Interlayer for matching crystal lattices in the domain walls of ferroelastic lead orthophosphate

The possible existence of an interlayer with the symmetry of the paraphase in the W and W′ domain walls of lead orthophosphate has been checked from the condition of matching the crystal lattices of adjacent domains. It is shown that two types of interlayers are possible, which possess different symmetries, strain tensors, matrices of the rotation of the coordinate system, thicknesses, and elastic energies. It is shown that the domain-wall energy is proportional to the squared spontaneous deformation.     

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.     

Refinement of the crystal structures of three fluorine-bearing elbaites

The crystal structures of three Li-Al natural tourmalines (elbaites) containing 0.88–1.39 wt % F are refined to R= 0.0294, 0.0308, and 0.0417. It is revealed that the W threefold anion site is split into two sites, namely, the W1 threefold site and W2 ninefold site (W1–W2 ～ 0.4 Å, Y-W₁ ≥ 1.94 Å, Y-W2 ≥ 1.75 Å). The following hypothesis is proposed and justified: the W1 and W2 sites are partially occupied by OH groups and fluorine anions, respectively. The ratio of the [YO₄(OH)₂] octahedra to the [YO₄(OH)F] octahedra depends on the fluorine content and varies from structure to structure. The fact that the W site is more than 50% occupied by fluorine in the structures of two tourmalines under investigation allows the conclusion that fluor-elbaite with the ideal formula Na(Li_1.5A1_1.5)A1₆(Si₆O₁₈)(BO₃)₃(OH)₃F is a new mineral species and that elbaite can be considered a superspecies.     

Domain structure in a nematic layer under oscillatory Couette flow

The formation of a spatially modulated structure (domains) in a homeotropically aligned layer of a nematic liquid crystal (NLC) under an oscillatory Couette flow are described theoretically by using the hydrodynamic equations for an NLC which are linearized with respect to perturbations. The shear frequencies at which the phenomenon is determined by the inertial properties of the medium are considered. It is shown that the domain size is equal to the thickness h of an NLC layer and that the dependence of the threshold value of the shear amplitude u _th on the frequency ω and the layer thickness h is described by the expression u _th = const/(ωh²). The calculated results are compared with the experimental data.     

Application of X-ray diffraction methods in the study of micrometer-sized porous Si layers

An X-ray analysis of porous silicon layers (Sb-doped n ⁺-Si(111)) obtained by anodic oxidation for different times with a current of 50 mA/cm² is performed by the methods of double-crystal rocking curves and total external reflection. A nondestructive method for monitoring the stationary process of the formation of micrometer-sized porous silicon layers and estimating their porosity and thickness is proposed. The parameters obtained for porous silicon layers with a thickness of ～6 μm are confirmed by the joint processing of diffraction curves for the 111 and 333 reflections on the basis of the developed model of dynamic scattering from layers while taking into account the strain profiles Δd(z)/d, the static Debye-Waller factor f(z), and the porosity P(z). The advantages and drawbacks of the proposed method are discussed.     

Crystal structure and microtwinning of monoclinic La<Subscript>3</Subscript>SbZn<Subscript>3</Subscript>Ge<Subscript>2</Subscript>O<Subscript>14</Subscript> crystals of the langasite family

The crystal structure of monoclinic La₃SbZn₃Ge₂O₁₄ crystals from the langasite family is determined by X-ray diffraction analysis [a = 5.202(1) Å, b = 8.312(1) Å, c = 14.394(2) Å, β = 90.02(1)°, sp. gr. A2, Z = 2, and R/R _w = (5.2/4.6)%]. The structure is a derivative of the Ca₃Ga₂Ge₄O₁₄-type structure (a = 8.069 Å, c = 4.967 Å, sp. gr. P321, Z = 1). The crystal studied is a polysynthetic twin with the twin index n = 2, whose monoclinic components are related by pseudomerohedry by a threefold rotation axis of the supergroup P321.     

Conditions for single-crystal formation during bridgman growth

The microstructure of IV–VI crystals grown by varying the shape of the conical bottom of a ampoule, the melt superheating ΔT⁺ with respect to the liquidus temperature of a blend, the axial temperature gradient in a furnace ΔT, and the pulling rate of a ampoule v were studied. It was found that the nucleation of a single-crystal seed is most likely under the following conditions: ΔT ≤ 20 K/cm, ΔT⁺ < 30 K, and v < 0.4 mm/h. The conical bottom of a ampoule should have walls of constant thickness. The principle of geometric selection of a single-crystal seed is not efficient. The obtained results are discussed with regard to data on the structure and behavior of IV–VI melts upon heating and cooling.     

