Showing papers on "Orthorhombic crystal system published in 1974"
TL;DR: Stroud et al. as mentioned in this paper reported the progress of analysis from the low-resolution (5 A) model, which shows the general molecular architecture, through high-resolution density maps, from which a model of the enzyme was built.
272 citations
TL;DR: In this paper, a gasketed diamond-anvil cell was used to make optical and x-ray observations of polyethylene at high pressures and temperatures using a diamond anvil cell, which confirmed the existence of the high pressure phase previously postulated by Bassett and Turner.
Abstract: Optical and x‐ray observations of polyethylene have been made at high pressures and temperatures using a gasketed diamond‐anvil cell. The experiments confirm the existence of the high‐pressure phase previously postulated by Bassett and Turner. The new phase is hexagonal, with orthohexagonal lattice parameters of a = 8.46 A and b = 4.88 A. Comparison with the previously measured volume change indicates that there is a decrease in the c dimension to 2.45 A per ethylene unit in transforming from orthorhombic to hexagonal structures. The likely implication is that the molecules in the hexagonal phase do not have an all‐trans conformation. Chain‐extended growth is the result of crystallization from the melt into the hexagonal phase, whereas chain‐folded growth is the familiar process of melt crystallization.into the orthorhombic phase. Chain‐extended lamellae are observed to grow outwards behind a growing edge with a permanent narrowed profile, showing that the lamellar thickness is determined in a region extending several microns behind the growth front.
254 citations
TL;DR: In this paper, a survey is given on lattice data and structure types of 40 compounds of the type Cu2II-II-IV-S4(Se4) with II = Mn, Fe, Co, Ni, Zn, Cd, Hg and IV = Si, Ge, Sn.
231 citations
TL;DR: In this paper, both natural and synthetic pyrope has been compressed to pressures above 300 kbar in a diamond-anvil press and heated to temperatures above 800°C by a continuous YAG laser.
Abstract: Both natural and synthetic pyrope have been compressed to pressures above 300 kbar in a diamond-anvil press and heated to temperatures above 800°C by a continuous YAG laser. After quenching and release of pressure, X-ray diffraction shows that the natural pyrope transforms to a single phase with perovskite-like structure whereas the synthetic pyrope disproportinates into a mixture of MgSiO3 (perovskite) plus Al2O3 (corundum). The orthorhombic perovskite lattice parameters for the natural pyrope are ao = 4.816 ± 0.004 A, bo = 4.973 ± 0.004 A, and co = 6.997 ± 0.006 A with Z = 4. The orthorhombic cell dimensions for MgSiO3 (perovskite) are ao = 4.818 ± 0.005 A, bo = 4.869 ± 0.005 A, and co = 6.906 ± 0.007 A with Z=4. The calculated density of MgSiO3 perovskite is thus 4.12 g/cm³, or 3.7% denser than the isochemical mixed oxides. These experiments are the first demonstration of static high pressure phase transformations in silicate garnets and the first examples of a pure silicate existing in the perovskite structure.
208 citations
TL;DR: In this paper, the phase transition at 130°C is caused by condensation of the M 3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensing of the doubly degenerate R 25 -like mode (Z 9 mode) at the Z point of a tetragonal Brilloupin zone.
Abstract: Structural phase transitions in perovskite-type CsPbBr 3 have been investigated by neutron diffraction method. Phase transitions occur at 88°C and 130°C, which are respectively second and first order. The phase transition at 130°C is caused by condensation of the M 3 mode at the M point of the cubic Brillouin zone, while the one at 88°C results from condensation of the doubly degenerate R 25 -like mode ( Z 9 mode) at the Z point of the tetragonal Brillouin zone. Group theoretical considerations based on these results reveal that the crystal trans-forms from cubic perovskite structure (O h 1 - P m 3 m ) to tetragonal D 4h 5 - P 4/ m b m at 130°C and further to orthorhombic D 2h 16 - P m b n at 88°C. Possible atomic displacements induced at the phase transitions are obtained from the eigenvectors of the condensing modes.
204 citations
192 citations
TL;DR: In this article, synthesis methods for cesium tin(II) trihalides via aqueous solution and from the melts of anhydrous halides, which ensure freedom from oxidation and the effects of traces of water, are described.
