A necessary condition for the formation of a bisystem molecular crystal

A hypothesis is formulated which states that the necessary condition for the existence of a bisystem molecular crystal is the approximate equality of the total energies of the interactions of two symmetrically independent molecules N and N′ with their environment in the crystal structure (U_Σ). The hypothesis was tested on four crystalline organic compounds of the structure class P2₁/c, Z = 8(1, 1). The computations in the atom-atom approximation demonstrated that the energies U_Σ(N) and U_Σ(N′) have very close values despite the different coordination numbers of the molecules and a difference in the structures of the molecular agglomerates that include the molecules N and N′.     

Hydrogen Bonding Network Assembled with 2-Amino-3-hydroxypyridium and Isonicotinate-N-oxide

The crystal structure of the compound (C₅H₇N₂O)·(C₆H₄NO₃)·H₂O (1) consisting of 2-amino-3-hydroxypyridium, isoniconate-N-oxide and lattice water was characterized by single-crystal X-ray diffraction analysis: monoclinic, P2₁/c, a = 9.6905(8), b = 6.0040(4), c = 21.039(2) Å, β = 101.062(8)°, V = 1201.34(18) Å³, Z = 4. In 1, the deprotonated isoniconate-N-oxide, the protonated 2-amino-3-hydroxypyridium and the lattice water are linked by a series of classical hydrogen bonds to form 2-D layer. Then, these 2-D sheets assemble to a 3-D network via weak hydrogen bonds.     

The crystal structure of vyuntspakhite: A redetermination

The crystal structure of the mineral vyuntspakhite (Y, TR)₆{Al₂(OH)₃[H_1.48Si_1.88O₇][SiO₄][SiO₃(OH)]}₂(a = 5.7551(11) Å, b = 14.752(3) Å, c = 15.906(4) Å, β = 96.046(4)°, sp. gr. P2₁/n, Z = 2), which had been established earlier in the pseudo-unit cell, is redetermined by X-ray diffraction (R = 0.040, T = 100 K). The redetermination of the structure shows that pronounced pseudotranslation along the axis c′ = c/3 is associated with the fact that Y(TR) atoms are related by a 1/3 translation along the [001] direction. Most of the hydrogen atoms are located. The crystal-chemical function of hydrogen bonds is analyzed. In the unit cell of vyuntspakhite, the cationic layers consisting of edge-sharing (Y,TR) eight-vertex polyhedra alternate along the b axis with mixed anionic layers composed of isolated Si tetrahedra (orthotetrahedra), Si₂O₇ double-tetrahedra (diortho) groups, Al five-vertex polyhedra, and Al₂O₈ double-tetrahedra groups linked by shared vertices and through hydrogen bonding.     

Crystal structure of <Emphasis Type="Italic">L</Emphasis>-serine phosphate

The crystal structure of L-serine phosphate (C₃O₃NH₇ · H₃PO₄) is determined by single-crystal X-ray diffraction. The unit cell parameters are as follows: a = 9.134(5) Å, b = 9.489(5) Å, c = 4.615(5) Å, γ = 99.54(5)°, space group P2₁, and Z = 2. The amino group of serine is protonated by a hydrogen atom of the phosphoric acid. The H₂PO ₄ ^1? ions are linked by hydrogen bonds into infinite ribbons aligned along the twofold screw axes. The ribbons form layers alternating with layers of serine molecules, which are directly linked by hydrogen bonds.     

Crystal chemistry of KCuMn<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>3</Subscript> in the context of detailed systematics of the alluaudite family

The crystal structure of new manganese potassium copper vanadate KCuMn₃(VO₄)₃, which was prepared by the hydrothermal synthesis in the K₂CO₃–CuO–MnCl₂–V₂O₅–H₂O system, was studied by X-ray diffraction (R = 0.0355): a = 12.396(1) Å, b = 12.944(1) Å, c = 6.9786(5) Å, β = 112.723(1)°, sp. gr. C2/c, Z = 4, ρ_calc = 3.938 g/cm³. A comparative analysis of the crystal-chemical features of the new representative of the alluaudite family and related structures of minerals and synthetic phosphates, arsenates, and vanadates of the general formula A(1)A(1)′A(1)″A(2)A(2)′M(1)M(2)₂(TO₄)₃ (where A are sites in the channels of the framework composed of MО₆ octahedra and TО₄ tetrahedra) was performed. A classification of these structures into subgroups according to the occupancy of A sites is suggested.     

