Showing papers on "Orthorhombic crystal system published in 1993"
TL;DR: In this article, a trivalent lanthanide and (fsaaep) 2- is the ligand deriving from 3-(N-2-(pyridylethyl)formimidoyl)salicylic acid.
Abstract: The hexanuclear clusters [Ln 2 C 4 (fsaaep) 4 (NO 3 ) 6 ].0.5(CH 3 OH.H 2 O) (abbreviated as [Ln 2 Cu 4 ]) have been synthesized; Ln is a trivalent lanthanide and (fsaaep) 2- is the ligand deriving from 3-(N-2-(pyridylethyl)formimidoyl)salicylic acid. The crystal structure of the compound with Ln=Pr has been solved. This compound crystallizes in the orthorhombic system, space group P2 1 2 1 2 1 . The lattice parameters are a=18.746(4) A, b=24.196(4) A, c=17.053(4) A, and Z=4
348 citations
TL;DR: Progressive departure from the ideal fluorite structure by reducing the Zr-O coordination and dilating or distorting the local ZR-Zr network is seen to be correlated to increased phase stability but not to long-range crystal symmetry.
Abstract: Characteristic extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra of zirconia polymorphs (monoclinic, orthorhombic, tetragonal, and cubic) have been identified at 10 K. For the host cations, these spectra are phase dependent but dopant independent because the dopant perturbation is short ranged. In the tetragonal structure, the first Zr-O shell is bifurcated into two tetrahedra, with outer O ions loosely bound and severely distorted. Although the tetragonal Zr cation network is nearly fcc and higher-order Zr-Zr coordinated shells are resolvable at distances up to 9 \AA{}, other evidence points to incoherent-distortion reflecting phase instability. In monoclinic, orthorhombic, and cubic structures, a similar Zr-O polyhedron with sevenfold coordination and a comparable bond length is observed, along with a large distortion or splitting of the next-nearest-neighbor Zr-Zr shell. Progressive departure from the ideal fluorite structure by reducing the Zr-O coordination and dilating or distorting the local Zr-Zr network is seen to be correlated to increased phase stability but not to long-range crystal symmetry.
258 citations
TL;DR: The results are described from the analyses of several known compounds starting with the crystal molecular structures as the MOLPAK search probes in the P1, P21, P 21/c, and P212121 space groups, in which the search probes were created by AM1 geometry optimization of preliminary molecular models.
Abstract: A procedure is reported for the prediction of dense crystal structures of C-, H-, N-, O-, and F-containing organic compounds in the primitive triclinic, monoclinic, and orthorhombic space groups with Z ≤ 4. The crystal environments of molecules in 242 crystal structures have been analyzed to determine the common coordination sphere pattens. This led to the development of the MOLPAK (MOLecular PAcKing) program, which uses a rigid-body molecular structure probe to build packing arrangements (possible crystal structures) in the various space groups. A MOLPAK search, which involves the investigation of all unique orientations of a central molecule and the construction of the appropriate coordination patterns about the central molecule, provides a 3-D map of minimum unit cell volume as a function of the orientation of the central molecule. MOLPAK uses a repulsion-only potential and a preset threshold to place molecules in contact with each other. The 5–10 smallest volume packing arrangements from a search are subjected to a lattice energy minimization refinement with the WMIN program to yield possible crystal structures. The results are described from the analyses of several known compounds starting with the crystal molecular structures as the MOLPAK search probes in the P1, P21, P21/c, and P212121 space groups. In addition, several examples are given in which the search probes were created by AM1 geometry optimization of preliminary molecular models. More extensive data are given in supplementary tables. © 1993 John Wiley & Sons, Inc.
219 citations
TL;DR: In this paper, the synthesis, structure, and spectroscopic and magnetic properties of three 2,2'-bipyrimidine (C 8 H 6 N 4, bpm)-containing copper(II) complexes of formula [Cu 2 (bpm)(NCO) 4 ] n (1), (Cu(bpm) 2 ] n(2) and [Cu 1 n 2, n(3) n (3) are reported.
