Refined structure of afwillite from the northern Baikal region

The crystal structure of the mineral afwillite [Ca₁₂(H₂O)₈][SiO₄]₄[SiO₂(OH)₂]₄ from the northern Baikal region was refined in a triclinic unit cell with the following parameters: a = 16.330(2) Å, b = 5.6389(6) Å, c = 11.685(1) Å, α = 90.08(1)°, β = 126.446(2)°, γ = 89.95(1)°, and sp. gr. P1. The triclinic unit cell is related to the monoclinic unit cell determined earlier (sp. gr. Cc, a = 16.278(1) Å, b = 5.6321(4) Å, c = 13.236(1) Å, and β = 134.898(3)°) by the transformation matrix [?100/010/101]. The structural model was determined using the phase-correction procedure and refined anisotropically to R = 2.8% based on 3270 independent reflections with F > 3σ(F). The crystal-chemical formula of afwillite was revised. The characteristic features of the IR spectrum of afwillite were explained based on the structural data.     

Synthesis and X-ray diffraction study of new uranyl malonate and oxalate complexes with carbamide

Two new malonate-containing uranyl complexes with carbamide of the formulas [UO₂(C₃H₂O₄)(Urea)₂] (I) and [UO₂(C₃H₂O₄)(Urea)₃] (II), where Urea is carbamide, and one uranyl oxalate complex of the formula [UO₂(C₂O₄)(Urea)₃] (III) were synthesized, and their crystals were studied by X-ray diffraction. The main structural units in crystals I are the electroneutral chains [UO₂(C₃H₂O₄)(Urea)₂]_∞ belonging to the crystal-chemical group AT¹¹M₂¹ (A = UO₂²⁺, T¹¹ = C₃H₂O₄^2-, M¹ = Urea) of uranyl complexes. Crystals II and III are composed of the molecular complexes [UO₂(L)(Urea)₃], where L = C₃H₂O₄^2- or C₂O₄^2-, belonging to the crystal-chemical group AB⁰¹M₃¹ (A = UO₂²⁺, B⁰¹ = C₃H₂O₄^2- or C₂O₄^2-, M¹ = Urea). The characteristic features of the packing of the uranium-containing complexes are discussed in terms of molecular Voronoi–Dirichlet polyhedra. The effect of the Urea: U ratio on the structure of uranium-containing structural units is considered.     

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′.     

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.     

Synthesis and crystal structure of Na<Subscript>3</Subscript>(H<Subscript>3</Subscript>O)[UO<Subscript>2</Subscript>(SeO<Subscript>3</Subscript>)<Subscript>2</Subscript>]<Subscript>2</Subscript>· H<Subscript>2</Subscript>O

Single crystals of the compound Na₃(H₃O)[UO₂(SeO₃)₂]₂ · H₂O (I) have been synthesized, and their structure has been investigated using X-ray diffraction. Compound I crystallizes in the triclinic system with the unit cell parameters a = 9.543(6)Å, b = 9.602(7)Å, c = 11.742(8)Å, α = 66.693(16)°, β = 84.10(2)°, γ = 63.686(14)°, space group P \(\bar 1\), Z = 2, and R = 0.0734. The uranium-containing structural units of the crystals are [UO₂(SeO₃)₂]^2? chains, which belong to the crystal-chemical group AB ² B ¹¹ (A = UO ₂ ²⁺ , B ² = SeO ₃ ^2? , B ¹¹ = SeO ₃ ^2? ) of the uranyl complexes. The structures of the compounds containing the [UO₂(SeO₃)₂]^2? anionic complexes are compared.     

