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.     

Structural chemistry of peroxo compounds of group VI transition metals: II. Peroxo complexes of molybdenum and tungsten: A review

The specific features revealed in the structure of the molybdenum and tungsten peroxo complexes with the ratios M: O₂ = 1: 1, 1: 2, and 1: 4 are considered. It is demonstrated that the geometry of the coordination polyhedron of the metal atom is primarily determined by the “metal: peroxo ligand” ratio. Formally, the pentagonal bipyramidal coordination polyhedra of the Mo(VI) and W(VI) oxo monoperoxo and oxo diperoxo complexes (the coordination numbers of the metal atoms are equal to seven) have different geometries, namely, the MO(O₂)A ₄ pseudooctahedral and MO(O₂)₂ A ₂ pseudotrigonal bipyramidal configurations.     

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.     

New Data on Isomorphism in Eudialyte-Group Minerals. I. Crystal Structure of Titanium-Rich Eudialyte from the Kovdor Alkaline Massif

An abnormally titanium-rich mineral of the eudialyte group was studied by IR spectroscopy and X-ray diffraction. The trigonal unit cell parameters are a = 14.165(1) Å, c = 30.600(5) Å, V = 5317.23(4) ų, sp. gr. R3m. The crystal structure was refined to R = 0.034 with anisotropic displacement parameters using 2530 reflections with F > 3σ(F). The idealized formula of the mineral (Z = 3) is Na₈(H₃O)₅(K,Ce,Sr)₂Ca₆Zr₂Ti_1.2(Fe,Mn)_0.6Si₂₆O₇₂(OH)₂Cl · 4H₂O. At the ratio Zr: Ti ~ 2: 1, titanium atoms lie in four sites and are not predominant in any of them. Another distinguishing feature of the mineral is the structural separation of chemical elements, such that K, Sr, and Ce cations and H₃O groups are randomly distributed between four split sites to form polyhedra with different volumes. The isomorphism of Zr and Ti in eudialyte-group minerals is discussed.     

Synthesis and an X-ray diffraction study of Rb<Subscript>2</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The synthesis and X-ray diffraction study of compound Rb₂[(UO₂)₂(C₂O₄)₃], which crystallizes in the monoclinic crystal system, are performed. The unit cell parameters are as follows: a = 7.9996(6) Å, b = 8.8259(8) Å, c = 11.3220(7) Å, β = 105.394(2)°, and V = 770.7(1) ų; space group P2₁/n, Z = 2, and R ₁ = 0.0271. [(UO₂)₂(C₂O₄)₃]^2? layers belonging to the AK _0.5 ⁰² T ¹¹ crystal chemical group of uranyl complexes (A = UO ₂ ²⁺ , K ⁰² = C₂O ₄ ^2? , and T ¹¹ = C₂O ₄ ^2? ) are uranium-containing structural units of the crystals. The layers are connected with outer-sphere rubidium cations by electrostatic interactions.     

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.     

Synthesis and structure of Cs[UO<Subscript>2</Subscript>(SeO<Subscript>4</Subscript>)(OH)] · <Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O (<Emphasis Type="Italic">n</Emphasis> = 1.5 or 1)

The synthesis and single-crystal X-ray diffraction study of Cs[UO₂(SeO₄)(OH)] · 1.5H₂O (I) and Cs[UO₂(SeO₄)(OH)] · H₂O (II) are performed. Compound I crystallizes in the monoclinic crystal system, a = 7.2142(2) Å, b = 14.4942(4) Å, c = 8.9270(3) Å, β = 112.706(1)°, space group P2₁/m, Z = 4, and R = 0.0222. Compound II is monoclinic, a = 8.4549(2) Å, b = 11.5358(3) Å, c = 9.5565(2) Å, β = 113.273(1)°, space group P2₁/c, Z = 4, and R = 0.0219. The main structural units of crystals I and II are [UO₂(SeO₄)(OH)]^? layers which belong to the AT ³ M ² crystal chemical group of uranyl complexes (A = UO ₂ ²⁺ , T ³ = SeO₄ ^2?, and M ² = OH^?). In structure I, johannite-like layers are found. Structure II is a topological isomer of I. The two structures differ in the number of U(VI) atoms bound to the central atom by all bridging ligands.     