Abstract: The synthesis, structure, and spectroscopic and magnetic properties of three 2,2'-bipyrimidine (C 8 H 6 N 4 , bpm)-containing copper(II) complexes of formula [Cu 2 (bpm)(NCO) 4 ] n (1), (Cu(bpm)(NCS) 2 ] n (2) and [Cu 2 (bpm)-(NCS) 4 ] n (3) are reported. Compounds 1 et 2 crystallize in the monoclinic system, space group C2/c and Z=4, with a=19.486, b=7.527, c=15.252A, β=137.8° for 1 and a=11.543, b=13.948, c=7.818A, β=93.26° for 2, whereas compound 3 is orthorhombic, space group Fddd, with a=12.811, b=22.473, c=24.259A, Z=16
162 citations
TL;DR: In this article, the crystal structure of NaMgF3 perovskite (Neighborite) has been studied at high temperature by X-ray powder diffraction using a position sensitive detector with a monochromatic synchroton radiation source.
158 citations
TL;DR: The crystal structure of a planar zigzag form of syndiotactic polystyrene (planar form I) obtained by annealing as-cast samples from solutions (molecular compounds with the solvents) was studied by X-rays as discussed by the authors.
153 citations
TL;DR: The structure of perovskite-like compound LaVO[sub 3] has been determined at room temperature and just above (150 K) and below (100 K) the transition as mentioned in this paper.
139 citations
TL;DR: In this paper, the structural properties of semi-crystalline high-molecular weight polyethylene and their changes with temperature were studied by analysis of Raman spectra and the results of temperature dependent measurements is indicative of a largely reversible surface melting process, the transition regions being located at the crystal surfaces.
Abstract: Structures of semi crystalline high-molecular weight polyethylene and their changes with temperature were studied by analysis of Raman spectra. Decomposition of spectra into contributions of an orthorhombic crystalline phase, a meltlike phase, and two kinds of transition regions yields the respective mass fractions. The results of temperature dependent measurements is indicative of a largely reversible surface melting process, the transition regions being located at the crystal surfaces. As-polymerized samples show a large fraction of the intermediate phase. Annealing leads to a reduction of this phase. © 1993 John Wiley & Sons, Inc.
121 citations
TL;DR: CeO2 transforms to an orthorhombic PbCl2-type structure at a pressure of about 31 GPa, accompanied by a 9.8% volume contraction as discussed by the authors.
Abstract: CeO2 transforms to an orthorhombic PbCl2-type structure at a pressure of about 31 GPa. The phase transition is accompanied by a 9.8% volume contraction. The bulk modulus of the low-pressure fluorite-type structure is 236(4) GPa. Comparisons are made with the high-pressure behaviour of UO2 and ThO2.
113 citations
TL;DR: In this paper, the oxygen concentration phase diagram of La2NiO4+δ has been constructed from room temperature X-ray powder diffraction data on specimens spanning a wide range of oxygen stoichiometries.
108 citations
TL;DR: In this article, the structural phase transition in NaMgF 3 perovskite can be modelled as a tricritical ferroelastic phase transition, and the coupling between the spontaneous strain and the octahedral tilts is in a linear-quadratic form.
TL;DR: The crystal structure of the superconductor HgBa2Ca2Cu3O8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K.
Abstract: The crystal structure of the superconductor HgBa2Ca2Cu3O8+δ has been determined by neutron powder diffraction techniques at room temperature and at 10 K. The compound crystallizes with the symmetry of space group P4/mmm and lattice parameters a=3.8501(1), c=15.7837(9) A . The structure is described by the sequence of layers: …[(CuO2)(Ca)(CuO2)(BaO)(HgOδ(BaO)(CuO2)(Ca)]… The compound, prepared by a solid-state reaction between HgO and the precursor Ba2Ca2Cu3Ox at a pressure of 18 kbar and 880°C, has an oxygen composition corresponding to δ=0.41 (2). This extra oxygen is located on the layer of the mercury atoms at the sites 1 2 , 1 2 , 1 2 . The superconducting transition temperature for the as-prepared sample is Tc=133 K. No evidence has been found of orthorhombic distortions, nor for substitution of some of the Hg atoms by Cu. The copper and oxygen atoms of the layers (CuO2) are coplanar for the Z=0 (CuO2 plane and almost perfectly coplanar for the others. No phase transition occurs down to 10 K.
TL;DR: In this paper, the lattice constants of Fe(Phen) 2 (NCS) 2 are a=12.656(3), b=9.848(2), and c=16.597(4) A.
