On the pressure-induced phase transformation in the structure II clathrate hydrate of tetrahydrofuran

A high-pressure phase of the clathrate hydrate of tetrahydrofuran was prepared by freezing a liquid phase of overall composition THF · 7 H₂O under a pressure of 3.0 kbar, or by pressurizing the solid structure II THF hydrate of 255K to 3.4 kbar. Unfortunately, the products recovered at 77K were always mixed phase materials as shown by X-ray powder diffraction. A number of diffraction lines could be indexed in terms of the cubic structure I hydrate with a slightly expanded lattice parameter, 12.08 Å, giving some support to Dyadin's idea that the high pressure phase transition involves a conversion of Structure II to Structure I. Other phases observed in the recovered product include Ice IX and amorphous materials. The reversion of the high pressure sample to the structure II hydrate was followed by differential scanning calorimetry. At ambient pressure, the high pressure sample converts slowly back to Structure II hydrate event at 77K.NRCC No. 35786.     

Dissociation behaviour of (tetra-n-butylammonium bromide + tetra-n-butylammonium chloride) mixed semiclathrate hydrate systems

The thermal properties of {tetra-n-butylammonium bromide + tetra-n-butylammonium chloride (TBAB + TBAC)} mixed semiclathrate hydrates prepared from aqueous solutions were investigated by dissociation temperature measurements and differential scanning calorimetry (DSC). The maximum dissociation temperature of the mixed hydrate crystals at 0.1 MPa is 288.5 K for x_TBAB = 0.2 {mole fraction of TBAB to (TBAB + TBAC)}, which is higher than that of the pure hydrates {T = (285.5 and 288.2) K for TBAB and TBAC hydrates, respectively}. In addition, the dissociation enthalpies of the mixed hydrates are higher than those of the pure hydrates {(5.55 ± 0.06) kJ ⋅ mol⁻¹ H₂O for pure TBAB hydrate and (5.30 ± 0.05) kJ ⋅ mol⁻¹ H₂O for pure TBAC hydrate}, with a maximum of (5.95 ± 0.12) kJ ⋅ mol⁻¹ H₂O recorded at approximately x_TBAB = 0.4. It was therefore suggested that the crystal distortion in (TBAB + TBAC) mixed hydrates, caused by replacing water molecules by both bromide and chloride anions, was smaller than that observed for each pure hydrate. Consequently, the hydration numbers in the mixed hydrates were hypothesized to be slightly higher than those of the pure hydrates.     

Thermal expansion of structure-H clathrate hydrates

The temperature dependence of the unit cell parameters of two newly identified hexagonal structure clathrate hydrates of hexamethylethane (HME) and 2,2-dimethylbutane (DMB) have been measured by X-ray powder diffraction. The thermal expansion of the two distinct crystallographic axes was found to be inequivalent. However, the coefficients of cubic expansion are comparable to that in the cubic structure I and II hydrates. The larger thermal expansivity in the clathrate hydrates relative to ice is attributed to the weakening of the host lattice due to the internal pressure generated by the rattling motions of the encaged guests.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Crystal structure of tetraethylammonium fluoride-water (4/11), 4(C2H5)4N+F− · 11H2O,a clathrate hydrate containing linear chains of edge-sharing (H2O)4F− tetrahedra and bridging water molecules

Crystals of 4(C₂H₅)₄N⁺F^– · 11H₂O are orthorhombic, space groupPna2₁, witha=16.130(3),b=16.949(7),c=17.493(7) Å, andZ=4. The structure was shown to be a clathrate hydrate containing infinite chains of edge-sharing (H₂O)₄F^– tetrahedra extending parallel to thea axis. The chains are laterally linked by bridging water molecules to form a three-dimensional hydrogen-bonded anion/water framework. The ordered (C₂H₅)₄N⁺ cations occupy the voids in two open channel systems running in theb andc directions. FinalR _F =0.091 for 2278 observed MoK data measured at 22°C. Supplementary Data: relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82010 (20 pages).Dedicated to Professor H. M. Powell.     