Crystal structure of <Emphasis Type="Italic">L</Emphasis>-alanine phosphate

The crystal structure of L-alanine phosphate (C₃O₂NH₇ · H₃PO₄) is determined by the single-crystal diffraction technique; a = 11.918(1) Å, b = 9.117(1) Å, c = 7.285(1) Å, γ = 104.7(1)°, space group P2₁, and Z = 4. The amino group of the alanine is protonated by the hydrogen atom of the phosphoric acid. Pairs of H₂PO ₄ ^? hydrogen-bonded ions are packed into layers alternating with layers of alanine molecules in the crystal. No hydrogen bonds are formed immediately between the alanine molecules.     

Hydrogen Bonding Patterns in 2,6-diamino-4-oxopyrimidinium Sulfosalicylate Monohydrate

The title compound, [C₄H₇N₄O⁺, C₇H₅O₆S^? . H₂O] contains one 2, 6-diamino-4-oxopyrimidinium cation, one sulfosalicylate anion and a water molecule. The crystal structure was determined by single crystal X-ray diffraction. This compound crystallized in the orthorhombic system; space group Pna2₁ with the unit cell parameters a = 13.402(3) Å, b = 16. 221(3) Å, c = 6.714(2) Å, V = 1459.6(6), Z = 4. The sulfonic acid group has protonated the aminopyrimidine moiety. The protonated N1 atom and N4 amino group are hydrogen bonded to the keto group (O1) of the neighbouring pyrimidine forming a six-membered ring with graph-set notation R₂ ¹(6) and a supramolecular chain along the c-axis. This supramolecular chain is further strengthened by one of the sulfonate oxygen atoms (O3), bridging the pyrimidines via hydrogen bonded rings, R₃ ²(10) involving N(3)–H(3)···O(3) and N(2)–H(2B)···O(3) hydrogen bonds. The other two oxygen atoms of the sulfonate groups are bridged by water molecules via O-H···O hydrogen bonds constituting a supramolecular chain. The water molecule also acts as hydrogen bond acceptor with respect to the carboxyl group.     

Catenated molecular pattern in the crystal structure of a copper(II) complex with pyrazine-2,6-dicarboxylic acid and chloride ligands

The crystals of catena-dichloro-(dihydrogen pyrazine-2,6-dicarboxylato-O,O′,N,-N′) copper(II) dihydrate Cu[H₂(2,6-PZDC)]Cl₂?2H₂O are monclinic, space group C2/c with a = 11.658(2) Å, b = 6.778(1) Å, c = 16.390(3) Å, β = 98.90(3)^°, and Z = 4. Two adjacent copper(II) ions are bridged by a fully protonated pyrazine-2,6-dicarboxylic acid molecule which uses for bridging its O,O′,N-bonding moiety on one side and a single hetero-ring nitrogen atom on the other. Two chloride ions in axial position complete the octahedral coordination around the metal ions with bond distances: Cu–N 2.027(3) ÅA, Cu–N′ 2.005(3) ÅA, Cu–Cl. 2.281(1) ÅA, and Cu–O 2.446(2) ÅA. Copper(II) ions are located in the center of symmetry and are coplanar with the ligand acid molecule forming flat molecular chains propagating in the direction of the b axis. Hydrogen bonds via solvation water molecule link the chains into molecular layers parallel to the ac plane. Weak van der Waals type interactions operate between the layers.     

Crystal structure of CoUO<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> · 5H<Subscript>2</Subscript>O at 293 K

Single crystals of cobalt uranyl sulfate are grown. The crystal structure is established by X-ray diffraction: the orthorhombic system, sp. gr. Pmc2₁, a = 6.452(2) Å, b = 8.295(2) Å, c = 11.288(3) Å, R₁ = 0.0303, wR₂ = 0.0735 for reflections with I > 2σ(I). The structure of CoUO₂(SO₄)₂ · 5H₂O consists of infinite two-dimensional uncharged [CoUO₂(SO₄)₂H₂O]_2∞ layers, which are linked to each other by hydrogen bonds.     

