Showing papers on "Orthorhombic crystal system published in 1983"

Vanadium pentoxide gels

Abstract: Vanadium pentoxide gels consist of entangled polymeric fibers. Electron diffraction experiments have shown that these fibers are actually arranged as flat ribbons about 103 A long, 102 A wide, and 10 A thick. A two-dimensional order is observed along these ribbons which does not vary upon swelling. The two-dimensional cell parameters a = 27.0 A and b = 3.6A suggest that the organization within the fibers is closely related to the lamellar structure of orthorhombic V2O5. The fiber is made of basic blocks containing 10 vanadium atoms along the a direction. These blocks seem to be linked together by water molecules which are strongly bonded to the structure and cannot be removed without crystallization of the xerogel into orthorhombic V2O5.

The crystal structure of CaGeO 3 perovskite and the crystal chemistry of the GdFeO 3 -type perovskites

Abstract: A single-crystal X-ray study indicates that the perovskite form of CaGeO 3 is orthorhombic (Pbnm), and isotypic with GdFeO 3 , although it was previously reported as a cubic form. The crystal structure is close to that of Sm-AlO 3 . Although the unit cell of CaGeO 3 is pseudocubic, the structure itself is very distorted through the tilting and disortion of polyhedra. The oxygen polyhedra are less tilted and less distorted than those of other GdFeO 3 -type perovskites. The structural deformation of the GdFeO 3 -type perovskite is determined primarily by the size-ratio of two kinds of cation occupying A and B sites. Some structural characteristics such as O(2)-O(2)-O(2) and A-O(1)-B angles and bond-length disortions exhibit systematic relationships as a function of the observed tolerance factor which is newly defined here. A strong correlation between the Goldschmidt tolerance factor and the observed tolerance factor has made possible some predictions for GdFeO 3 -type perovskites.--Modified journal asbstract.

Intermediate valence state of cerium in CeNi

Abstract: Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3 , an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.

Polymer single crystals of poly(4‐hydroxybenzoate). II. A contribution to crystal structure and polymorphism

Abstract: Electron diffraction has been used to investigate the structure of a wide range of as-polymerized crystals of poly(4-hydroxybenzoate) [systematic name: poly(1,4-oxybenzoyl)]. The chemical composition and the degree of polymerization (DP) have been varied and some samples have been thermally treated. At room temperature two crystalline modifications with orthorhombic unit cells coexist. The chains adopt a 21 helical conformation in both forms, but there are differences for oligomer and polymer crystals. Oligomers of low DP have an extended chain-conformation, whereas in polymers a shortening of the repeat distance along the chain is observed as a function of both the DP and the crystallization conditions. From the most extensive data sets we have derived the lattice parameters a = 7.52, b = 5.70, and c = 12.49 A for polymer crystals of phase I, and the subcell parameters for oligomer crystals of phase II a = 3.77, b = 11.06, and c = 12.89 A. Both phases contain two chains per unit cell. In addition to modifications I and II several defect structures exist the unit cells of which contain more than two chains. At temperatures which depend on the degree of polymerization, a phase transition to a third modification takes place. The large difference between the densities of phase III as compared to both phase I and II suggests that torsional degrees of freedom exist in phase III which allow a certain mobility of the phenyl and ester groups. This mobility enables the end groups of adjacent layers in interlamellar regions of oligomer crystals to undergo transesterification reactions and therefore to increase the molecular weight of the samples.

Molecular dynamics and structures of amine boranes of the type R3N.BH3. I. X-ray investigation of H3N.BH3 at 295K and 110K

Abstract: The results of an X-ray diffraction investigation of powdered H3N.BH3 are reported. A first-order rotational order-disorder phase transition has been observed at 225K. In the high-temperature tetragonal phase the space group permits twelvefold reorientations of GH3 and NH3 groups. The cell parameters are linear functions of temperature in this phase. The low-temperature structure is orthorhombic. The length of the c axis is independent of temperature while the a axis decreases non-linearly in length as the temperature is increased. The length of the b axis increases non-linearly with increasing temperature. With the exception of the c axis which changes very little in length at the phase transition, all cell dimensions as well as the volume of the unit cell show drastic changes at the phase transition. The temperature dependences of the cell parameters are explained qualitatively in terms of the reorientations and relative orientations of the BH3 and NH3 groups.