On pseudo-clathrate hydrate structure in aqueous solutions of tetrahydrofuran

Dielectric relaxation in aqueous solutions of tetrahydrofuran is reported and the results interpreted in terms of the pseudo-clathrate hydrate model of the structure of the liquid state. The lifetime of any given pseudo-clathrate configuration is 10^–11 sec.Issued as NRCC Publication No. 17436     

Superconductivity and crystal structure of the solid solutions of Ba8−δSi46−xGex (0?x?23) with Type I clathrate structure

The solid solutions of barium containing Type I clathrate, Ba_8−δSi_46−xGe_x (0?x?23) were prepared under high-pressure and high-temperature conditions of 3 GPa at 800°C. All the solid solutions showed superconductivity, and the transition temperature (T_c) decreased from 8.0 to 2.0 K as the germanium content increased from x=0 to 23 in Ba_8−δSi_46−xGe_x. The single crystals with five different compositions were obtained and the structures, compositions, and site occupancies were determined from X-ray single-crystal analysis. A slight barium deficiency was observed at Ba1 (2a) sites for all the clathrates. The Ge atoms replaced the Si atoms at the Si3 (24k) site in the composition range of x<8, and then at the Si2 (16i) site. The crystals had a slight deficiency in the covalent (Si, Ge) network and the deficiency increased with the increase of the Ge content.     

Crystal structure analysis of the carbon tetrabromide clathrate of hexakis(phenylseleno)benzene

The 12 inclusion compound of hexakis(phenylseleno)benzene (1) with CBr₄ is trigonal, space groupR , witha=14.474(3),c=21.487(4) Å, and three host and six guest molecules in a unit cell referred to hexagonal axes. A true clathrate structure is found and in each closed cage there are two carbon tetrabromide guest molecules, orientated such that a C–Br bond of each is colinear with thec-axis of the crystal. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82002. To obtain copies, see page ii of this issue.     

Cubic structure II double clathrate hydrates with tetra(n-propyl)ammonium fluoride

A double clathrate hydrate with the composition THF·0.5(n-Pr)₄NF·16H₂O and cubic structure II (CS-II,a=17.67 Å) has been obtained. Its experimental density is 1.053±0.001 g/cm³; its melting point is 8.1°C, i.e. 3.1°C higher than that of the THF·17H₂O hydrate. The double hydrates of acetone, 1,4-dioxan, trimethyleneoxide and 1,3-dioxolane with (n-Pr)₄NF have melting points of –14.8, –5.5, –2.6 and –9.6°C, respectively. With pressure increase up to 6 kbar the melting points of the double hydrates increase monotonously in contrast to common CS-II hydrates. The friability of the structure of the hydrates (the packing coefficient) and their sensitivity to pressure (dT/dP) are compared.The results of this work have been reported at the International Seminar on Inclusion Compounds, Jaszowiec (Poland), 24–26th September 1987.     

Structure determination from powder diffraction data and thermal behaviour of layered lead nitrate oxalate hydrate, Pb2(NO3)2(C2O4).2H2O

The mixed lead nitrate oxalate, Pb₂(NO₃)₂(C₂O₄).2H₂O, has been obtained in a polycrystalline form in the course of a study on precursors of nanocrystalline PZT-type oxides. Its crystal structure has been solved from powder diffraction data collected using a monochromatic radiation from a conventional X-ray source. The symmetry is monoclinic, space group P2₁/c (No. 14), the cell dimensions are a=10.623(2) Å, b=7.9559(9) Å, c=6.1932(5) Å, β=104.49(1)° and Z=4. The structure consists of a stacking of complex double sheets parallel to (1 0 0), forming layers held together by hydrogen bonds. The sheets result from the condensation of PbO₁₀ polyhedra, in which the oxalate and nitrate groups, as well as water molecules, play a major role. The structure is discussed in terms of Pb---O distances, polyhedra shape and lead coordination, with emphasis on the dimensional polymerisation role of water molecules. The thermal behaviour of this layered compound is carefully described from temperature-dependent powder diffraction and thermogravimetric measurements. The enthalpy, Δ_rH=232(3) kJ mol⁻¹, and entropy, Δ_rS=532(8) J K⁻¹ mol⁻¹, of the dehydration reaction have been determined. The high value of Δ_rH demonstrates that the water molecules are strongly bonded in the structure. The complex decomposition proceeds through the crystallisation and decomposition of Pb(NO₃)₂(C₂O₄) into Pb(NO₃)₂ and PbC₂O₄, and, finally, various lead oxides.     

Short-Chain Mono-Alkyl β-D-Glucoside Crystals—Do They Form a Cubic Crystal Structure?