Structure and some properties of [UO<Subscript>2</Subscript>(C<Subscript>5</Subscript>H<Subscript>12</Subscript>N<Subscript>2</Subscript>O)<Subscript>5</Subscript>](ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

Compound [UO₂(C₅H₁₂N₂O)₅](ClO₄)₂ is synthesized and characterized by thermogravimetry, IR spectroscopy, and X-ray diffraction. The compound crystallizes in the monoclinic crystal system; a = 15.2985(9) Å, b = 26.9676(15) Å, c = 20.6962(11) Å, β = 100.697(1)°, space group P2₁/c, Z = 8, and R = 0.0445. Discrete [UO₂(C₅H₁₂N₂O)₅]²⁺ groups belonging to the AM ₅ ¹ crystal chemical group of uranyl complexes (A = UO ₂ ²⁺ and M ¹=C₅H₁₂N₂O) are uranium-containing structural units of the crystals.     

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.     

Complexes of Ambidentate N,O-Donor Ligands with Rhenium(I) and -(V)

The reaction of the potentially bidentate ambidentate N,O-donor ligands 3-hydroxy-2-pyridinecarboxylic acid (Hhpc) and 3-hydroxy-2-(hydroxymethyl)pyridine (Hhhp) with trans-[ReOCl₃(PPh₃)₂] led to the isolation of the products [ReOCl(hpc)₂] (1) (from acetonitrile) and [ReOCl₂(hhp)(PPh₃)] (2) (from ethanol) respectively. In both complexes hpc and hhp are coordinated as bidentate N,O-donor chelates, rather than as O,O-donor ligands. From the reaction of [Re(CO)₅Cl] and Hhhp·HCl in ethanol the neutral complex fac-[Re(CO)₃Cl(Hhhp)] (3) was obtained, with N,O-coordination of Hhhp. Complex 1 crystallizes in the monoclinic space group P2₁/n with a?=?6.8782(3), b?=?20.0647(8), c?=?10.8692(4) Å, β?=?107.545(1)°, and Z?=?4. Complex 2 crystallizes in the triclinic space group P-1 with a?=?7.3523(4), b?=?8.1047(5), c?=?19.591(1) Å, α?=?91.133(2)°, β?=?93.656(2)°, γ?=?93.074(2)° and Z?=?2. Complex 3 has monoclinic P2₁/c symmetry with the cell parameters a?=?10.6452(5), b?=?11.1372(5), c?=?9.7229(5) Å, β?=?106.107(2)°, and Z?=?4.

Graphical Abstract

The reaction of the potentially bidentate ambidentate N,O-donor ligands 3-hydroxy-2-pyridinecarboxylic acid (Hhpc) and 3-hydroxy-2-(hydroxymethyl)pyridine (Hhhp) with trans-[ReOCl₃(PPh₃)₂] led to the isolation of the products [ReOCl(hpc)₂] (from acetonitrile) and [ReOCl₂(hhp)(PPh₃)] (from ethanol) respectively. In both complexes hpc and hhp are coordinated as bidentate N,O-donor chelates, rather than as O,O-donor ligands. From the reaction of [Re(CO)₅Cl] and Hhhp·HCl in ethanol the neutral complex fac-[Re(CO)₃Cl(Hhhp)] was obtained, with N,O-coordination of Hhhp. Spectroscopic data and the X-ray crystal structures of the complexes are reported.

