Phase Transition and Structure Change of Urea Adducts with n-Paraffins and Paraffin-type Compounds

The phase transition temperatures of urea adducts with many n-paraffins and several paraffin-type compounds were measured by differential thermal analysis and the general structure change causing the phase transitions was revealed by X-ray structure analysis. The ordinary hexagonal adducts (high-temperature form) transform commonly into orthorhombic adducts (low-temperature form) except for a few adducts with guest molecules possessing cylindrical shapes. The phase transition can be regarded as an order-disorder transition with respect to the orientation of the guest molecules about the urea channel axis with a cooperative deformation of the urea channels resulting in the orderly orientation of guest molecules below the transition temperatures. The changes in orientation and motion of the guest molecules through the phase transitions are discussed on the basis of X-ray analysis, potential energy and broad-line NMR.     

Montecarlo Simulation of Intercalated Atomic Distributions in Layered Dichalcogenide

Abstract

Atomic distributions of intercalated guest atoms into layered dichalcogenide have been simulated taking into account the interactions between the first and second nearest neighboring guest atoms in the triangular lattices stacked with 6 layers (lattice size 18 × 18 × 6) using a MonteCarlo method. With these distributions, the X-ray patterns of Fe _x TiS₂ with x = 0.25 having the 2√a × 2a × 2c superlattice with the fractional site occupancy have been reproduced qualitatively by averaging the intensities for different patterns obtained by different random seeds.     

Structural Phase Transition of an Intercalation Compound Mn1/4NbS2

Abstract

A structural phase transition of an intercalation compound Mn_1/4NbS₂ has been investigated by X-ray diffraction at high temperatures. The lattice parameter c exhibited a discontinuous change at 640K. The superlattice reflections observed below 640K disappeared suddenly above 640K. The phase transition at 640K took an aspect of the first-order phase transition. The precise structure analyses were performed at various temperatures above and below the phase-transition temperature. It was revealed that Mn atoms were arranged in disorder in the high-temperature phase, while the Mn atoms were ordered forming the 2a ₀ × 2a ₀ × c ₀ superlattice in the low-temperature phase. The Nb and S atoms around the ordered Mn atoms slightly shifted from the high-symmetry position in the low-temperature phase. The order parameters were the degree of order of the Mn atoms and the degree of displacement of the Nb and S atoms.     

Synthesis and Crystal Structure of <Emphasis Type="Italic">N</Emphasis>′-(6-Chloropyridin-3-formyl)-<Emphasis Type="Italic">N</Emphasis>-<Emphasis Type="Italic">t</Emphasis>-Butyl Urea

Abstract The crystal structure of N′-(6-chloropyridin-3-formyl)-N-t-butyl urea, C₁₁H₁₄ClN₃O₂, has been established, and which belongs to monoclinic crystal system, space group Pn with unit cell dimensions a = 9.335(3) ?, b = 12.715(3) ?, c = 21.813(6) ?, β = 95.417(6)°, V = 2577.6(12) ?³ and Z = 8. An intramolecular N–H···O hydrogen bond forms a six-membered ring in the central part of the molecule. Both two N–H atoms of single unit participate in intermolecular hydrogen bonds and an intramolecular hydrogen bond, respectively. Index Abstract The crystal structure ofN′-(6-chloropyridin-3-formyl)-N-t-butyl urea, C₁₁H₁₄ClN₃O₂, has been established, and which belongs to monoclinic crystal system withZ = 8, space group Pn.      

ESR study of Mn2+ in GeTe and GeTeSi glasses

The electron spin resonance spectra of Mn²⁺ ions have been studied in Ge_xTe_100?x with x = 15, 17.5 and 20, and Ge_20?xTe₈₀Si_x with 0 ?x? 20. All samples are found to exhibit six hyperfine lines centered at g = 4.3 with hyperfine interaction constant A = 56 × 10^?4cm^?1. The g = 4.3 line is interpreted as being caused by Mn²⁺ ions incorporated in the amorphous network and surrounded by four Te atoms in an arrangement of orthorhombic symmetry. Some of the samples of GeTe show a g = 2.0 line. This line also appears after heat treatment in air at temperatures above the glass transition temperature. It is concluded that the g = 2.0 line is caused by Mn²⁺ ions in phase separated microcrystalline or concentrated regions of MnO in the glass.     