X-ray study of the low-temperature phase transitions in LiKSO4

Abstract: Lattice parameters of LiKSO4 have been measured in the temperature range 155–300 K with an X-ray single-crystal Bond diffractometer. Below 180 K LiKSO4 transforms on cooling into an orthorhombic phase. Above this temperature there is a broad range of an intermediate hexagonal phase which depends on the real structure of the sample and differs considerably on cooling and on heating. The anomalies in the physical properties of LiKSO4 reported in the literature have been explained. The crystal structure of the low-temperature and intermediate phase, as well as the mechanism of the phase transitions, are suggested and discussed.

The crystal structure and some properties of ReSi2

Abstract: ReSi2 was found not to be isotypic with MoSi2 but to crystallize in space group Immm (no. 71) with the orthorhombic cell a = 3.128(1) A, b = 3.144(1) A, c = 7.677 (2) A and Z = 2. ReSi2 is isopuntal (identical space group and site sets but different atomic coordination) with MoPt2 although the distortions of the MoSi2 type are very weak. Measurements of the electrical resistance and the optical reflectivity characterize ReSi2 as a semiconductor with an energy gap of about 0.2 eV. This property appears to require localized Re-Re bonds which, however, were not found.

Residual stresses in cubic materials with orthorhombic or monoclinic specimen symmetry: influence of texture on ψ splitting and non‐linear behaviour

Abstract: A quantitative formulation is given of the X-ray diffraction determination of (elastic) lattice deformation in a direction determined by two angles ψ and ϕ with respect to the specimen frame: the so-called sin2ψ method giving full credit now to crystallographic texture. In most cases of practical X-ray stress analysis, (residual) stresses are evaluated by the traditional sin2ψ, method using quasi-isotropic X-ray elastic constants depending on the lattice plane {hkl} chosen for the measurement but not on the directions ϕ and ψ. For the case of a textured specimen, however, the single-crystal elastic bitensor components should be coupled to the orientation distribution function (o.d.f.). At variance with most papers published on this subject, which describe the texture using ideal texture components, a general method is developed making full use of relevant o.d.f. theory. It is demonstrated that for orthorhombic specimen symmetry no ψ splitting occurs in contrast with the case of monoclinic specimen symmetry. The theory developed is used to explain some experimental results published previously. All calculations use Reuss's theory of elasticity which neglects the mutual coupling of the crystallites.

Structure and Properties of Dichloro(L-proline)cadmium(II) Hydrate

Abstract: An X-ray diffraction study of the title complex has been carried out. The crystal is orthorhombic, with the space group P212121; Z=4, a=10.021(3), b=13.562(4), c=7.298(3) A. Block-diagonal least-squares refinements have led to the final R value of 0.035. The structure is very similar to that of dichloro(4-hydroxy-L-proline)cadmium(II), which has a one-dimensional polymer bridged by chlorine atoms and a carboxyl group like an infinite folding screen. The thermal behavior is, however, different from that of dichloro(4-hydroxy-L-proline)cadmium(II). The difference is likely to be due to a difference of the crystal structure, whether it contains intermolecular hydrogen bonds or not.

Molecular inclusion in functionalized macrocycles. Part 6. The crystal and molecular structures of the calix[4]arene from p-(1,1,3,3-tetramethylbutyl)phenol and its 1 : 1 complex with toluene

Abstract: The cyclic tetramer (1a) isolated from the reaction mixture of the base-catalysed reaction of p-(1,1,3,3-tetramethylbutyl)phenol and formaldehyde crystallizes from acetone without including the solvent whereas in the presence of toluene a 1 : 1 complex between (1a) and toluene is formed. Crystals of the empty phase are monoclinic, space group P21/a, a= 12.753(8), b= 38.778(16), c= 11.370(7)A, β= 101.98(9)°, Z= 4, final R value 0.092; the toluene complex is orthorhombic, space group Fmm2, a= 21.227(16), b= 21.227(17), c= 14.522(14)A, Z= 4, final R value 0.069. The macrocycle exists in the cone conformation both in the empty form and in the complex, where the guest molecule occupies intermolecular cavities of channel type in the host lattice.

Piezoelectricity and pyroelectricity in polyvinylidene fluoride: Influence of the lattice structure

Abstract: Piezoelectric and pyroelectric responses of β‐phase (Phase I) polyvinylidene fluoride are predicted for a model system of polarizable point dipoles. The model incorporates the influence of the orthorhombic crystal structure by including the dependence of the internal electric field on the lattice parameters. Strong anisotropy in the piezoelectric response under uniaxial stress is predicted as a consequence of the orthorhombic lattice structure. Predictions are found to be in reasonable agreement with room‐temperature experimental data.