165 citations
TL;DR: In this article, the paraffin C33H68 undergoes three solid solid solid phase transitions between room temperature and the melting point, and the orthorhombic modification (A) stable at low temperatures is followed by two different monoclinic forms (B: 54.5°C < T < 65.6°C; C: 65.5C
Abstract: The paraffin C33H68 undergoes three solid‐solid phase transitions between room temperature and the melting point. The orthorhombic modification (A) stable at low temperatures is followed by two different monoclinic forms (B: 54.5°C < T < 65.5°C; C: 65.5°C < T < 68°C) and by the ``rotator phase'' (D: 68°C < T < Tf=71.8°C). Defect structures as they occur in the three high temperature modifications were determined with the aid of small angle x‐ray scattering experiments. A method is applied which permits us to decide about the presence of different types of defects after a measurement of the absolute intensities of the 00l reflections. It was found that each one of the modifications B, C, D is characterized by a specific type of disorder. One observes orientational disorder of the skeletal planes in modification B, a superposition of longitudinal shifts of the chains in modification C, and an additional introduction of intrachain defects in modification D.
162 citations
TL;DR: The phase formulated previously as BaTi 3 O 7 is shown to be Ba 4 Ti 13 O 30 based on structural and density considerations, phase equilibria, and single crystal and powder X-ray diffraction data as mentioned in this paper.
158 citations
TL;DR: In this article, the symmetries and splittings of the uppermost valence bands in orthorhombic and chalcopyrite AgIn${\mathrm{S}}_{2}$ have been determined from electroreflectance measurements on oriented crystals using polarized radiation.
Abstract: The symmetries and splittings of the uppermost valence bands in orthorhombic and chalcopyrite AgIn${\mathrm{S}}_{2}$ have been determined from electroreflectance measurements on oriented crystals using polarized radiation. The principal features of the valence-band structures in both phases are dominated by the deviations of the lattice constants from ideal wurtzite and chalcopyrite, respectively. On the other hand, the small splitting of the lowest two valence bands in orthorhombic AgIn${\mathrm{S}}_{2}$ is attributed to the "wurtzite-like" potential. The lowest band gaps at 300 \ifmmode^\circ\else\textdegree\fi{}K in the orthorhombic and chalcopyrite phases are 1.98 and 1.86 eV, respectively.
132 citations
TL;DR: In this paper, the crystal structure of the red form of 2,2′-bipyridyl (PtCl2) was determined from X-ray diffractometer data.
Abstract: The crystal structure of the red form of [(2,2′-bipyridyl)PtCl2] has been determined from X-ray diffractometer data. Crystals are orthorhombic, space group Cmcm, with a= 17·666(2), b= 9·086(1), c= 6·803(1)A, Z= 4. The structure was derived from 554 intensities, by Patterson and Fourier methods, and refined by least squares to R 0·024. The structure comprises layers of monomeric molecules lying parallel to (001) and separated by 3·40 A. The platinum atoms are nearly superimposed and are separated by 3·45 A. There is marked pleochroism, the maximum absorption of the broad line at 520 nm occurring when the electric vector is parallel to the strings of platinum atoms.Unit-cell parameters for the yellow form are a= 15·93, b= 7·22, c= 18·38 A; space group Pbca, Z= 8.
TL;DR: In this article, the nonlinear optical properties of KNbO3 single crystals were investigated in detail in the orthorhombic phase by using YAG/Nd laser.
Abstract: Nonlinear optical properties of KNbO3 single crystals were investigated in detail in the orthorhombic phase by using YAG/Nd laser. The refractive indices for some laser wavelengths were measured. The nonlinear optical coefficients of this crystal at room temperature were obtained by means of the Maker fringes technique. Results were as follows: d31=35, d32=-40 and d33=-61 relative to d11 of quartz. These values are comparable with those of Ba2NaNb5O15. The temperature dependences of dij coefficients were measured. Critical and noncritical phase-matching conditions were determined. At room temperature, critical phase-matching was possible for d31 and d32 coefficients. The observed phasematching angles were found to be in good agreement with the values calculated from the refractive index data.
TL;DR: The crystal structure of triphenylborane has been determined in this article, and the compound crystallizes in the orthorhombic space group Pbcn with Z = 4 and a = 17.265, b = 10.355, c = 8.03 A.