Molecular and crystal structures of mesomorphic aromatic esters: II. The structure of <Emphasis Type="Italic">p</Emphasis>-ethoxyphenyl <Emphasis Type="Italic">p</Emphasis>′-pentylbenzoate

p-Ethoxyphenyl p′-pentylbenzoate (I), which exhibits a mesophase (K 62.6 N 63.3 I), was studied by X-ray diffraction analysis at 120.0 and 296.0 K. In the molecule I, one of the benzene rings is almost coplanar with the carboxy group COO, whereas another benzene ring is twisted with respect to the carboxy group by 65.9°. The geometric parameters of the molecule I are indicative of possible conjugation between the carboxy group and the former benzene ring and the absence of conjugation between this group and the second benzene ring. In the crystal packing, extended molecules I are oriented along the bc diagonal. The mutual orientation of the nearest benzene rings of any two adjacent molecules is T-shaped, which corresponds to a weak C-H...π-type interaction.     

Flow of space-charge-limited currents in cubic boron nitride

Current-voltage characteristics of polycrystal cubic nitride samples are studied at various temperatures. The samples were undoped crystals obtained by the method of direct phase transformation of pyrolytic graphite-like boron nitride and crystals doped with sulfur to obtain n-type conductivity. The current-voltage characteristics consist of three different parts: linear and quadratic segments and the region of dramatic increase in the current, which indicates the existence of space-charge-limited currents. The activation energy, E_t, and concentration of the trapping centers, N_t, are determined. The experimental data obtained allowed us to calculate, for the first time, the mobility and concentration of free carrier in the conduction band. Possible mechanisms giving rise to a dramatic increase in are made the current on the current-voltage characteristics are discussed.     

Conditions for stabilization of the ordered state of cations in complex oxides with perovskite structure

Analytical expressions for the coefficients in the Landau potential for six types of ordering of complex oxides with perovskite structure in terms of the sums of ordering energies in different coordination spheres are obtained in the approximation of effective pair interactions. On the basis of the pair interaction potentials of the form v(r) = ?1/r⁶ + Acos(qr)/(qr)³, the conditions for q and R are established, which lead to stabilization of the ordered phases.     

<Emphasis Type="Italic">P</Emphasis>-wave ray velocities and the inverse acoustic problem for anisotropic media

The specific features of the calculation of ray velocities of quasi-longitudinal waves in anisotropic media have been considered. A technique for calculating elastic constants using P-wave ray velocities measured in an ultrasonic experiment on spherical samples is presented. It is shown by an example of tabular data that elastic constants С₁₁, С₂₂, and С₃₃ and combinations of constants (С₁₂ + 2С₆₆), (С₁₃ + 2С₅₅), (С₂₃ + 2С₄₄), (С₁₄ + 2С₅₆), (С₂₅ + 2С₄₆), and (С₃₆ + 2С₄₅) can be calculated most accurately for the general case of anisotropic media with elastic properties of arbitrary symmetry. Since the determining system of equations is illconditioned, the values of elastic constants entering these combinations depend on the choosed initial approximation.     

Synthesis and characterization of 9-(4-nitrophenyl)3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-1,8(2H,5H) acridinedione and its methoxyphenyl derivative

9-(4-Nitrophenyl)-3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-1,8(2H,5H) acridinedione (NTHA) crystallizes in orthorhombic space group P2₁2₁2₁ with a = 5.9716(1) Å, b = 18.0476(3) Å, c = 19.2445(2) Å, V = 2074.04(5) ų, Z = 4, D_cal = 1.263 Mg m^?3 and R = 0.0521 (wR = 0.1326) for 4078 observed reflections. 9-(4-Nitrophenyl)-10-(4-methoxyphenyl)-3,3,6,6-tetramethyl-3,4,6,7,9,10-hexahydro-1,8(2H,5H) acridinedione NMTHA, crystallizes in monoclinic space group P2₁/c with a = 15.669(5) Å, b = 10.652(4) Å, c = 18.337(6) Å, β = 108.25(1)^°, V = 2906.66(2) ų, Z = 4, D_cal = 1.245 Mg m^?3 and R = 0.0725 (wR = 0.1847) for 5105 observed reflections. The experimental values are compared with the theoretical values calculated based on the semiemperical methods. The structures are stabilized by N–H?sO and C–H?sO types of intermolecular interactions in addition to van der Waals forces.     