Synthesis and crystal structures of coordination compounds of pyridoxine with zinc and cadmium sulfates

The pyridoxine complexes with zinc and cadmium sulfates are synthesized. The IR absorption spectra and thermal behavior of the synthesized compounds are described. Crystals of the [M(C₈H₁₁O₃N)₂(H₂O)₂]SO₄ · 3H₂O (M = Zn, Cd) compounds are investigated using X-ray diffraction. In the structures of both compounds, the M atoms are coordinated by the oxygen atoms of the deprotonated OH group and the CH₂OH group retaining its own hydrogen atom, as well as by two H₂O molecules, and have an octahedral coordination. The nitrogen atom of the heterocycle is protonated, so that the heterocycle acquires a pyridinium character. The cationic complexes form layers separated by the anions and crystallization water molecules located in between. The structural units of the crystals are joined together by a complex system of hydrogen bonds.     

Raman spectroscopy study of the structure of gallium nitride epitaxial layers of different orientations

The composition nonstoichiometry and structural quality of undoped gallium nitride layers grown by the hydride vapor phase epitaxy on sapphire substrates of different orientations have been estimated using Raman spectroscopy. It is found for the first time that the peak position of the phonon mode E₂(high) in the Raman spectra of gallium nitride films at a wave vector of 572 cm^–1 depends on the initial orientation of sapphire substrate and is low-frequency shifted when passing from Ga-polar to partially N-polar orientation. Additional modes are found in the spectra of GaN layers grown on substrates with m and r orientations. It is shown that a decrease in the composition deviation from stoichiometry, caused by reducing the HCl flow through the gallium source during the growth of GaN layers, leads to an increase in the phonon-mode intensity in the Raman spectrum.     

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.     

Complete derivation of space groups of magnetic symmetry of crystals by numerical construction

The solution is given to the problem regarding the derivation of the so-called space groups of magnetic symmetry with crystallographic angles of rotation of the spin vector, which was formulated as far back as the early 1970s. The crystallographic P-symmetries that should be used in the generalization with the three-dimensional space Fedorov groups G ₃ are revealed. The space groups G ₃ are generalized with each of these P-symmetries.     

Crystal structure of the (Mg,Fe)[UO<Subscript>2</Subscript>(P,As)O<Subscript>4</Subscript>]<Subscript>2</Subscript> · 10H<Subscript>2</Subscript>O solid solution—A novel mineral variety of saléeite

The crystal structure of a novel variety {[(Mg_0.81Fe_0.19)(H₂O)₆](H₂O)₄}{(UO₂)[(P_0.67As_0.33)O₄]}₂ of the mineral saléeite is determined using X-ray diffraction (Bruker Smart diffractometer, λMoK _α, graphite monochromator, 2θ_max = 56.62°, R = 0.0321 for 2317 reflections, T = 100 K). The main crystal data are as follows: a = 6.952(6) Å, b = 19.865(5) Å, c = 6.969(2) Å, β = 90.806(4)°, space group P12₁/n1, Z = 2, and ρ_calcd = 3.34 g/cm³. It is shown that the structure is formed by alternating (along the [010] direction) anionic layers, which are composed of uranium bipyramids and T(P,As) tetrahedra, and cation layers consisting of M(Mg,Fe) octahedra and water molecules, which are joined through a system of asymmetric hydrogen bonds. The hydrogen atoms are located, the scheme of hydrogen bonds is established, and their geometric characteristics are calculated.     

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.     

X-ray diffraction study of Rb<Subscript>2</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(CrO<Subscript>4</Subscript>)<Subscript>3</Subscript>(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>] · 4H<Subscript>2</Subscript>O

The compound Rb₂[(UO₂)₂(CrO₄)₃(H₂O)₂] · 4H₂O was studied by X-ray diffraction. The crystals are monoclinic, a = 10.695(2) Å, b = 14.684(3) Å, c = 14.125(3) Å, β = 108.396(4)°, sp. gr. P2₁/c, Z = 4, V = 2104.9(7) ų, and R = 0.0491. The main structural units are layers consisting of [(UO₂)₂(CrO₄)₃(H₂O)₂]^2? anions belonging to the crystal-chemical group A ₂ T ₂ ³ B ²M ₂ ¹ (A = UL ₂ ²⁺ , T ³ and B ² are CrO ₄ ^2? , and M ¹ is H₂O) of uranyl complexes. The uranium-containing layered groups are held together by electrostatic interactions with rubidium cations, as well as by hydrogen bonds with the participation of inner- and outer-sphere water molecules.     