TL;DR: In this paper, the structure of the low temperature orthorhombic phases β1, β2 (indistinguishable on the basis of powder photographs) has been determined from a single crystal X-ray analysis, the most probable space group of these phases is Bb21 m, the mirror planes required of the molecules perpendicular to their length being obtained by disordering.
Abstract: The n-eicosane, C20H42: n-docosane, C22H46 system has been studied by X-ray crystallo-graphic and calorimetric techniques. The phase diagram exhibits no less than six distinct solid phases. The two terminal solid solutions, γ1 and γ2 are triclinic in keeping with the pure components. An orthorhombic phase, β0, exists at all compositions just below the solidus and has a centered (C or F) lattice. The structure of the low temperature orthorhombic phases β1, β2 (indistinguishable on the basis of powder photographs) has been determined from a single crystal X-ray analysis. The most probable space group of these phases is Bb21 m, the mirror planes required of the molecules perpendicular to their length being obtained by disordering.
TL;DR: In this article, the formation of chain-extended polyethylene correlates with the occurrence of two exothermic processes (unresolved below 4 kbar) during crystallization, which leads to the hypothesis that whereas chain folded crystallization occurs by direct transformation from melt to orthorhombic solid, chain extended polyethylenes results when the intermediate phase intervenes in crystallization.
Abstract: The melting and crystallization of polyethylene have been examined in the pressure range 0 to 6 kbar. Volumetric and thermal measurements show that fractionated polymer melts and crystallizes in two clearly resolved stages above 4 kbar. It is suggested that these are due to the formation of a new, high pressure phase of polyethylene with a triple point at ∼ 3 kbar. This phase is intermediate in specific volume and entropy between orthorhombic solid and melt, but closer to the latter, and is either liquid-crystalline or solid. It is shown further that the formation of chain-extended polyethylene correlates with the occurrence of two exothermic processes (unresolved below 4 kbar) during crystallization. This leads to the hypothesis that whereas chain folded crystallization occurs by direct transformation from melt to orthorhombic solid, chain-extended polyethylene results when the intermediate phase intervenes in crystallization. This hypothesis appears capable of accounting for most of the known f...
TL;DR: In this paper, single crystals of lithium neodymium ultraphosphate (probably LiNdP4O12) were grown by the Kyropoulos method from a melt.
Abstract: Single crystals of lithium neodymium ultraphosphate (probably LiNdP4O12) were grown by the Kyropoulos method from a melt. The crystal belongs to the orthorhombic system with space group Imma (D2h28), and the Nd content is 4.37×1021 cm−3. Fluorescence and optical absorption were examined. The lifetime of 4F3/2 levels was measured to be 120±10 μs. The effective emission cross section at the line peak (1.047 μ) was found to be (9.0±0.4)×10−19 cm2, which is very close to that of Nd:YAG.
TL;DR: In this article, optical birefringence, X-ray and neutron diffraction methods with single crystals were used to study the structural phase transitions of the perowskite-type layer structures of (CH3NH3)2MeCl4 with Me=Mn, Fe.
TL;DR: In this article, a least square refinement of these data did not converge and the thermal motions and orientations of the ammonium groups at low temperature (10 and 78°K) were determined by means of a constrained refinement which treated ammonium group as a rigid body; these results were compared with the results obtained by the conventional least square method.
Abstract: The crystal structure of ammonium perchlorate has been studied at 298, 78, and 10°K by means of neutron diffraction. The NH4ClO4 crystal has an orthorhombic unit cell, space group Pnma, with four formula units per cell at all three temperatures. The unit cell dimensions at 298°K are a=9.20, b=5.82, c=7.45 A, as determined previously; at 78°K, a=9.02, b=5.85, c=7.39 A; and a=8.94, b=5.89, c=7.30 for 10°K. Initial positions of H atoms were determined from a difference Fourier map of the room temperature data. However, a least‐square refinement of these data did not converge. The thermal motions and orientations of the ammonium groups at low temperature (10 and 78°K) were determined by means of a constrained refinement which treated the ammonium group as a rigid body; these results are compared with the results obtained by the conventional least‐squares method. Both refinements show that the librational motions about one of the three principal axes have particularly large rms amplitudes: 21° for the 10°K structure and 30° for the 78°K structure. Each ammonium group is surrounded by 10 oxygen atoms with short N ⋯ O distances ranging from 2.9 to 3.25 A. The ClO4− group and NH4+ group each have essentially ideal tetrahedral structure. They are linked together by N–H ⋯ O type hydrogen bonds, one for each hydrogen, to form a three‐dimensional network. Examination of the rms amplitudes for libration and the hydrogen bonding of the NH4+ ions indicates that two of the four hydrogens are bound identically, one hydrogen is bound more rigidly, and the fourth more weakly. These results suggest that the rotational motions of the ammoniums are quite complex even at 10°K.