Three-dimensional liquid crystal (LC) phases, cubic LC phases, have been extensively studied as fascinating molecular assembled systems formed by amphiphilic compounds. However, similar structures have only been seen in rare instances in lipid crystal states in glycolipid crystal studies. In this study, we prepared short-chain n-alkyl β-D-glucosides (CnG) with an alkyl chain length n ranging from 4 to 6 and investigated their crystal structures. First, differential thermal analysis (DTA) and thermogravimetric analysis (TG) measurements showed the formation of hydrated crystals for C4G and C5G, respectively. Second, the crystal structures of CnG (n = 4, 5, 6) in both anhydrous and hydrated states were examined using a temperature-controlled powder X-ray diffraction (PXRD) measurement. Both hydrate and anhydrous crystals of C4G and C5G with critical packing parameters (CPPs) less than 0.33 formed cubic crystal phases. Bilayer lengths, calculated from the main diffraction peaks in each PXRD profile, depended on crystalline moisture for C5G, but no significant change was confirmed for C4G, indicating that the properties of each hydrophilic layer differ. However, C6G with a CPP of 0.42 formed a crystal structure with a modulated lamellar structure similar to C7G and C8G with similar CPP values. Thus, a glycolipid motif concept with a cubic crystal structure was demonstrated.     

The structure of CeAlO3 by Rietveld refinement of X-ray powder diffraction data

The room temperature structure of perovskite CeAlO₃ has been reinvestigated by X-ray powder diffraction. The Rietveld refinement has confirmed the tetragonal symmetry; but revealed a super cell, a=5.32489(6) Å and c=7.58976(10) Å, with the space group I4/mcm. In CeAlO₃, the distortion from the ideal cubic perovskite is caused by the cooperative tilting of the AlO₆ octahedra around the primitive cubic [001]_p-axis.     

Crystal structure of Ca2Ln3Sb3O14 (Ln=La, Pr, Nd and Y): A novel variant of weberite

The crystal structures of Ca₂Ln₃Sb₃O₁₄ (Ln=La, Pr, Nd and Y) and Ca₂Sb₂O₇ at room temperature were refined by the Rietveld method using combined X-ray and neutron powder diffraction data. Ca₂Sb₂O₇ adopts the weberite structure having the space group Imma. The structures of Ca₂Ln₃Sb₃O₁₄ are, however, neither the orthorhombic nor the tetragonal chiolite as has been suggested previously. They crystallize in the monoclinic space group I2/m11 belonging to a hitherto unknown type of deformation of the parent (orthorhombic) weberite structure.     

Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure

We have successfully synthesized a high-purity polycrystalline sample of tetragonal Li₇La₃Zr₂O₁₂. Single crystals have been also grown by a flux method. The single-crystal X-ray diffraction analysis verifies that tetragonal Li₇La₃Zr₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.134(4) Å and c=12.663(8) Å. The garnet-type framework structure is composed of two types of dodecahedral LaO₈ and octahedral ZrO₆. Li atoms occupy three crystallographic sites in the interstices of this framework structure, where Li(1), Li(2), and Li(3) atoms are located at the tetrahedral 8a site and the distorted octahedral 16f and 32g sites, respectively. The structure is also investigated by the Rietveld method with X-ray and neutron powder diffraction data. These diffraction patterns are identified as the tetragonal Li₇La₃Zr₂O₁₂ structure determined from the single-crystal data. The present tetragonal Li₇La₃Zr₂O₁₂ sample exhibits a bulk Li-ion conductivity of σ_b=1.63×10⁻⁶ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=5.59×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.54 eV in the temperature range of 300–560 K.     

Synthesis and crystal structure of the A6B5O18 perovskite-like compounds

New members of the A_nB_n−1O_3n perovskite-like family (Ba₅KNb₅O₁₈ and Sr₆Nb₄SnO₁₈ compounds) with n = 6 have been synthesized and studied by the X-ray powder diffraction. Their crystal structures were found to belong to the Ba₆Nb₄TiO₁₈-type with a = 0.57840(7) nm, c = 4.2532(5) nm and a = 0.5661(1) nm, c = 4.186(1) nm for Ba₅KNb₅O₁₈ and Sr₆Nb₄SnO₁₈, respectively. It was shown that Ba and K (A-atoms) are completely disordered in the crystal structure of Ba₅KNb₅O₁₈ compound. But Nb and Sn atoms (B-atoms) in the crystal structure of the Sr₆Nb₄SnO₁₈ compound are quite ordered with the preferred Sn⁺⁴ and Nb⁵⁺ cations localization in the center of perovskite-like block and on the boundaries of these blocks, respectively. Temperature and frequency dependencies of the real components of electric conductivity σ^I and dielectric permeability ɛ^I; specific electric conductivity at the direct current σ_dc have been obtained by the impedance spectroscopy method for Sr₆Nb₄SnO₁₈.     