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X-ray diffraction investigation of <Emphasis Type="Italic">trans</Emphasis>-2,8-dihydroxy-2,4,4′, 6,6′,8,10,10′,12,12′-decamethyl-5,11-dicarbacyclohexasiloxane and <Emphasis Type="Italic">trans</Emphasis>-1,4-dihydroxy-1,4-dimethyl-1,4-disilacyclohexane

The crystal structures of two organosilicon compounds are studied by X-ray diffraction. Crystals of trans-2,8-dihydroxy-2,4,4′,6,6′,8,10,10′,12,12′-decamethyl-5,11-dicarbacyclohexasiloxane, C₁₂H₃₆O₆Si₆, (I) are studied at 293 K [a = b = 16.310(4) Å, c = 9.849(3) Å, V = 2620(1) ų, d_calcd = 1.128 g/cm³, space group P4(2)/n, Z = 4, 3370 reflections, wR2 = 0.1167, R1 = 0.0472 for 2291 reflections with F > 4σ(F)]. Crystals of trans-1,4-dihydroxy-1,4-dimethyl-1,4-disilacyclohexane, C₆H₁₆O₂Si₂, (II) are studied at 110 K [a = 6.8253(5) Å, b = 9.5495(8) Å, c = 12.0064(10) Å, α = 101.774(2)°, β = 102.203(2)°, γ = 95.068(2)°, V = 741.8(1) ų, d_calcd = 1.184 g/cm³, space group \(P\bar 1\), Z = 3, 6267 reflections, wR2 = 0.1052, R1 = 0.0421 for 3299 reflections with F > 4σ(F)]. It is found that the conformation of the ring in compound I, which contains two methylene groups in the cyclohexasiloxane ring, differs from those in its analogues containing only oxygen atoms or one methylene group in the ring. The noticeable difference between the SiCSi angle [123.0(2)°] and the tetrahedral angle is characteristic of cyclohexasiloxanes. Structure II contains three independent molecules with very close conformations. The cyclohexane rings adopt a chair conformation. The methylene groups in II, in distinction to those in I, are characterized by a standard tetrahedral coordination.     

X-ray crystal structures of two polymorphic forms,monoclinic and triclinic,of: 1-[(<Emphasis Type="Italic">E</Emphasis>)-2-(4-bromophenyl)1-diazenyl]-3-({3-[(<Emphasis Type="Italic">E</Emphasis>)-2-(4-bromophenyl)-1-diazenyl]-6-ethylhexahydro-1-pyrimidinyl}methyl)-4-ethylhexahydropyrimidine

1-[(E)-2-(4-bromophenyl)-1-diazenyl]-3-({3-[(E)-2-(4-bromophenyl)-1-diazenyl]-6-ethylhexahydro-1-pyrimidinyl}methyl)-4-ethylhexahydropyrimidine (1) has been synthesized by reaction of a mixture of formaldehyde and 1,3-pentanediamine{DYTEK^®EPdiamine} with p-bromobenzenediazonium chloride. This compound crystallizes in two polymorphic forms 1-α and 1-β whose crystal structures have been determined by single crystal X-ray diffraction analysis. Both polymorphs 1-α and 1-β display crystallographic disorder within the hexahydropyrimidine rings. The molecule of 1 is built up of two equivalent 3-(aryldiazenyl)-6-ethylhexahydro-1-pyrimidinyl groups in the s-trans orientation around the central methylene group (C13). In both structures the triazene moieties adopt the anti configuration around the N=N bonds, displaying significant π-conjugation. The crystal packings are determined only by van der Waals interactions. The crystal structures of 1-α and 1-β are compared with the previously reported structure of the 5,5-dimethylhexahydropyrimidine analogue 3. Compounds 1 and 3 are isomeric with respect to the hexahydropyrimidine moiety. The structures of 1 and 3 are very different in one respect; in 1 the aryldiazenyl-hexahydropyrimidinyl groups are in the s-trans orientation around the central methylene group, whereas in 3 the arrangement of the aryldiazenylhexahydropyrimidinyl groups is the s-cis orientation. Crystal data: 1-α C₂₅H₃₄N₈Br₂, monoclinic, space group P2(1)/c, a = 9.2150(3), b = 19.4059(6), c = 15.4324(5) Å, β = 98.738(1)^°, V = 2727.7(2) ų, for Z = 4; 1-β C₂₅H₃₄N₈Br₂, triclinic, space group P-1, a = 9.6009(3), b = 10.7509(4), c = 14.2169(5) Å, α = 99.830(2), β = 105.973(3), γ = 95.578(1)^°, V = 1373.9(1) ų, for Z = 2.     