The Structure of the Cyclodextrin Complex. XVI. Crystal Structure of Heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin-p-Iodophenol (1 : 1) Complex Tetrahydrate

Abstract: The crystal structure of heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin–p-iodophenol (1 : 1) tetrahydrate, C63H112O35·C6H5OI·4H2O, was determined by the X-ray method. The crystal is orthorhombic with the space group P212121, Z=4, a=14.997(2), b=21.368(2), and c=28.205(3) A. The structure was solved by the heavy atom method combined with the rigid-body least-squares technique and refined by the block-diagonal least-squares method to the final R-value of 0.094 for 3268 reflections (sinθ⁄λ<0.49). Heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin is in the shape of a truncated elliptical cone. The macrocyclic ring is remarkably distorted from a regular heptagonal structure. The iodophenyl group of the guest is inserted into the host cavity. The phenolic hydroxyl group protrudes from the cavity, forming hydrogen bonds with two water molecules. The iodine atom is located at the center of the macrocyclic ring, while the benzene ring occupies its position at the base side of the cone. The crystal is built up of columns wh...

Infrared spectra of crystalline n-alkanes. Changes observed during the phase I → phase II transition

Abstract: Detailed measurements of the changes in the infrared spectra of n-nonadceanc and n-heneicosane on transition from the orthorhombic to the so-called "hexagonal" or "rotator" phase are reported.

Superconductivity and crystal structure of a new class of ternary transition metal phosphides TT/t'P (TZr, Nb, Ta and T/t'Ru, Rh)

Abstract: A new series of superconducting ternary transition metal phosphides TT/t'P (TZr, Nb, Ta and T/t'Ru, Rh) was synthesized. A single-crystal structural determination for ZrRuP yielded an orthorhombic structure (space group, Pnam) with a final R value of 0.036. The superconducting transition temperature is strongly dependent on the electronic structure with TaRhP possessing the highest Tc of 4.7 K. The compound ZrRuP crystallizes in two modifications: the hexagonal phase has a Tc of 13.0 K and the orthorhombic structure has a Tc of 3.5 K. The relationship between the crystal chemistry and superconducting properties for these two modifications is discussed.

Balangeroite, a new fibrous silicate related to gageite from Balangero, Italy

Abstract: It is orthorhombic with a 13.85(4), b 13.58(3), and c 9.65(3)Aa; a sub-cell with c9 = c/3 is strongly evident. The X-ray powder pattern demonstrates an isostructural relationship with gageite; the strongest reflections are: 9.59(40)(110), 6.77(80)(020), 3.378(45)(410), 3.278(40)(140), 2.714(100)(050,510), 2.674(75)(150,223), and 2.516(40)(250). Suggested formula of (Mg,Fe (super 2+) ,Fe (super 3+) ,Mn (super 2+) , ) 42 Si 15 (O,OH) 90 .--Modified journal abstract.

X-ray diffraction study of the crystal structures of K2CuF4 and K2CuxZn1-xF4

Abstract: The investigation of the crystal structures of K2CuF4 and its diluted system K2CuxZn1-xF4 has been made by means of an X-ray camera and computer method. Both the pure (x = 1) and the diluted compounds were found to have two kinds of structures, an ordered and a disordered type. The ordered K2CuF4 generally shows a multi-domain structure, which means that a single domain sample is orthorhombic, D182h-Bbcm. On the contrary, the ordered K2CuxZn1-xF4 with x < ≈0.9 has a tetragonal symmetry due to the absence of a correlation between the displacement of F- ions around the body-centered Cu2+ ion with respect to those in the basal plane. Some comments concerning the magnetic structures of the pure and the diluted systems are given on the basis of the present crystallographic analyses.

Evidence for Tunneling of Charge-Density Waves in Ta S 3

Abstract: The charge-density-wave contribution to ac and dc conductivity, rectification, and harmonic mixing has been measured on a single crystal of orthorhombic Ta${\mathrm{S}}_{3}$. A modified tunneling theory based on the Zener model of charge-density-wave conduction provides a detailed and quantitative account of these experiments. This evidence strongly supports the hypothesis that macroscopic quantum effects are observed in the megahertz frequency region at temperatures as high as 200 K.

Neutron diffraction study of LaNi4Co and LaNi4CoD4

Abstract: The compound LaNi4Co is isostructural with LaNi5. The distribution of the Co atoms in the 2c and 3g special positions of the space group D6h1-P6/mmm is not random. Deuterisation yields LaNi4CoD4 with structure that belongs to the orthorhombic space group D2h19-Cmmm. The deuterium atoms occupy the 4e and 4h special position of this symmetry. The reduced deuterium content in LaNi4CoD4 as compared with that in LaNi5D6 is consistent with geometrical considerations and the different symmetries.