X-ray crystallography of cholest-3,5-diene-7-one

The molecular structure of cholest-3,5-diene-7-one (C₂₇H₄₂O) is determined by X-ray diffraction. The compound crystallizes in the orthorhombic crystal system (space group P2₁2₁2₁) with unit cell parameters a = 11.281(5) Å, b = 11.350(5) Å, c= 18.518(5) Å, and Z = 4. The structure is solved by direct methods and refined to an R-value of 0.054 for 1070 observed reflections [F_o > 4σ(F_o)]. Ring A adopts a distorted half-chair conformation, ring B exists in sofa conformation, ring C acquires a chair conformation, and the five-membered ring D occurs in distorted half-chair conformation. The crystal structure is stabilized by van der Waals forces.     

New data on vlasovite: Refinement of the crystal structure and the radiation damage of the crystal during the x-ray diffraction experiment

The crystal structure of vlasovite obtained from the Kipawa alkaline complex in Quebec is refined to R _F = 0.053 for 1515 unique reflections with |F| > 4σ(F). The parameters of the monoclinic unit are as follows: a = 11.063(8) Å, b = 10.15(1) Å, c = 8.60(1) Å, β = 100.3(1)°, space group C2/c, and Z = 4. The X-ray diffraction, electron microprobe, and IR spectroscopic data indicate that, under X-ray radiation, the specimen suffers radiation damage, which is accompanied by a partial removal of Na atoms, incorporation of H₂O molecules into the structure, and, possibly, a change in the configuration of the (Si,O) framework.     

Domain structure and properties of triglycine sulfate crystals with profile <Emphasis Type="Italic">D</Emphasis>,<Emphasis Type="Italic">L</Emphasis>-α- and <Emphasis Type="Italic">L</Emphasis>-α-alanine-doped layers

A complex investigation of the domain structure and dielectric properties of triglycine sulfate (TGS) crystals containing profile layers doped with D,L-α-alanine (DLATGS) and L-α-alanine (LATGS) impurities is carried out. The images of the DLATGS and LATGS layers and ferroelectric domains are obtained by piezoelectric force microscopy; the parameters of the domain structure and the degree of unipolarity are determined. It is established that DLATGS layers are multidomain and LATGS stripes are mainly single-domain. The experimental data on the macroscopic dielectric properties of the crystals are compared with the results of a microscopic analysis of the domain structure.     

X-ray mapping in heterocyclic design: XII. X-ray diffraction study of 2-pyridones containing cycloalkane fragments annelated to the C(5)-C(6) bond

The structures of 4,6-dimethyl-1H-pyridin-2-one [a = 6.125(2) Å, b = 15.153(4) Å, and c = 14.477(4) Å, Z = 8, space group Pbca], the 2: 1: 1 complex of 4-methyl-1,5,6,7-tetrahydro-2H-cyclopenta[b]pyridin-2-one with phosphoric acid and methanol [a = 11.181(2) Å, b = 14.059(6) Å, c = 13.593(4) Å, β = 97.78(2)°, Z = 8, space group P2₁/n], 4-methyl-1,5,6,7,8,9-hexahydro-2H-cyclohepta[b]pyridin-2-one [a = 12.565(6) Å, b = 5.836(6) Å, c = 13.007(3) Å, β = 93.10(3)°, Z = 4, space group P2₁/n], and 4-methyl-5,6,7,8,9,10-hexahydrocycloocta[b]pyridin-2(1H)-one [a = 12.955(3) Å, b = 6.1595(15) Å, c = 13.038(3) Å, β = 95.50(2)°, Z = 4, space group P2₁/n] are determined by single-crystal X-ray diffraction. The structures are solved by direct methods and refined by the full-matrix least-squares procedure in the anisotropic approximation to R = 0.0755, 0.0644, 0.0754, and 0.0569, respectively. The structures of 4-methyl-1,5,6,7-tetrahydro-2H-cyclopenta[b]pyridin-2-one [a = 7.353(4) Å, b = 8.176(4) Å, c = 13.00(1) Å, β = 105.64(2)°, Z = 4, space group P2₁/c] and 2-oxo-1,2,5,6,7,8-hexahydroquinoline-3-carbonitrile [a = 5.9870(2) Å, b = 16.5280(5) Å, c = 9.6540(3) Å, β = 111.52(4)°, Z = 4, space group P2₁/c] are studied by the powder diffraction technique. The structures are solved using the grid search procedure and refined by the Rietveld method to R _wp = 0.108 and 0.058, R_exp = 0.032 and 0.027, R _p = 0.076 and 0.043, R _b = 0.123 and 0.077, and χ² = 7.9 and 3.64, respectively. In all the structures, hydrogen bonds involving the N, H, and O atoms of the pyridone fragment are formed.     