Coordination dimers constructed from metal(II) halides and the organic ligand 1,2-dimethoxy- 4,5-bis(2-pyridylethynyl)benzene

A series of new coordination compounds has been synthesized using the organic ligand 1,2-dimethoxy-4,5-bis(2-pyridylethynyl)benzene (dmpeb). The compounds all form dimers consisting of two metal cations bridged by two ligand molecules. Charge balance is provided by halide ligands, and the four-coordinate metal centers are distorted from the ideal tetrahedral environment. [CoCl₂(dmpeb)]₂ (1) crystallizes in the monoclinic space group P2₁/n with a = 8.5272(6) Å, b = 18.3653(13) Å, c = 13.3493(9) Å, β = 103.574(2)^°, V = 2032.2(2) ų, Z = 2. [ZnCl₂(dmpeb)]₂ (2) is isostructural to 1 and has the cell parameters a = 8.5495(4) Å, b = 18.4049(8) Å, c = 13.3692(6) Å, β = 103.4460(10)^°, V = 2046.01(16) ų, Z = 2. [ZnBr₂(dmpeb)]₂ (3) is also isostructural to 1 with a = 8.7882(5) Å, b = 18.7260(12) Å, c = 13.3857(8) Å, β = 102.5990(10)^°, V = 2149.8(2) ų, Z = 2. Additionally, the compounds [ZnI₂(dmpeb)]₂ (4, cell parameters: a = 8.9650(5) Å, b = 19.1251(10) Å, c = 13.4160(7) Å, β = 101.1660(10)^°, V = 2256.7(2) ų, Z = 2), [HgCl₂(dmpeb)]₂ (5, cell parameters: a = 8.8457(7) Å, b = 18.4030(15) Å, c = 13.3711(11) Å, β = 104.246(2)^°, V = 2109.7(3) ų, Z = 2), and [HgBr₂(dmpeb)]₂ (6, cell parameters: a = 9.0576(5) Å, b = 18.8634(11) Å, c = 13.4535(8) Å, β = 102.9780(10)^°, V = 2239.9(2) ų, Z = 2) are also isostructural to 1. A seventh dimeric compound, [HgI₂(dmpeb)]₂, not isostructural to the others was also characterized by X-ray crystallography. [HgI₂(dmpeb)]₂ (7) crystallizes in the triclinic space group P-1 with a = 8.8028(5) Å, b = 12.0990(7) Å, c = 12.4082(7) Å, α = 109.7240(10)^°, β = 107.3680(10)^°, γ = 93.0880(10)^°, V = 1169.57(12) ų, Z = 1.     

Investigation of X-ray natural circular dichroism in a CsCuCl<Subscript>3</Subscript> single crystal: Theory and experiment

X-ray natural circular dichroism in the near-Cu K edge in a CsCuCl₃ single crystal, caused by dipole-quadrupole (E1E2) transitions between electronic states of different parity, has been experimentally and theoretically investigated. Numerical simulation of the experimental X-ray absorption spectrum and circular dichroism has been performed. It is established that a circular dichroism signal is observed in the pre-edge spectral region; therefore, it is mainly due to the localized electronic p-d states.     

The crystal structure of calcium aqua{ethylenediaminetetraacetato(3-)}nickelate(II) tetrahydrate,Ca[Ni(H<Emphasis Type="Italic">Edta</Emphasis>)(H<Subscript>2</Subscript>O)]<Subscript>2</Subscript>·4H<Subscript>2</Subscript>O

The crystal structure of Ca[Ni(HEdta)(H₂O)]₂ · 4H₂O (HEdta ^3?is a protonated anion of ethylenediaminetetraacetic acid) is determined. The unit-cell parameters are as follows: a = 7.706(1), b = 14.923(9), and c = 26.570(5) Å; V = 3055.6(2.4) ų; Z = 8; and space group Pbcn. Crystals are shaped as tetragonal prisms and colored green, which is not characteristic of this class of compounds. The structure is built of Ca²⁺ cations, octahedral [Ni(HEdta)(H₂O)]^? complex anions, and crystallization water molecules. In the complex anion, one of the acetate branches is free of coordination, whereas the remaining three branches coordinate the central atom forming glycinate metallocycles. The structure is based on sandwich layers, which are formed by networks of flat tetragonal Ca[Ni(HEdta)(H₂O)]₄(H₂O)₂ fragments adjoining one another by vertices.