Abstract: Crystal structures of Fe(Phen) 2 (NCS) 2 , form II (phen=1,10-phenanthroline) and Fe(Btz) 2 (NCS) 2 (Btz=2,2'-bi-4,5-dihydrothiazine) were determined at room temperature by X-ray diffraction, under a pressure of P≃1.0 GPa. For both compounds, the space group is orthorhombic, Pbcn (Z=4). Lattice constants of Fe(Phen) 2 (NCS) 2 are a=12.656(3), b=9.848(2), and c=16.597(4) A. Data were refined (159 parameters) to R w =0.043 for 631 observed reflections (F 0 >4σ(F 0)); for Fe(Brz) 2 (NCS) 2 , a=12.839(4), b=10.454(3), and c=16.362(4) A, with R w =0.056 (154 parameters for 662 observed reflections)
TL;DR: In this article, the Monte Carlo simulations for the Ising spin tiangular lattice antiferromagnet were used to reproduce the observed successive transition in the case of the exchange parameters 4 J 3 <2 J 2 ∼ J 1 <0.
Abstract: Magnetic ordering in a rhombohedrally stacked triangular lattice antiferromagnet, CuFeO 2 was investigated on powder and single crystals by measurements of neutron diffraction, magnetic susceptibility, Mossbauer effect together with Monte Carlo simulations. CuFeO 2 was found to exhibit transitions at T N1 =16±0.5 K and T N2 =11±0.5 K. The low temperature phase has an orthorhombic magnetic unit cell with collinear moments along the c -axis, whereas the intermediate temperature phase is a partially disordered phase with a monoclinic magnetic unit cell where 1/5 of moments remain paramagnetic. The Monte Carlo simulations for the Ising spin tiangular lattice antiferromagnet can reproduce the observed successive transition in the case of the exchange parameters 4 J 3 <2 J 2 ∼ J 1 <0. This combination of exchange parameters gives an account for the stability of the collinear magnetic structure in CuFeO 2 .
TL;DR: The phase transformations and pressure-volume dependence of HfO 2 have been investigated at room temperature by angle-dispersive powder x-ray diffraction under high pressure to 50 GPa in a diamond anvil cell.
Abstract: The phase transformations and pressure-volume dependence of ${\mathrm{HfO}}_{2}$ have been investigated at room temperature by angle-dispersive powder x-ray diffraction under high pressure to 50 GPa in a diamond anvil cell. The phase transformation from the monoclinic I (baddeleyite) to orthorhombic phase II was observed around 10 GPa. This phase is stable up to 26 GPa where it transforms to a new phase III with another orthorhombic unit cell. At about 42 GPa, a third phase transition occurs to phase IV of tetragonal symmetry. The pressure dependences of the cell parameters and volume have been determined. The successive volume discontinuities are 2.5%, 2.5%, and 5%, respectively. The bulk moduli of all the phases have been calculated from Birch's equation of state and are discussed. The high-pressure phases were found to be metastable at normal pressure. No orthorhombic cotunnite-type structure was observed under pressure at room temperature. Although the structural properties of ${\mathrm{HfO}}_{2}$ and ${\mathrm{ZrO}}_{2}$ are similar at lower pressures, their evolutions are different above 20 GPa.
TL;DR: A new, better-ordered orthorhombic crystal form of bacteriorhodopsin was formed by heterogeneous nucleation on benzamidine by the influence of different detergents, additives and precipitants on the formation of pseudohexagonal needles.
TL;DR: In this article, an angle-dispersive X-ray diffraction was used to study anatase up to 49 GPa in a diamond anvil cell by angle dispersive diffraction with B 0 values of 59(5) and 98(9) GPa, respectively.
Abstract: TiO 2 (anatase) was studied up to 49 GPa in a diamond anvil cell by angle-dispersive X-ray diffraction. Two phase transitions were observed, the first to the orthorhombic α-PbO 2 -type phase and the second to a monoclinic phase, which appears to be baddeleyite-structured. Birch-Murnaghan equations of state were calculated for both anatase and the α-PbO 2 -type phase yielding B 0 values of 59(5) and 98(9) GPa, respectively. Accordingly, anatase is four times and the α-PbO 2 phase twice as compressible as rutile. The high-pressure, monoclinic phase, in contrast, was found to be much harder than rutile.
TL;DR: It is found that the 2.4% epitaxial compression nearly doubles the ferromagnetic Tc of Dy with little change in the Neel temperature, indicating that a helimagnetic phase exists over a narrow temperature range.
Abstract: We have grown high quality superlattices of Dy with nonmagnetic Lu and find that the 2.4% epitaxial compression nearly doubles the ferromagnetic ${\mathit{T}}_{\mathit{c}}$ of Dy with little change in the N\'eel temperature. A helimagnetic phase exists over a narrow temperature range. Below ${\mathit{T}}_{\mathit{c}}$ in superlattices, 300 \AA{} orthorhombic domains form despite epitaxial constraints, each with a magnetostrictive distortion comparable to that of bulk Dy. For the thinnest intervening Lu layers, individual ferromagnetic Dy blocks have parallel alignment; the remaining samples show antiparallel alignment, coherent over many bilayer periods.
