the previous studies, but the formal double-bond length still exceeds the formal single bond lengths. Comparisons of crystal bond lengths with ab initio derived bond lengths of an isolated nitroguanidine molecule and hydrogen-bonded clusters of nitroguanidine molecules suggest that observed bond-length abnormalities are caused by intermolecular forces in the crystal. The crystal is orthorhombic, with a = 17.6390(5), b = 24.8730(7), c = 3.5903(1) Å, V = 1575.19 ų, Z = 16, and D _calc = 1.736 g cm^–3.     

A modular model of the crystal structure of the pyrochlore-murataite polysomatic series

This paper reports on the results of a structural investigation of an important group of synthetic compounds with derivative structures of the mineral murataite, a potential matrix for immobilization of radioactive wastes. A model describing the structure of the entire group of compounds is proposed on the basis of analyzing the results obtained using chemical (electron microprobe) analysis methods, high-resolution transmission electron microscopy (high-resolution electron images and microdiffraction patterns), and X-ray diffractometry of a large array of artificial murataite ceramics. In essence, the proposed structural model is as follows: in the structure, layers of two (pyrochlore and murataite) packing types alternate along the [111] direction of the cubic supercell, so that the resulting structures can undergo substitutional and displacive modulations. Experiments on irradiation of different murataite modifications with heavy ions demonstrate that these modifications have close values of the radiation resistance, which is higher than that of the pyrochlore coexisting with them.     

Restoration of structure of thin multilayer films from reflectivity data by stepwise model refinement

The computational experiment on the restoration of the model parameters of the cross-section profile of the scattering density of thin multilayer films is described. It is shown that the use of the stepwise model of the structure, which is described by the thickness, density, absorption, and roughness parameters makes the problem of profile restoration multimodal and ill-conditioned. The analysis of the numerical experiments performed allowed us to propose a procedure of consecutive refinement of the initial-model parameters using the results of the local minimization of the discrepancy functional applied for the characteristic segments of the reflectivity curve. The methods proposed were used to restore the structure of a multilayer film deposited on a silicon substrate by the Langmuir-Blodgett method.     

Strontium in the collinsite structure: Rietveld refinement

The crystal structure of a strontium variety of a rare phosphate—mineral collinsite (Ca_{2 ? x}Sr_x)₂Mg[PO₄] · 2H₂O was solved from powder X-ray diffraction data (λ CuK_α radiation, Ni filter, 12.36° ≤ 2θ ≤ 100.00°, scan step 0.02°, exposure time per step 15 s) by the Rietveld method (R_wp = 4.15%, R_F = 1.03%, R_B = 2.46%); a = 5.8219(1) Å, b = 6.8319(2) Å, c = 5.4713(1) Å, α = 96.965(2)°, β = 108.846(2)°, γ = 107.211(2)°, sp. gr., \(\bar P1\), Z = 1, ρ_calcd = 3.12 g/cm³ (at x = 0.72). The new mineral was discovered in carbonatites from the Kovdor alkaline-ultrabasic massif. The crystallochemical data for collinsite were analyzed and compared with those for isotypic minerals of the fairfieldite group. Characteristic features of the low-temperature geochemistry of strontium were established.     

Principles and criteria for the choice and refinement of structural models in inorganic crystal chemistry

A hierarchical approach to the choice of the most probable structural models of a compound with a specified chemical composition and to the subsequent refinement of these models with the use of different methods is analyzed. Most attention is given to one of the first stages of the choice of the starting model (on the basis of the principles of total and local crystal electroneutrality) and to the solution of the problem of construction and determination of the bond graphs and connectivity matrices. Simple criteria for the most stable configurations of chemical bonds are introduced reasoning from the principles of minimum dissymmetrization and maximum informational entropy.     

An X-ray diffraction refinement of the structure of natural thomsonite

Thomsonite Na₂Ca₄Al₁₀Si₁₀O₄₀, orthorhombic Pncn, a = 1.3124 nm, b = 1.3078 nm, c = 0.662 nm, V = 1.1369 nm³, Z = 2, D_m = 2.30, D_c = 2.26, μ = 12.4 cm⁻¹. The average length of the bond T–O = 0.1685 nm. The final R-value for 456 independent observed reflections is 0.085.     

An X-ray diffraction refinement of the crystal structure of natural tetragonal analcime (NaAlSi2O6. H2O)

An X-ray diffraction analysis was performed on a single crystal of natural tetragonal analcime: (Na_0.99K_0.01)/(Al_1.05Si_2.02) O₆/0.99 H₂O, space group I 4₁/acd, a = 13.726(2) Å, c = 13.684(1) Å, V = 2578.1 ų, Z = 16, D_obs = 2.26. Refinement of 260 independent reflections yields R = 0.028 (R_w = 0.033). The mean Si(Al) O distance is 1.636 Å and tetrahedral angle is 109.35°. The framework of tetragonal analcime is topologically the same as the cubic modification.     