The 7.25-hydrate oftert-butylamine. A semi-clathrate and complex variant of the cubic 12 Å structure type

Of the various hydrates oftert-butylamine, the title compound has been identified as the second-highest, melting incongruently at –19°C. Its crystal structure (orthorhombic, space groupPca2₁,Z=32 formula units per unit cell,a=24.80,b=16.440,c=25.29 Å) and the exact composition have been determined from X-ray diffraction at –150°C. The hydrate is a rather complex semi-clathrate, with the amine molecules not merely encaged, but also hydrogen-bonded, in a three-dimensional water host structure, which in turn is not fully four-connected. Nevertheless, it bears a clear relationship to the basic and genuine clathrate-hydrate cubic 12 Å type.Dedicated to Professor G. A. Jeffrey on his 80th birthday. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82196 (8 pages).Part 14 of the seriesHydrates of Weak and Strong Bases; for part 13 see [1].     

Crystal structure of the monoclinic and cubic polymorphs of BiMn7O12

The complex perovskite BiMn₇O₁₂ occurs with two polymorphic structures, cubic and monoclinic. Currently their crystal structures are investigated with high-resolution synchrotron powder X-ray diffraction at room temperature. Rietveld analysis reveals unusual behavior for, respectively, the oxygen and bismuth atoms in the monoclinic and cubic phases. Bond valence calculations indicate that all the Mn atoms in both the phases are in trivalent state. Possible roles of the 6s² lone-pair electrons of Bi³⁺ in BiMn₇O₁₂ are discussed in comparison with the LaMn₇O₁₂ phase that is isomorphic to monoclinic BiMn₇O₁₂. Multiple roles of the lone-pair electrons are revealed, causing (i) A-site cation deficiency, (ii) octahedral tilting, (iii) A-site cation displacement, and (iv) Mn³⁺ Jahn-Teller (JT) distortion. Relationships between the monoclinic and cubic phases are discussed with emphasis on the MnO₂ and MnO₆ local structural aspects. All Mn atoms in the monoclinic polymorph have distorted coordination consistent with JT-active Mn(III) high spin, whereas for the cubic polymorph, the B-site Mn atoms show regular octahedral coordination.     

Ab initio structure determination via powder X-ray diffraction

Structure determination by powder X-ray diffraction data has gone through a recent surge since it has become important to get to the structural information of materials which do not yield good quality single crystals. Although the method of structure completion when once the starting model is provided is facile through the Rietveld refinement technique, the structure solutionab initio is still not push-button technology. In this article a survey of the recent development in this area is provided with an illustration of the structure determination of α-NaBi₃V₂O₁₀.     

黄芩苷的晶体结构研究

用乙醇-水溶液体系从黄芩中提取黄芩苷, 纯化得黄芩苷纯度为95.2%. 在步长0.0167°, 停留时间7.62 ms条件下对其粉末进行扫描, 获得较高质量粉末衍射谱图. 在运用密度泛函方法获得其分子低能量构象基础上, 采用直接空间搜索法解析其晶体结构, 得该样品属于单斜晶系, P21空间群, 晶胞参数为: a＝13.02 Å, b＝17.22 Å, c＝4.49 Å, α＝ γ＝90°, β＝95.40°, Z＝2, V＝1016.1 Å3. 结合自然键轨道理论分析, 表明结构中苷元部分杂环可能具有芳香性. 红外振动光谱结果发现, 共轭效应导致了羰基伸缩振动频率红移, 氢键的存在导致了羟基伸缩振动频率红移. 由分子间氢键的影响, 预测了其晶体生长特性.     

Crystal structure and electron density in the apatite-type ionic conductor La9.71(Si5.81Mg0.18)O26.37

Crystal structure and electron density in the apatite-type ionic conductor La_9.71(Si_5.81Mg_0.18)O_26.37 have been investigated at 302, 674 and 1010 K by Rietveld refinement and a whole-pattern fitting approach based on the maximum-entropy method (MEM) using synchrotron X-ray powder diffraction data. Second harmonic generation measurements indicated that the space group of this material is centrosymmetric. Among the possible hexagonal groups P6₃/m, P6₃ and the former is correct for La_9.71(Si_5.81Mg_0.18)O_26.37. Rietveld refinements suggested an oxygen interstitial site (0.03,0.15,0.85) near the hexagonal axis. MEM analyses revealed that the Si_0.97Mg_0.03 atom has covalent bonds with four adjacent oxygen atoms to form a tetrahedron. The oxygen O4 atom located at the 2a site (0.0,0.0,1/4) exhibited large atomic displacement parameters along the c axis and electron density mapping also indicated the wide distribution consistent with migration of oxygen ions in this direction.