Thermoelastic and Piezoelastic Properties of Hexagonal RbMnCl3

Temperature and pressure derivatives of the elastic constants, indices of refraction, dielectric constants, Faraday effect and thermal expansion of RbMnCl₃ near T_c = 278 K confirm the existence of a second-order phase transition. A strongly anomalous behaviour is observed with the elastic shear stiffness c₄₄ which possesses a positive temperature derivative and a negative pressure derivative. These properties and also the pressure derivative of the transition temperature, dT_c/dp = 6.2 K kbar⁻¹ closely resemble the anomalous behaviour of two other ferroelectric species, betaine borate and betaine hydrogen maleinate.     

Structural Studies of Antimony Pentahalide Graphite Intercalation Compounds Using 001 Reflections

X-ray diffraction studies at room temperature are reported of stages 1, 2 and 3 SbCl_mF_5-m graphite intercalation compounds (m = 0, 2, 4, 5). The analysis of the 001 reflection inten-sities shows the periodic stacking of carbon, chlorine and antimony layers in all compounds. The results are in good agreement with the structural model recently found in other metal halide graphites. But we have found that the antimony atoms are not located on the mirror plane (z = O) in the centre of the intercalate layer. In case of dilute SbCl₅ graphites, the identity period along c axis I_c increases with SbCl₅ concentration in the stage 2 com-pound. Interlayer distances were determined in dependence on SbCl₅ concentration. A molecular layer form consisting of SbX₅ molecules which are linked by halogen bridges is suggested to fill the galleries in the graphite lattice.     

Mathematical Description of Strongly Nonlinear Pressure Dependence of Phase Transition Temperatures

In order to describe mathematically the pressure dependence of phase transition temperatures we have derived an equation using the following assumption: i. The transition enthalpy Δ_tH = const. ii. The transition volume Δ_tV depends on the pressure p according to: The modified Simon-Glatzel-equation T = T₀(1 + p/b)^a exp (cp) allows to calculate the transition temperatures T also in cases of strongly non-linear relations.     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Phase Transition and Phase Diagram of Anion Radical Salts of [(C6H5)3PCH3]+ [(C6H5)3AsCH3]+ ×(TCNO)− 2 (0 ≤×≤ 1)

Abstract

Much attention has been paid to the solid anion radical salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their prominent electronic properties.^1–4 In particular, the salts containing mixed cations represented by [(C₆H₅)₃PCH₃]⁺ _1–x [(C₆H₅)₃AsCH₃]⁺ _× (TCNQ)^? ₂, (0 ≤×≤ 1), are known to undergo phase transitions at 1 atm pressure in the solid state.^1–4 The phase transition of pure methyltriphenylphosphonium salt, (x = 0.00), takes place at 315.7 K. Heat-capacity measurements of this phase transition have been made by Kosaki et al. ³ The transition has thus been found to be of the first order. The enthalpy and the total entropy change associated with the phase transition were experimentally determined to be 485.18 cal/mol and 1.7206 cal/deg.mol, respectively. For the solid solutions, it was found that the transition temperature (T_c ) is increased, while the magnitude of the heat of transition (δH) is decreased, progressively with an increase in the composition parameter (x) and that pure methyltriphenylarsonium salt, (x = 1.00), has no such phase transition up to the decomposition temperature of about 480 K at 1 atm pressure.^1–3 Figure 1 shows the experimental relation between T _c and x, together with the relation between δH and x.⁴ In the present paper, we attempted to explain thermodynamically the phase diagram of Figure 1 for the solid solutions of those TCNQ anion radical salts.     

Electrical Characterization of Irradiation plus Annealing – induced VAsZnGa Pairs in p-type GaAs (Zn) Crystals

Effect of fast electron irradiation (E =2.2 Mev, ϕ_c = 1 × 10¹⁶ el/cm²) and subsequent annealings (T = 150 to 350 °C, t = 10 to 600 min) of zinc-doped p-type GaAs crystals on the formation and dissociation of V_AsZn_Ga, pairs is studied. An analysis of the formation and dissociation kinetics of V_AsZn_Ga pairs permitted to find the diffusion coefficient of radiation-induced arsenic vacancies D(D = 1.5 × 10⁻¹⁸, 1 × 10⁻¹⁷ and 5 × 10⁻¹⁷ cm²/s at 150, 175 and 200 °C accordingly), their migration energy ϵ_m(ϵ_m = 1.1 eV), the binding energy of V_AsZn_Ga, pairs ϵ_b(ϵ_b = 0.5 eV), and also their dissociation energy ϵ_d(ϵ_d = 1.6 eV).     