On the capability of revealing the pseudosymmetry of the chalcopyrite‐type crystal structure

The tetragonal crystal‐structure type of chalcopyrites (chemical formula A^IB^IIIC^VI₂) is a superstructure of sphalerite type. The c /a ratio differs generally from the ideal value 2, i.e., the crystal structure is pseudocubically distorted. For CuInSe₂ and CuGaSe₂ thin films, simulations demonstrate that it is theoretically possible to reveal the tetragonality in electron backscatter‐diffraction (EBSD) patterns for CuGaSe₂, whereas it may not be possible for CuInSe₂. EBSD experiments on CuGaSe₂ thin films using the ”Advanced Fit” band‐detection method show that it is possible to extract accurate misorientation‐angle distributions from the CuGaSe₂ thin film. Pole figures revealing the texture of the CuGaSe₂ thin film are shown, which agree well with X‐ray texture measurements from the same layer. (© 2008 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Refined crystal structure of kuzmenkoite

The structure of the mineral kuzmenkoite found in the Lovozero massif (the Kola Peninsula) was reinvestigated by the X-ray diffraction analysis. This mineral belongs to the labuntsovite family and differs from other members of this family by the absence of Na atoms, a high Mn content, and the new type of filling frame-work channels because of the replacement of Na atoms by H₂O molecules and H₃O groups. It was found that the disorder of atoms in the channels leads to lowering of the symmetry to Cm.     

Disordered crystal structure of chlorodifluoroacetamide

The crystal structure of chlorodifluoroacetamide has been determined from three-dimensional counter data, and refined by full-matrix least-squares techniques. The crystals belong to the monoclinic space groupP2₁/n witha = 7.401(6),b = 8.123(6),c = 8.431(6) Å, = 104.48(5) °, andD = 1.52 gcm^–3 forZ = 4. The finalR factor for 650 observed reflections is 0.059. The compound exists with a rotational disorder in the -CClF₂ group about the C-C bond. The occupancy factors for the major and minor conformations are 0.74 and 0.24, respectively.     

Principle of modular crystal structure

The principle of modular crystal structure has been formulated and the possibility of choosing a module of a specified structure type for its subsequent modular design is discussed. The modular characteristics of some structure types based on cubic close packing (fundamental and basic modules and the module that can be used to obtain a certain variety of modular structures related to the initial parent type) are described. Possible ways to obtain such a module and the algorithm for choosing it are considered. A comparative analysis of the theoretically derived module with the modules of experimentally determined modular structures is performed by the example of spinelloids (materials with spinel-like structure).     

The crystal structure of hexabenzoylhexaazaisowurtzitane

The crystal and molecular structure of an interesting cage compound is described. Crystal data: C₄₈H₃₆N₆O₆·(CH₃)₂CO; monoclinic; space group: P2 ₁ /n; a = 14.948(3) Å, b = 15.079(3) Å, c = 19.539(4)Å, = 93.93(3)°, V = 4394(2)ų; and Z = 4.     

The crystal structure of dodecyltrimethylammoniumbromide

C₁₅H₃₄BrN, monoclinic, P 2₁/m, a = 21.603(9), b = 7.261(3), c = 5.636(2) Å, β = 86.85(2)°, Z = 2, V = 882.72 ų, D_x = 1.160 g/cm³, μ(MoKα) = 2.45 cm⁻¹, λ(MoKα) = 0.71069 Å. The structure of this cationic surfactant was solved with Patterson methods and refined to R = 0.091 for 2057 measured reflections. The dodecyl chains are slightly screwed around their chain axes. The hydrocarbon chains are statistically disordered. The polar regions satisfy the mirror plane which is by remaining parts only statistically fulfilled. In the process of structure refinement statistical disorder has been included by separation of some atom positions. The sublattice of the hydrocarbon chains is nearly O⟂ with a_s = 5.14, b_s = 7.26, c_s = 2.52 Å.     

Refinement of the crystal structure of sanidine-like feldspar

The crystal structure of sanidine-like feldspar of the composition KAlSi₃O₈ from the Khibiny alkaline massif (the Kola Peninsula) has been refined (X-ray diffraction analysis; automated Syntex $P\bar 1$ diffractometer; 2θ: θ scanning technique; 2320 reflections; R _(hkl) = 0.0409; anisotropic refinement; AREN program package). The data obtained for KAlSi₃O₈ are: a = 8.615(9), b = 13.030(7), c = 7.200(5) Å, α = 89.99(5)°, β = 116.01(6)°, γ = 89.98(7)°, Z = 4, sp. gr. $C\bar 1$ . Microtwinning revealed in the crystal structure of the mineral explains the simultaneous existence of two structural-optical types in one sample—“high” and “low” sanidines.     

Refinement of the crystal structure of lithium-bearing uvite

The crystal structure of a natural calcium tourmaline, i.e., uvite with a high lithium content (0.51 au per formula (aupf) at the Y site, is refined to R = 0.019, R _w = 0.020, and S = 1.11. It is shown that, in nature, there exist uvites in which the charge balance in the case where the Z site is occupied by trivalent cations is provided by the replacement of part of the divalent magnesium cations at the Y site by univalent cations, divalent calcium cations at the X site by sodium cations, and univalent anions at the W site by oxygen anions. The W site is found to be split into two sites, namely, the W1 and W11 sites (the W1–W11 distance is 0.14 Å), which are partially occupied by the fluorine and oxygen anions, respectively. An analysis of the results obtained in this study and the data available in the literature on the crystal structure of uvites allows the conclusion that uvite can be considered a superspecies and that the nomenclature of this mineral group needs refinement with the use of structural data.