TL;DR: In this article, an orthorhombic unit-cell has been proposed to account for the observed reflections, the cell constants are: a = 21·48 ± 0·02 A ; b = 11·55 ± 0.03 A ; c = 7·096 ± 0 ·003 A.
TL;DR: In this article, the molecular and crystal structures of the synthesized racemic poly-(β-ethyl-β-propiolactone) [CH(C2H5)CH2COO)n were studied by X-ray diffraction.
Abstract: The molecular and crystal structures of the synthesized racemic poly-(β-ethyl-β-propiolactone) [—CH(C2H5)CH2COO—]n were studied by X-ray diffraction. The unit cell is orthorhombic, P212121(D24), with a=9.32A, b=10.02A, and c(fiber axis)=5.56A; two molecules pass through the unit cell. The molecule has a conformation shown by the Fischer projection for the case of the rectus polymer, giving a left-handed (2/1) helix: the O—C bond is nearly skew, 136°, and the C—C(O) bond is far from the gauche form, −21°. The internal rotation angle about the sequence O—CH—CH2—CH3is 64°. From the results of the X-ray analysis, it may be concluded that (1) the crystalline polymer prepared from the racemic monomer, β-ethyl-β-propiolactone, is isotactic and (2) the bulk sample of this racemic polymer consists of two kinds of crystallites, one composed only of left-handed helices of rectus polymer chains, the other only of right-handed helices of sinister polymer chains.
TL;DR: In this paper, a revised phase equilibrium diagram is presented, showing that Pb3SiO5 is thermodynamically stable relative to Pb2SiO4 and 4PbO. The new phases include Pb5Si8O21, an orthorhombic polymorph of PbSiO3, and a high-SiO2 phase containing ∼ 60 mol% SiO2.
Abstract: Compound formation in the system PbO-SiO2 was studied; the results are contrasted with those previously reported. Fifteen binary phases, 4 of which had not been reported, were prepared in the present study. The new phases include Pb5Si8O21, an orthorhombic polymorph of PbSiO3, Pb3SiO5, and a high-SiO2 phase containing ∼ 60 mol% SiO2. It was shown that Pb3SiO5 is thermodynamically stable relative to Pb2SiO4 and 4PbO.SiO2 below 640±5°C. A revised phase equilibrium diagram is presented.
TL;DR: The crystal and molecular structure of a series of oxo-molybdenun di-n-propyldithiocarbamates have been studied by X-ray diffraction methods as discussed by the authors.
Abstract: The crystal and molecular structure of a series of oxo-molybdenun di-n-propyldithiocarbamates have been studied by X-ray diffraction methods. The compounds studied were di(oxo-di-n-propyldithiocarbamato) molybdenum VI (A), μ-oxo-bis-(di(oxo-di-n-propyldithiocarbamato) molybdenum V) (B), and oxo-bis-di-n-propyldithiocarbamato molybdenum IV (C). Compound A is orthorhombic, space group Pbca, with eight molecules in a unit cell having dimensions a =16.577(3) A, b =18.771(3) A and c =13.702(2) A. The molybdenum V compound (B) crystallizes in the orthorhombic non-centrosymmetric space group Pca21, with four molecules in a unit cell having parameters a =19.605(6) A, b =13.491(3) A and c =16.342(4) A. The last compound (C) is triclinic, space group P1 with two molecules in the unit cell. Its crystal parameters are a =10.183(6) A, b =13.012(9) A, c =9.208(5) A' a =71.57(5)°, β =105.81(5)° and γ =81.82(5)°. The intensities were collected by counter techniques using MoKa radiation. All structures were refin...