Crystal structure of zinc(II) hydrated trinuclear complex with monoprotonated ethylenediaminetetraacetate anion(3−), [Zn<Subscript>3</Subscript>(H<Emphasis Type="Italic">Edta</Emphasis>)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)<Subscript>6</Subscript>]

The crystal structure of the [Zn₃(HEdta)₂(H₂O)₆] complex (I) is determined by X-ray diffraction analysis. The crystals are orthorhombic, a = 14.780 Å, b = 29.699 Å, c = 7.032 Å, Z = 4, and space group Pna2₁. The structural units of crystals I are trinuclear linear molecules, in which the peripheral atoms Zn(1) and Zn(2) each coordinate two N atoms and three O atoms of the HEdta ^3? ligand and the O(w) atom of the H₂O molecule, whereas the central Zn(3) atom coordinates four O(w) atoms of the H₂O molecules and two terminal O atoms of the two HEdta ^3? ligands. The HEdta ^3? ligand fulfills a hexadentate chelating—bridging function. The bond lengths are as follows: Zn-O(HEdta), 2.006(4)–2.123(4) Å; Zn-N, 2.214(6) and 2.128(5) Å; and Zn-O(w), 2.006(6)–2.225(5) Å. In structure I, there is a specific contact formed by hydrogen bonds, owing to which the distance between the central atoms of individual molecules appears to be shorter than that in covalently bonded complexes.     

New representative in the sodalite structure type with extraframework anions [AlF<Subscript>6</Subscript>]<Superscript>3−</Superscript>

The crystal structure of new synthetic aluminosilicate |Na_7.38(AlF₆)_0.70(H₂O)_4.88|[(Si_6.74Al_5.26)O₂₄]-SOD, which was obtained by hydrothermal synthesis (T = 650° C, P = 2 Kbar) in the Si-Al-Na-F-H₂O system, has been found by X-ray diffraction (Xcalibur-S-CCD diffractometer, 2θ_max = 64.99°, R = 0.037 for 440 reflections): a = 9.0461(1) Å, sp. gr. P \(\bar 4\)3m, Z = 1, and ρ_calcd = 2.370 g/cm³. The disordered Si,Al-tetrahedral framework (the structural basis of the new compound) is topologically identical to the framework of mineral sodalite. Na⁺ cations, [AlF₆]^3? anions, and H₂O molecules occupy framework voids. The form of fluorine incorporation into the sodalite crystal structure (as octahedral aluminofluoride complexes) has been reliably established for the first time.     

Crystal and molecular structure of barium cyclohexane-1,2-diamine-<Emphasis Type="Italic">N,N,N′,N′</Emphasis>-tetraacetatonickeloate decahydrate Ba[Ni(<Emphasis Type="Italic">Cdta</Emphasis>)] · 10H<Subscript>2</Subscript>O