High-pressure optical spectra of condensed oxygen

Abstract: Optical-absorption spectra (1.5-3.5 eV) of the fluid and three solid phases of ${\mathrm{O}}_{2}$ have been recorded up to 40 GPa near 298 K. The principal feature of the spectra of the fluid and rhombohedral $\ensuremath{\beta}\ensuremath{-}{\mathrm{O}}_{2}$ is the $2^{1}\ensuremath{\Delta}_{g}\ensuremath{\leftarrow}2^{3}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$ band; in $\ensuremath{\beta}\ensuremath{-}{\mathrm{O}}_{2}$, it is polarized perpendicular to the $c$ axis of the hexagonal cell. The energies of this and other transitions among the $^{3}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$, $^{1}\ensuremath{\Delta}_{g}$, and $^{1}\ensuremath{\Sigma}_{g}^{+}$ states increase with increasing pressure. The spectra and visual observations of the transformation between orthorhombic and $\ensuremath{\epsilon}\ensuremath{-}{\mathrm{O}}_{2}$, whose structure has not been determined, suggest that $\ensuremath{\epsilon}\ensuremath{-}{\mathrm{O}}_{2}$ has a layered structure. Absorption spectra of orthorhombic and $\ensuremath{\epsilon}\ensuremath{-}{\mathrm{O}}_{2}$ for light propagating perpendicular to the layers are dominated by a moderately strong band which moves from the uv through the visible with increasing pressure and a very strong ultraviolet band. These bands can be assigned as excitations from ${\ensuremath{\pi}}^{4}\ensuremath{-}{\ensuremath{\pi}}^{*2}$ states to ${\ensuremath{\pi}}^{3}\ensuremath{-}{\ensuremath{\pi}}^{*3}$ states which are mixed with intermolecular-charge-transfer excitations. Both bands move toward longer wavelengths at high pressures. At pressures above 13 GPa, absorption features for light propagating in the planes of the $\ensuremath{\epsilon}$-phase structure can be assigned as $2^{1}\ensuremath{\Delta}_{g}\ensuremath{\leftarrow}2^{3}\ensuremath{\Sigma}_{g}^{\ensuremath{-}}$. However, between 10 and 13 GPa, this feature is absent, and the pressure dependence of the Raman spectrum also is unusual. Both observations suggest that, between these pressures, the molecules reorient with respect to the crystallographic axes.

Magnetic Properties and Crystal Distortion of Hexagonal Mn 3 Ga

Abstract: Magnetization measurements and X-ray diffraction experiments have been made on e -Mn 3 Ga (71.4 at%Mn) having a triangular spin structure with a weak ferromagnetic component below 460 K. It has been found that the D0 19 -type hexagonal structure is slightly distorted to an orthorhombic structure below 170 K, and that the transition is accompanied by a steep increase of spontaneous magnetization. The crystal distortion may enhance the deviation from the ideal triangular spin configuration.

Structure and stereochemistry in f-block complexes of high co-ordination number. Part 5. Ten-co-ordination : the crystal structures of tetrapotassium tetraoxalatouranate(IV) tetrahydrate(orthorhombic and triclinic phases), bicapped square antiprismatic and sphenocoronal stereochemistries

Abstract: The title compond, K4[U(C2O4)4]·4H2O, has been prepared in two crystalline modifications; the crystal structures of both have been determined by single-crystal X-ray diffraction methods at 295 K. One form is orthorhombic, space group Fdd2, with unit-cell dimensions a= 30.09(2), b= 22.18(1), c= 12.747(6)A, and Z= 16; R= 0.043 was obtained for 3 127 ‘observed’ reflections; the other form is triclinic, space group P, with unit-cell dimensions a= 9.595(5), b= 12.998(4), c= 10.329(5)A, α= 115.47(3), β= 80.49(4), γ= 113.09(13)°, and Z= 2; R= 0.034 for 6 017 independent reflections. In each case, the formula unit is the asymmetric unit; some disorder occurs among the cations in the orthorhombic phase. In both structures the uranium atom is ten-co-ordinate, one of the ligands in each case being bridging and linking the uranium atoms in an infinite one-dimensional polymeric array. For the orthorhombic phase U–O range between 2.37(1) and 2.58(1)A, while in the triclinic form the range is 2.406(5)–2.641(4)A. The co-ordination stereochemistries about the uranium atoms are examined in terms of a repulsion model.