TL;DR: In this paper, high pressure and high temperature in situ X-ray observations of MgSiO3 perovskite were carried out using a newly developed Drickamer-type apparatus combined with synchrotron radiation.
Abstract: High pressure and high temperature in situ X-ray observations of MgSiO3 perovskite were carried out using a newly developed Drickamer-type apparatus combined with synchrotron radiation. Variations of the unit cell parameters of the perovskite were observed as a function of temperature up to about 1900 K at 36 GPa. The orthorhombic distortion changed very little and no indication of transformations to higher symmetry phases was observed up to 1900 K. This result indicates that the orthorhombic perovskite is the stable phase of MgSiO3 in the most part of the lower mantle. The thermal expansivity of the perovskite is determined to be larger than 1.7(2) × 10−5 /K at 36 GPa and room temperature. This value will give an important constraint to the chemical composition of the lower mantle.
TL;DR: In this article, the crystal structure of the Al3Mn phase, considered to be closely related to that of decagonal auasicrystais, has been determined by a combination of single-crystal X-ray analysis and high-resolution electron microscopy.
Abstract: The crystal structure of the Al3Mn phase, considered to be closely related to that of decagonal auasicrystais, has been determined by a combination of single-crystal X-ray analysis and high-resolution electron microscopy. The atomic model, obtained from a Patterson synthesis and high-resolution electron microscopy, was refined by the least-sauares methods with the Unics III system to a final R value of 0.066. There are 156 atoms in an orthorhombic unit cell with the lattice parameters a=1.483 nm, b=1.243 nm and c=1.251 nm, and the space group is Pnma (No. 62). Based on the structure of Al3Mn, an atomie arrangement for the Al-Pd-Mn decagonal quasicrystal is discussed.
TL;DR: In this paper, a linear thermal expansion is observed for CaTiO3 to 1373 K; extrapolation of these data suggest a possible transition to a tetragonal or pseudo-tetragonal phase near 1600 K.
Abstract: CaTiO3 perovskite has been studied at high temperature and P = 1 bar by powder x-ray diffraction and its structure refined subsequently by the Rietveld method. This Pbnm perovskite shows a decrease of orthorhombic distortion with increasing temperature as manifested by the increasing of the inter-octahedral angles towards 180°. Linear thermal expansion is observed for CaTiO3 to 1373 K; extrapolation of these data suggest a possible transition to a tetragonal or pseudo-tetragonal phase near 1600 K.
01 Jul 1993
TL;DR: In this paper, the role of structural transition in perovskite elasticity has been examined and it was shown that the role is small in the sense that a single well-defined general trend exists in pervskites with a wide range of tilting angles.
Abstract: Elasticity of CaTiO3, SrTiO3 and BaTiO3 perovskites has been experimentally investigated as a function of pressure up to 3.0 GPa in a liquid-medium piston cylinder apparatus using a high precision ultrasonic interferometric technique. Specimens used are hot-pressed fine-grained (3–10 μm) polycrystalline aggregates with low porosity (<1.5%). Compressional and shear wave velocities and their pressure derivatives have been measured. The results are compared with previous studies on other perovskites and the role of structural transitions is examined. We find that the role of Ti-O6 polyhedral tilting (such as observed in CaTiO3) is small in the sense that a single well-defined general trend exists in perovskites with a wide range of tilting angles, although there is suggestion that cubic perovskites have slightly higher bulk modulus than orthorhombic perovskites. In contrast, cation-anion displacement that changes crystal symmetry from cubic to tetragonal in BaTiO3 has very large effects on elasticity. This distortion significantly reduces the bulk modulus (but not much the shear modulus) and results in an unusually large pressure derivative of bulk modulus (dK/dP∼10). A large change in elasticity in BaTiO3 associated with the structural transition (without a significant volume change) is a clear example of the breakdown of the Birch's law between densities and elastic wave velocities.
TL;DR: In this paper, single crystals of low temperature form of BiNbO4 (α-form) were grown from BiOF flux below 900°C and solved by means of single crystal x-ray diffraction and refined to R = 0.031.