The X-ray diffraction study of Ba[Ni(Cdta)] · 10H₂O is performed (R1 = 0.0441 for 5136 observed reflections). The crystals are triclinic, a = 8.833(2) Å, b = 9.025(2) Å, c = 16.922(3) Å, α = 80.56(3)°, β = 82.77(3)°, γ = 76.98(3)°, Z = 2, and space group \(P\bar 1\). The crystal is built of the [Ni(Cdta)]^2? anionic complexes, the [Ba(H₂O)₆]²⁺ hydrated cations, and crystallization water molecules. The distorted octahedral coordination of the Ni atom includes two N and four O atoms of the Cdta^4? ligand (mean Ni-N, Ni-O_G, and Ni-O_R re 2.080, 2.082, and 2.036 Å, respectively). The irregular nine-fold coordination of the Ba atom consists of six O atoms of water molecules and three O(Cdta) atoms from three anionic complexes (Ba—O, 2.715–3.090 Å). With consideration for the bonds with three Ba atoms, the Cdta^4? ligand is octadentate (2N + 6O) and fulfills the pentadentate μ₄-bridging function. The structural units are linked through an extended network of hydrogen bonds.     

Crystal structure of modular sodium-rich and low-iron eudialyte from Lovozero alkaline massif

The structure of the sodium-rich representative of the eudialyte group found by A.P. Khomyakov at the Lovozero massif (Kola Peninsula) is studied by X-ray diffraction. The trigonal cell parameters are: a = 14.2032(1) and c = 60.612(1) Å, V = 10589.13 Å3, space group R3m. The structure is refined to the final R = 5.0% in the anisotropic approximation of atomic displacement parameters using 3742|F| > 3σ(F). The idealized formula (Z = 3) is Na₃₇Ca₁₀Mn₂FeZr₆Si₅₀(Ti, Nb)₂O₁₄₄(OH)₅Cl₃ · H₂O. Like other 24-layer minerals of the eudialyte group, this mineral has a modular structure. Its structure contains two modules, namely, “alluaivite” (with an admixture of “eudialyte”) and “kentbrooksite,” called according to the main structural fragments of alluaivite, eudialyte, and kentbrooksite. The mineral found at the Lovozero alkaline massif shows some chemical and symmetry-structural distinctions from the close-in-composition labyrinthite modular mineral from the Khibiny massif. The difference between the minerals stems from different geochemical conditions of mineral formation in the two regions.     

Crystal structure of SrFe(<Emphasis Type="Italic">EDTA</Emphasis>)Cl · 5H<Subscript>2</Subscript>O

The crystal structure of the SrFe(Edta)Cl · 5H₂O (I) complex is determined. The crystals are monoclinic, a = 7.530(4) Å, b = 10.575(3) Å, c = 23.308(10) Å, β = 95.75(4)°, Z = 4, and space group P2₁/c. The structural units of I are infinite ribbons of the molecular type that are formed by tetranuclear fragments. A tetranuclear fragment involves the centrosymmetric positively charged dimer group [Sr(H₂O)₄Cl] ₂ ²⁺ at the center and the [Fe(Edta)(H₂O)]_? anionic complexes, which compensate for the positive charge of the dimer group, at the periphery. These constituents are bound via bridging oxygen atoms of the Edta ligands. The coordination number of the Sr atom is nine. The Sr-O bond lengths lie in the range between 2.552 and 2.766 Å, the Sr-Cl bond length is 3.216(3) Å, and the Sr?Sr distance is 4.371(1) Å. The parameters of the [Fe(Edta)(H₂O)]^? group are within the range of values observed in such complexes: Fe-O, 1.996–2.086(3) Å; Fe-O(w), 2.110(4) Å; and Fe-N, 2.289(4) and 2.327(4) Å. Separate ribbons are linked by hydrogen bonds involving all H₂O molecules and terminal oxygen atoms of the Edta ligand.     