Solid-Solid Phase Transitions of Long-Chain n-Alkyltrimethylammonium Halides

Transition properties of some compounds of the type, n-C _n H_2n+1N⁺(CH₃)₃X_-, with n = 10, 12, 14, 18, 22, and X = Cl, Br, and I, were investigated. All these compounds show a solid-solid phase transition with a large enthalpy change in the range of temperature 350 ～ 400K. The transition behaviors were investigated by the aid of DSC, TG, X-ray, and optical microscope. The solid-solid phase transition is regarded to be caused by melting of the layer of hydrocarbon chains of the compounds, while the rigid ionic layer retains the regular arrangement. Some relationships between the physical properties and the structure of the compounds are discussed.     

Crystal structure determination of bis-dimethyl heptamethine cyanine chloride,C11N2H19Cl · 4 H2O

Mr = 286.59, monoclinic superposition structure C 121, Z = 4, a = 15.248(3), b = = 6.942 (3), c = 9.074(1) Å, β = 120.1(1)°, V = 827(1) ų, D_m = 1.13(1) Mg m⁻³, D_x = = 1.15 Mg m⁻³, λ(MoK_α) = 0.71069 Å, μ = 0.243 mm⁻¹, F (000) = 312, T = 296 K, final R = 0.061 for 802 reflections. All hydrogen atoms were located. The structure consists of chloride anions, bis-dimethyl heptamethine cyanine cations and water molecules. The structure of bis-dimethyl heptamethine cyanine chloride is disordered, e.g. there exist partial coincidence operations, leading to more than one stacking mode of the bis-dimethyl heptamethin cyanine chains. The crystal structure consists of rows of molecule along the a-axis. The C C bonds show the bond length compensation, typical of polymethines, towards a partial double bond (mean value 1.38 Å) between formally sp²-hybridized C atoms. The C N bond lengths to the methyl carbon atoms (mean value 1.49 Å) correspond to normal C N single bonds. The valence angles at the C atoms of the heptamethine chain clearly alternate and deviate systematically from 120°. This is also observed in other structures of a similar type. The structure determined by X-ray analysis confirms the chemical results.     

Carrier Scattering Mechanisms in GaS0.5Se0.5 Layered Crystals

Systematic dark electrical resistivity and Hall mobility measurements have been carried out in the temperature range 150‐400 K on n‐type GaS_0.5Se_0.5 layered crystals. The analysis of temperature dependent electrical resistivity and carrier concentration reveals the extrinsic type of conduction with a donor impurity level located at 0.44 eV, donor and acceptor concentrations of 3.4 ×10¹⁷ and 4.1×10¹⁶ cm^‐3, respectively, and an electron effective mass of 0.41 m₀. The Hall mobility is limited by the electron‐phonon short‐range interactions scattering at high temperatures combined with the ionized impurity scattering at low temperatures. The electron‐phonon short‐range interactions scattering mobility analysis reveals an electron‐phonon coupling constant of 0.25 and conduction band deformation potential of 5.57 eV/Å.     

Synthesis and crystal structure of a new dysprosium(III) complex with 2-fluorobenzoato and 1,10-phenanthroline

The dysprosium complex with 2-fluorobenzoato (2-FBA) and 1,10-phenanthroline (phen) was synthesized and characterized by X-ray diffraction. The complex crystallizes in the triclinic system with space group , lattice parameters: a = 11.233(7)  Å, b = 12.598(8)  Å, c = 22.718(14)  Å, α = 81.045(13)°, β = 78.213(12)°, γ = 80.506(11)°, V = 3079(3) ų, Z= 2, Dcalc = 1.689 Mg/m³. The complex contains two independent binuclear molecules, namely, [Dy(2-FBA)₃·phen·CH₃CH₂OH]₂ (I) and [Dy(2-FBA)₃·phen]₂ (II). In molecule (I), Dy1³⁺ ion is coordinated by eight atoms, five O atoms from five 2-FBA groups, one O atom from an ethanol molecule and two N atoms from the phen ligand. The 2-FBA groups adopt monodentate and bridging two-coordination modes. In molecule (II), the Dy2³⁺ ion is coordinated by nine atoms, seven O atoms from five 2-FBA groups and two N atoms from the phen ligand. The 2-FBA groups adopt chelating, bridging and chelating-bridging three-coordination modes.Supplementary material CCDC-257186 for the compound contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK; fax: (44(0) 1223-336033; email: deposit@ccdc.cam.ac.uk.     

EXAFS and XANES Studies on 3d Transition Metal Intercalation Compounds M x TiS2

Abstract

EXAFS and XANES spectra of Ti K-edge have been measured for 3d transition metal intercalation compounds M _x TiS₂ (M = Mn, Fe, Co and Ni; x ≤ 0.33). We have found that the interatomic distance between Ti and the first nearest neighbor S atoms, R(Ti-S), increases with the guest concentration x. The variation in XANES spectra with x reveals the reduction of the valence state of Ti atoms upon intercalation of M. From these results as well as the M K-edge EXAFS data studied previously, we have proposed a simple model on the local structure of M _x TiS₂ to reproduce the observed values of R(Ti-S) by averaging local shift of S atoms caused by intercalation.     