TL;DR: In this paper, single crystals of boron-rich binary B-C compounds were prepared by CVD on Ta and BN substrates, in which interesting compositional and structural effects were frozen in.
TL;DR: In this article, Patterson and Fourier methods were used to solve the crystal and molecular structures of the title compounds of Ru(NH3)5(NO) by using diffractometer data, and the structure was refined to R 0·047 (971 observed reflections).
Abstract: The crystal and molecular structures of the title compounds have been reinvestigated and solved by Patterson and Fourier methods by use of diffractometer data. Crystals of [Ru(NH3)5(NO)]Cl3,H2O are orthorhombic, space group Pn21a, a= 11·864(7), b= 6·878(5), c= 14.l92(9)A, Z= 4. The structure was refined by full-matrix methods to R 0·047 (971 observed reflections). The ruthenium ion is octahedrally co-ordinated by six nitrogen atoms, with Ru–NO 1·770(9), N–O l.172(14), and Ru–NH3 2·017–2·133 A. Because of correlation effects only two distances (cis-Ru–NH3) are regarded as true measurements of the bond lengths. Crystals of trans-[Ru(OH)(NH3)4(NO)]Cl2 are monoclinic, space group C2/m(assumed), a= 11·422(3), b= 7·365(2), c= 1l·157(3)A, β= 109·09(2)°, Z= 4. Full-matrix refinement for 2398 observed reflections, gave R 0·031. The ruthenium ion is octahedrally co-ordinated with Ru–NO 1·735(3), Ru–NH3 2·099(3) and 2·106(3), Ru–OH, 1·961 (3), N–O l·159(5)A, and Ru–N–O 173·8(3)°. The Ru–OH distance is shorter than expected from covalent radii considerations.
TL;DR: Inelastic neutron scattering measurements on single-domain orthorhombic KNbO3 at room temperature show no evidence of overdamped scattering, in contrast with previous neutron results in the cubic phase as discussed by the authors.
Abstract: Inelastic neutron scattering measurements on single-domain orthorhombic KNbO3 at room temperature show no evidence of overdamped scattering, in contrast with previous neutron results in the cubic phase. As in the case of KTaO3, the observed diffuse x-ray planes are shown to correspond to a highly anisotropic transverse acoustic branch with an unusually flat dispersion in the 'soft' plane. In addition, a well defined optic branch with a polarization related to the orthorhombic-rhombohedral phase transition is seen to become soft as q to 0 in the soft plane. A mode-eigenvector determination confirms the x-ray result that essentially only one atom (niobium) moves in the flat transverse acoustic mode. The connection with other experimental results and with the independent chain model is discussed.
TL;DR: In this article, a probable model for the structure of the orthorhombic A -type neodymium hydroxycarbonate is presented, which relies on an optical determination of the site symmetry of OH−, CO2−3, and Nd3+ atoms from infrared and visible absorption spectra, together with a computation of 50 X-ray diffraction lines intensities of a high resolution Guinier powder pattern.
TL;DR: In this article, the structures of AuTeCl and AuTe2I have been determined, which consist essentially of corrugated two-dimensional nets of gold and tellurium atoms, with interleaving halogen atoms.
TL;DR: The crystal structure of polyisobutylene was determined by x-ray analysis as discussed by the authors, which is essentially an (8/3) helix, but deviates appreciably from the exact 8/3 helix symmetry.
Abstract: The crystal structure of polyisobutylene was determined by x-ray analysis. The orthorhombic cell, with a = 6.88 A, b = 11.91 A, c (fiber axis) = 18.60 A (space group: P212121 − D), contains two molecular chains each consisting of eight monomeric units in the fiber identity period. The chain conformation is essentially an (8/3) helix, but deviates appreciably from the exact (8/3) helix symmetry. The symmetry of the molecular chain is only a twofold screw axis in exact sense, and a crystallographic asymmetric unit consists of four monomeric units. The torsional angles are
where M denotes the methyl group. The averaged skeletal CCH2C and CCM2C bond angles are 128° and 110°, respectively. The large CCH2C bond angles may be due to steric respulsion between the adjacent methyl groups, giving intramolecular distances larger than 3.09 A.