Molecular structures of chalcone podands: A prognosis of photoinduced transformations in the crystals

A series of chalcone podands with the propenone group in the ortho position of the bridging aryl substituent with respect to the oxyethylene fragment is synthesized. The influence of the preorganization of the chalcone podand molecules in crystals on their ability to participate in topochemical reactions is investigated. From analyzing the X-ray structural data, the highest probability of the solid-state photochemical [2 + 2]cycloaddition is predicted for podands with phenyl substituents and the oxyethylene fragment containing two or three oxygen atoms. The X-ray structural data for the chalcone podand C₃₂H₂₆O₄ (3a) are as follows: a = 7.904(9) Å, b = 14.92(2) Å, c = 21.30(3) Å, β = 91.7(1)°, monoclinic system, space group P2₁/c, Z = 4, V = 2510(5) ų, ρ = 1.26 g/cm³, and R = 0.046; C₃₄H₃₀O₅ (3b): a = 15.738(9) Å, b = 11.889(2) Å, c = 15.0830(15) Å, β = 105.47(14)°, monoclinic system, space group C2/c, Z = 4, V = 2720.0(9) ų, ρ = 1.266 g/cm³, and R = 0.0418; C₃₂H₂₄N₂O₈ (4a): a = 17.9416(18) Å, b = 10.9703(8) Å, c = 41.699(2) Å, β = 105.970(11)°, monoclinic system, space group P2₁/c, Z = 4, V = 2781.4(5) ų, ρ = 1.348 g/cm³, and R = 0.0426; C₃₆H₃₂N₂O₁₀ (4c): a = 7.6286(5)Å, b = 17.9398(10) Å, c = 11.5890(3)Å, β = 95.287(4)°, monoclinic system, space group P2₁/n, Z = 2, V = 1579.27(14) ų, ρ = 1.372 g/cm³, and R = 0.0377; and C₂₈H₂₂O₆ (5a): a = 15.6032(10) Å, b = 8.1131(5) Å, c = 17.7334(11) Å, β = 91.381(5)°, monoclinic system, space group C2/c, Z = 4, V = 2244.2(2) ų, ρ = 1.345 g/cm³, and R = 0.0309.     

Synthesis,physical properties,and atomic structure of niobium-doped RbTiOPO<Subscript>4</Subscript> crystals

Single crystals of solid solutions Rb_1?xTi_1?xNb_xOPO₄(RTP: Nb) were grown and the temperature dependences of their dielectric and nonlinear optical properties and electric conductivity were studied. The maximum possible niobium content in these crystals is close to x = 0.1. The niobium impurities decelerate growth of {100} faces, and crystals take a plate-like habit. With increasing doping level, ferroelectric phase transitions diffuse and their temperature decreases. A specific feature of the dielectric properties of RTP: Nb crystals is the appearance of a broad relaxation maximum ε₃₃ in the temperature range 200–600°C caused by the formation of vacancies in the rubidium cation sublattice. The intensity of second-harmonic generation under laser irradiation decreases with increasing niobium content. The atomic structure of a crystal with x = 0.01 is studied and it is established that niobium substitutes for titanium only in Ti(1) positions.     

Crystal structure of 2,4,6-trinitropyridine and its N-oxide

The structures of 2,4,6-trinitropyridine (TNPy) and its N-oxide were determined by X-ray single crystal diffraction. TNPy and 2,4,6-trinitropyridine-1-oxide (TNPyO) crystallize in space groupPbcn andPnma, respectively. The crystallographic parameters are as follows: TNPy,a = 28.573(6) Å,b = 9.7394(19) Å, andc = 8.7566(18) Å, α = β = γ = 90^°, μ = 0.164 mm^?1,V = 2436.8(8) ų,z = 12,Dx = 1.751 mg/mm³,F(000) = 1296,T = 293(2) K, 1.43^°≤ θ≤ 27.40^°, the finalR factor:R ₁ = 0.0574,wR ₂ = 0.1337. TNPyO,a = 9.6272(19) Å,b = 14.128(3) Å, andc = 5.9943(12) Å, α = β = γ = 90^°, μ = 0.179 mm^?1,V = 815.3(3)ų,z = 4,Dx = 1.875 mg/mm³,F(000) = 464,T = 293(2) K, 2.88^°≤ θ≤ 27.44^°, the finalR factor:R ₁ = 0.0497,wR ₂ = 0.1515.