Structure of complexes of uranyl succinate with carbamide and dimethylurea

Three new succinate-containing complexes of uranyl with carbamide (Urea) and N,N'-dimethylurea (s-Dmur) are synthesized and studied by IR spectroscopy and X-ray diffraction. Structures of the same type, [UO₂(Urea)₄(H₂O)][(UO₂)₂(C₄H₄O₄)3] · 3H₂О and [UO₂(Urea)₄(H₂O)][(UO₂)₂(C₄H₄O₄)3] · 2Urea contain two sorts of uranium-containing complex groups, namely, mononuclear [UO₂(Urea)₄(H₂O)]²⁺ cations and two-dimensional [(UO₂)₂(C₄H₄O₄)₃]^2– anions described by crystal-chemical formulas AМ ₅ ¹ and A ₂ Q ₃ ⁰², respectively (A = UO₂ ²⁺, M ¹ = Urea or H₂O, Q ⁰² = C₄H₄O₄ ^2-), and differ only in the nature of noncoordinated molecules—water and carbamide. The main structural groups of the [(UO₂)₂(C₄H₄O₄)₂(s-Dmur)₃] crystals are [(UO₂)₂(C₄H₄O₄)₂(s-Dmur)₃] chains belonging to the А ₂ Q ₂ ⁰² M ₃ ¹ (A = UO₂ ²⁺, Q ⁰² = C₄H₄O₄ ^2-, M ¹ = s-Dmur) crystal-chemical group. Specific features of intermolecular interactions in the crystal structures are revealed using the Voronoi–Dirichlet method of molecular polyhedra.     

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.     

Characterization of defects generated by di-and trivalent cations in the potassium-dihydrophosphate structure and their influence on growth kinetics and face morphology

Different types of defect sites generated by the impurities of divalent (M²⁺) and trivalent (M³⁺) metals in the structure of potassium dihydrophosphate KH₂PO₄ (KDP) were revealed by crystal-chemical analysis and computer simulation. These sites cause different deformations of the crystal matrix by generating different local strains, which enhance the inhibiting effect of impurity atoms adsorbed on the surface. This fact accounts for the different influence of di-and trivalent cations on the growth kinetics and face morphology of KDP crystals. The effect of the M³⁺ ions is associated primarily with their adsorption on the face surfaces, whereas the influence of the M²⁺ ions results from their insertion into the surface layer of the crystal.     

Crystal chemistry and prediction of compounds with a structure of skutterudite type

Crystallochemical analysis of skutterudite-type structures in the BX ₃, AB ₄ X ₁₂, and AB′₃ B ₄O₁₂ compositions (A and B are metals; X = P, As, Sb) has been performed. Probable regions of structure formation are determined, thereby indicating that more than 270 new compounds of the AB ₄ X ₁₂ composition can be synthesized.     

Crystallization,crystal-structure refinement,and IR spectroscopy of a synthetic hexahydroborite analog

The crystal structure of the hexahydroborite analog Ca[B(OH)₄]₂ · 2H₂O (a = 7.9941(3) Å, b = 6.6321(2) Å, c = 7.9871(3) Å, β = 104.166(4)°, V = 410.58(3) ų, sp. gr. P2/c, Z = 2, ρ_calc = 1.891 g/cm³; Xcalibur S CCD automated diffractometer, 1196 reflections with I > 2σ(I), λMoK _α), which was synthesized by the hydrothermal method via the recrystallization of calciborite CaB₂O₄ (M) in the M ? B₂O₃ ? H₂O system (t = 250°C and P = 70–80 atm), was refined by the least-squares method with anisotropic displacement parameters (H atoms were located; R ₁ = 0.0260). The structure of synthetic hexahydroborite consists of infinite columns running along the c axis. The columns are formed by Ca polyhedra linked together and to [B(OH)₄] orthotetrahedra by sharing edges. Along the two other axes, the translationally equivalent columns are linked only by hydrogen bonds. The presence of a stronger bond between the discrete (Ca-B-O) columns along the shortest (b = 6.6 Å) axis accounts for the possibility of the shift of the columns by 1/2T _b and the formation of the second modification of Ca[B(OH)₄]₂ · 2H₂O. The crystals of synthetic hexahydroborite were studied by IR spectroscopy. A crystal-chemical analysis was performed for a series of natural metaborates with the general formula CaB₂O₄ · nH₂O (CaO: B₂O₃ = 1: 1, n = 0–6), including calciborite CaB₂O₄ and hexahydroborite CaB₂O₄ · 6H₂O as the end members.     

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.     

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.     