X-ray and Raman studies of single crystals of CsSbF6

Crystals of CsSbF₆ belong to the rhombohedral space groupR¯3-C _3i ² witha=7.904(1)andc=8.261(1) Å,V=446.95 ų,Z=3,D _c=4.11 gcm^–3. The antimony atom is surrounded by six fluorine atoms in a nearly perfect octahedral configuration with Sb-F 1.875(9) Å, while 12 fluorine atoms surround the cesium atom with closest contact 3.116 Å. Polarized Raman spectra of single crystals of CsSbF₆ have been obtained, and it is shown that these results can be interpreted in terms of a unimolecular rhombohedral structure. The small distortion from an octahedral arrangement for the SbF ₆ ^– group is clearly reflected in the spectra. The Raman results are in better agreement with the space groupR¯3m-D _3d ⁵ than withR¯3-C _3i ² , but this conclusion must be regarded with caution since the two features in the vibrational spectra of CsSbF₆ which can be used to distinguish between the two structures are weak and ill-defined.     

Crystal Structure Study of Co(II) Complexes with 2,2′-(1,4-Ethylenedioxyl) Dibenzoic Acid-[Co · L · (H2O)3]n · nH2O· 0.2 nH2O

The structure ofS the title complex [Co. L. (H₂O)₃]_n. nH₂O. 0.2 nH₂O has been established by single crystal X-ray diffraction. Crystals of the complex are monoclinic, space group C2/c with cell constants α = 45.29(4), b=10.631(6), c = 8.015(6) Å, β = 90.65(8)°, Z = 8, D_c = 1.489 g. cm⁻³ The structure was solved and refined to R =0.0478 (wR = 0.0577). In the chain structure, Co(II) ions are hexacoordinated by 0 atoms in an octahedral arrangement. CoO₆ octahedra share corners (bridged) through O atoms of water, with each L^2--ligand binding two adjacent Co atoms.     

A dinuclear cyano-bridged complex based on hexacyanoferrate(III) and samarium(III) nitrate: synthesis and crystal structure

A new complex, [Sm(DMF)₄(H₂O)₄Fe(CN)₆]·H₂O (DMF = N, N-dimethylformamide), has been synthesized and characterized by X-ray single crystal structure and thermogravimetric analyses. The complex crystallizes in the P2₁/n space group, with lattice parameters a = 17.583(4) Å, b = 8.870(2) Å, c = 19.845(6) Å, = 95.98(3)°, V = 3078(1) ų, D _x = 1.679 Mg m^–3, D _m = 1.65(1) Mg m^–3, Z = 4. The molecular structure shows that a cyano-bridged bimetallic structure is obtained. The Sm atom is coordinated by eight oxygen atoms of four water molecules and four DMF molecules and one nitrogen atom of the bridging cyanide ligand. The iron atom assumes approximately an octahedral environment surrounded by six CN ligands. The hydrate water molecule is hydrogen-bonded to one of the O atoms bound to Sm. Each terminal CN ligand of the Fe(CN) ₆ ^3– entity is hydrogen-bonded to some O atoms of water molecules. An infrared spectrum is also reported.     

Molecular complexes of 3-amino-1,2,4-triazole: The crystal structure of 3-amino-1,2,4-triazolium 5-nitrofuran-d2-carboxylate

3-Amino-1,2,4-triazolium (3-AT) 5-nitrofuran-2-carboxylate has been prepared and its x-ray crystal structure reported (a = 12.363(3) Å, b = 12.5720(10) Å, c = 12.8550(10) Å, Pbca, orthorhombic, Z = 8, and D _c = 1.604 mg m^–3). The solid-state packing of this organic acid-base salt consists of a three-dimensional hydrogen-bonded network which exhibits a cage-like repeat unit involving both the N(2)–H and N(4)–H sites of 3-AT. A review of five such structures reveals that in all cases the carboxylate groups interact with either of these sites [and the N(31)H₂] and form a R ² ₂(8) graph set interaction. Analysis of all 3-AT complexes which form this interaction reveals that the hydrogen-bonding distances between the non-hydrogen atoms are not equal [av. 2.712(5) Å and 2.897(5) Å], with the shorter distance being the interaction between the heterocyclic nitrogen and the corresponding carboxylate oxygen. Furthermore, a review of the interactive modes of all 3-AT adducts has been undertaken.