Synthesis and Crystal Structure Studies of a Novel Bioactive Heterocycle: 1-Benzhydryl-4-phenylmethane Sulfonyl Piperazine

1-Benzhydryl-4-phenylmethane sulfonyl piperazine was synthesized from 1-benzhydryl piperazine with phenylmethane sulfonyl chloride. The product obtained was characterized by ¹H NMR, MS and IR techniques and finally confirmed by X-ray crystallography. The title compound C₂₄H₂₆N₂O₂S, M _r   =   406.53, crystallizes in the orthorhombic crystal class in the space group Pbca with unit cell parameters a  =  11.1240(10)Å, b  =   9.4940(15)Å, c  =  40.239(4)Å, Z  =  8 and V = 4249.7(9)ų. The structure was solved by direct methods and refined to R ₁=0.0561 for 2,445 reflections with [I > 2 σ(I)]. The piperazine ring adopts a chair conformation. The sulfonyl moiety is in a distorted tetrahedral configuration.     

Disaccharide nucleosides: The crystal and molecular structure of 2′-<Emphasis Type="Italic">O</Emphasis>-β-<Emphasis Type="Italic">D</Emphasis>-ribopyranosylcytidine

The crystal structure of a disaccharide nucleoside, 2′-O-β-D-ribopyranosylcytidine, is studied using X-ray diffraction (space group P2₁, a = 6.827(2) Å, b = 12.813(3) Å, c = 9.532(2) Å, β = 92.934(5)°, V = 832.7(4) ų, Z = 2). The stereochemical features of the molecular structure of 2′-O-β-D-ribopyranosylcytidine are analyzed, and the structural data are compared with those obtained for the previously studied disaccharide nucleoside 2′-O-β-D-ribofuranosyluridine.     

X-ray mapping in heterocyclic design: 16. X-ray diffraction study of 1-(4-chlorophenacyl)-4-methyl-1,5,6,7-tetrahydro-2<Emphasis Type="Italic">H</Emphasis>-cyclopenta[<Emphasis Type="Italic">b</Emphasis>]pyridin-2-one

The structure of 1-(4-chlorophenacyl)-4-methyl-1,5,6,7-tetrahydro-2H-cyclopenta[b]pyridin-2-one is studied using single-crystal X-ray diffraction. The structure (a = 37.006(8) Å, b = 8.967(3) Å, c = 27.911(3) Å, β = 96.52(2)°, Z = 24, space group P2₁/c) is solved by direct methods and refined to R₁ = 0.0608 and wR₂ = 0.1170. Six crystallographically independent molecules differ in the dihedral angle between the phenyl and heterocycle planes. The formation of Cl?Cl aggregates is discussed.     

6<Emphasis Type="Italic">S</Emphasis>,8α-Dihydroxy-14,15-Dihydrocarapin (Khayanone) from the Stem Bark of <Emphasis Type="Italic">Khaya Senegalensis</Emphasis> (Meliaceae): Isolation and its Crystal Structure

Khayanone was isolated from the stem bark of African mahogany, Khaya senegalensis (Meliaceae), and characterized as 6S,8α-dihydroxy-14,15-dihydrocarapin on the basis of spectral analysis and single crystal X-ray diffraction study. The compound crystallizes in the tetragonal space group P4 ₁ 2 ₁ 2 with unit cell parameters: a = 13.2315(19) Å, c = 29.118(6) Å, Z = 8. The crystal structure has been solved by direct methods and refined to R = 0.0375 for 4552 unique reflections. The six-membered rings A and B exist in boat conformations, rings C and D in chair conformations, and the furan ring is planar. The crystal structure is stabilized by O-H···O and C-H···O interactions. This is the first report about 6S configuration of mexicanolide revealed by X-ray diffraction analysis. The configuration of oxygenated C-6 in mexicanolide-group limonoids is discussed.