Crystal structure of complex natural aluminum magnesium calcium iron oxide

The structure of a new natural oxide found near the Tashelga River (Eastern Siberia) was studied by X-ray diffraction. The pseudo-orthorhombic unit cell parameters are a = 5.6973(1) Å, b = 17.1823(4) Å, c = 23.5718(5) Å, β = 90°, sp. gr. Pc. The structure was refined to R = 0.0516 based on 4773 reflections with |F| > 7σ(F) taking into account the twin plane perpendicular to the z axis (the twin components are 0.47 and 0.53). The crystal-chemical formula (Z = 4) is Ca₂Mg ₂ ^IV Fe ₂ ^(2+)IV [Al ₁₄ ^VI O₃₁(OH)][Al ₂ ^IV O][Al^IV]AL^IV(OH)], where the Roman numerals designate the coordination of the atoms. The structure of the mineral is based on wide ribbons of edge-sharing Al octahedra (an integral part of the spinel layer). The ribbons run along the shortest x axis and are inclined to the y and z axes. The adjacent ribbons are shifted with respect to each other along the y axis, resulting in the formation of step-like layers in which the two-ribbon thickness alternates with the three-ribbon thickness. Additional Al octahedra and Mg and Fe²⁺ tetrahedra are located between the ribbons. The layers are linked together to form a three-dimensional framework by Al tetrahedra, Ca polyhedra, and hydrogen bonds with the participation of OH groups.     

Unstrained bond lengths and corresponding cation radii in crystals with perovskite structure

A method for determining average lengths of unstrained bands A-X (l_0AX) and B-X (l_0BX) and the ratio of the rigidity constants of these bonds for ABX₃ compounds with perovskite structure is proposed. The values of l_0AX and l_0BX correspond to the minimum energies of cation-anion interaction of the crystal sublattices. Values of l_0AX and l_0BX are obtained for several groups of halide and oxide compounds: A⁺B²⁺F₃, Cs⁺B²⁺Cl₃, A⁺B⁵⁺O₃, A²⁺B⁴⁺O₃, and A³⁺B³⁺O₃. It is ascertained that, for most compounds studied, the values of l_0AX and l_0BX are equal or close to the interatomic distances in crystals of binary compounds. The values of l_0AX and l_0BX are compared with the sums of the radii of the corresponding cations (R _A , R _B ) and anions (\(^{VI} R_{O^{2 - } } ,^{VI} R_{F^ - } ,^{VI} R_{Cl^ - }\)). It is found that the differences \(l_{0AO^ - } ^{VI} R_{O^{2 - } } (L_{0AF^ - } ^{VI} R_{F^ - } )\) and \(l_{0BO^ - } ^{VI} R_{O^{2 - } } (l_{0BF^ - } ^{VI} R_{F^ - } )\), regarded as the radii of the A and B cations in unstrained bonds, are close to the Shannon radii for a coordination number of six. It is shown that the rigidity constant for A-X bonds is several times smaller than that for B-X bonds.     

Relationship between the rotation angles of octahedra and bond-strength energy in crystals with perovskite structure

The dependence of the strain energy of anion-cation bonds on the sum Ψ² of squares of the angles of ordered rotations of octahedra around the coordinate axes of the reduced cell has been determined for ABX ₃ compounds with perovskite structure in the two-particle approximation. A relation determining the change in the thermodynamic Gibbs potential with allowance for the change in the bond-strain energy in the dependence on Ψ² is proposed. An expression for the equilibrium value of Ψ² as a function of the bond-strain energy and temperature is obtained. Analysis of Ψ² is performed for a homologous series of LnVO₃, LnFeO₃, and LnAlO₃ compounds (Ln are rare earth elements) and a series of CaBO ₃ and SrBO₃ compounds has been performed. It is shown that the calculated values of Ψ² for each of these series of compounds are in agreement with the values experimentally obtained from the structural data. The transition temperatures of the compounds belonging to the series studied into the cubic phase are estimated and compared with the experimental data.     

Structural features and isomorphous substitutions in chromium- and magnesium-containing beryls and chromium-containing beryllian indialite grown by the flux method

Beryls and beryllian indialite {the general formula M ^VI ₂ T(2)^IV ₃ T(1)^IV ₆O₁₈} synthesized in magnesium-containing flux systems saturated with chromium are investigated using X-ray diffraction. The isovalent schemes of the isomorphous incorporation of chromium into Moctahedra of these compounds and the simultaneously realized heterovalent schemes with the participation of other components are revealed from the occupancies of the positions. It is demonstrated that an increase in the average bond lengths in the M positions leads predominantly to an increase in the parameter α. In the beryllian indialites, the T(1) tetrahedra are substantially closer to perfect tetrahedra, the T(2) tetrahedra are distorted to a lesser extent, and the M octahedra are distorted to a greater extent than those in beryls. The structural indications of the ability of compounds with a beryl structure to congruently melt are distinguished.     

Relationship between the temperature dependence of the induced helical pitch and the anisometry of molecules of chiral dopants

The regularities of changes in the temperature dependence of the helical pitch P and the parameter dP/dt are analyzed for induced cholesteric liquid-crystal systems based on nematic 4-pentyl-4′-cyanobiphenyl, including chiral arylidene derivatives of (S)-1-phenylethylamine and (1R),(4R)-isomenthone. In the molecules of these compounds, the length of the π-electron arylidene fragment, the nature of the bridge group between its benzene rings and of the lateral substitution in the cyclohexanone ring, and the length of the terminal alkyl or oxyalkyl substituent are varied. The statistical molecular anisometry of the chiral dopants is calculated using the density functional method and the semiempirical AM1 method. It is shown that the temperature dependence of the induced helical pitch and, correspondingly, its quantitative characteristic dP/dt and the parameter dt _t /dC, characterizing the effect of a chiral dopant on the thermal stability of the mesophase, are determined by the molecular anisometry of the chiral component.     

Dimorphism of <Emphasis Type="Italic">R</Emphasis>F<Subscript>3</Subscript> (<Emphasis Type="Italic">R</Emphasis> = La–Nd) crystals based on the data of X-ray diffraction studies

The ratio of two forms of tysonite in nominally pure single crystals of RF₃ (R = La–Nd) obtained from melts under identical conditions (the as-grown state) is studied for the first time by X-ray diffraction. Crystals of RF₃ with R = La–Nd belong to the β-LaF₃ structural type (space group \(P\bar 3c1\), Z = 6) and form twins. Samples 0.2–0.4 in diameter contain inclusions of the high-temperature a form (space group P6₃/mmc, Z = 2). It is shown that twinning and dimorphism of the RF₃ crystals (R = La–Nd) have a common structural basis.     

Structural chemistry of peroxo compounds of group VI transition metals: I. Peroxo complexes of chromium (a review)

The specific features revealed in the structure of the d ³ Cr(III), d ² Cr(IV), d ¹Cr(V), and d ⁰ Cr(VI) peroxo complexes with the ratios M:O₂ = 1:1, 1:2, and 1:4 are considered. It is noted that, in eleven compounds of the general formula Cr(O₂)_nO_m A _p (n = 1, 2, 4; m = 0, 1; p = 0–4), the metal atoms can be in four oxidations states: +3 (d ³), +4 (d ⁴), +5 (d ¹), and +6 (d ⁰). This property distinguishes chromium peroxo compounds from molybdenum and tungsten dioxygen complexes, which, with one exception, are represented by the d ⁰ M(VI) compounds.     

Crystal structures of complexes of the <Emphasis Type="Italic">cys-syn-cys</Emphasis> isomer of dicyclohexano-18-crown-6 with oxonium hexafluorotantalate and oxonium hexafluoroniobate

The crystal structures of [(cys-syn-cys-dicyclohexano-18-crown-6 · H₃O)][TaF₆] and [(cys-syn-cys-dicyclohexano-18-crown-6 · H₃O)][NbF₆] complex compounds are determined using X-ray diffraction analysis. The tantalum complex has two polymorphic modifications, namely, the monoclinic (I) and triclinic (II) modifications. The unit cell parameters of these compounds are as follows: a = 8.507(4) Å, b = 11.947(5) Å, c = 27.392(12) Å, β = 93.11(1)°, Z = 4, and space group P2₁/n for modification I; and a = 10.828(1) Å, b = 11.204(1) Å, c = 12.378(1) Å, α = 72.12(1)°, β = 79.40(1)°, γ = 73.70(1)°, Z = 2, and space group P-1 for modification II. The triclinic niobium complex [(cys-syn-cys-dicyclohexano-18-crown-6 · H₃O)][NbF₆] (III) with the unit cell parameters a = 10.796(3) Å, b = 11.183(3) Å, c = 12.352(3) Å, α = 72.364(5)°, β = 79.577(5)°, γ = 73.773(4)°, Z = 2, and space group P-1 is isostructural with tantalum complex II. The structures of all three complexes are ionic in character. The oxonium cation in complexes I–III is encapsulated by the crown ether and thus forms one ordinary and two bifurcated hydrogen bonds with the oxygen atoms of the crown ether. This macrocyclic cation is bound to the anions through the C-H...F contacts (H...F, 2.48–2.58 Å). The conformation of the macrocycle in complex I differs substantially from that in complex II (III).