TL;DR: The crystal structures of the title compounds have been determined from X-ray intensity data collected at 200 K with a CAD 4 diffractometer as mentioned in this paper, where the gold-gold distances indicate metal-metal bonding.
TL;DR: The pressure-induced phase sequence of sulfur up to 90 GPa has been studied by an angle-dispersive x-ray-diffraction method and the highest-pressure phase (S-III) can be explained as a base-centered orthorhombic lattice that is isomorphous to that in the metallic high- pressure phase of selenium.
Abstract: The pressure-induced phase sequence of sulfur up to 90 GPa has been studied by an angle-dispersive x-ray-diffraction method. The orthorhombic sulfur (S-I) with ${\mathrm{S}}_{8}$ cyclic molecules transforms to a high-pressure crystal phase (S-II) between 6 and 55 GPa. The second structural phase transition takes place at 83\ifmmode\pm\else\textpm\fi{}2 GPa. The highest-pressure phase (S-III) can be explained as a base-centered orthorhombic lattice. This lattice is isomorphous to that in the metallic high-pressure phase of selenium. Metallization for sulfur reported by Luo et al. is ascribed to the second transition.
TL;DR: Raman scattering studies of perovskite-structured BaCeO 3 acceptor-doped with the rare earth ions Yb 3+, Gd 3+, and Nd 3+ are presented in this paper.
TL;DR: In this paper, an optically nonlinear octupolar guanidinium cation in a host polyanionic L-monohydrogentartrate lattice leads to a non-centrosymmetric orthorhombic P2 1 2 1 2 2 1 guest-host crystalline structure with cell parameters.
Abstract: Encapsulation of an optically nonlinear octupolar guanidinium cation in a host polyanionic L-monohydrogentartrate lattice leads to a noncentrosymmetric orthorhombic P2 1 2 1 2 1 guest-host crystalline structure with cell parameters a=11.347 (2), b=11.162 (2), c=6.668 (2) A with Z=4. Final R=0.049 from 1229 independent reflections. The packing shows strong interlocking between the anionic and cationic sublattices by a multidirectional hydrogen-bonding network. The optimally oriented octupolar crystalline structure for the 222 crystal point group is defined and the corresponding reference maximal nonlinear susceptibility is compared to that of the present guanidinium crystal structure thus pointing out possible improvements via appropriate structural modifications
TL;DR: In this paper, the authors show that the CaTiO3 perovskite has two overlapping phase transitions at 1384 and 1520 K. The 1384 K transition shows a λ-type Cp variation with a very sharp Cp decrease after the transition; in contrast, the 1520 k transition exhibits a unusual λ shape with a long high-temperature tail spanning more than 400 K.
Abstract: Drop-calorimetry measurements performed on CaTiO3 perovskite between 400 and 1800 K have shown the occurrence of two overlapping phase transitions at 1384 and 1520 K. The 1384 K transition shows a λ-type Cp variation with a very sharp Cp decrease after the transition; in contrast, the 1520 K transition exhibits a unusual λ shape with a long high-temperature tail spanning more than 400 K. By comparison with previous structural studies, we suggest that the 1384 K transition may be due to an orthorhombic Pbnm to orthorhombic Cmcm transition and that the peak centered at 1520 K represents the effects of overlapping orthorhombic to tetragonal and tetragonal to cubic phase transitions. The large anomaly of specific heat above 1520 K suggests that the cubic phase produced may be strongly disordered up to the melting point.
TL;DR: In this article, a new route has been found for producing the Bi2Sr2CaCu2Oy, phase with Pb substituted for part of the Bi (denoted Bi,Pb(2212)).
Abstract: A new route has been found for producing the Bi2Sr2CaCu2Oy, phase with Pb substituted for part of the Bi (denoted Bi,Pb(2212)). By this novel route, Bi,Pb(2212) has been prepared in air at 860 degrees C. For comparison, this phase has also been prepared in argon at 740 degrees C by the previously known route. Bi,Pb(2212) transforms from a tetragonal to an orthorhombic structure as the Pb content grows. Once this phase was obtained with a nominal composition of Bi2-xPbxSr2CaCu2Oy, with Pb contents x=0, 0.2, 0.4 and 0.6, the Bi(2223) phase was formed starting from Bi(2212) and Bi,Pb(2212). Single-phase samples were obtained after two heat treatments at 849 degrees C with Bi,Pb(2212) and a proportion of Pb of x=0.4. This proportion corresponds to the one used previously by the authors to obtain high critical current carrying tapes in Ag-clad Bi(2223) tapes.
TL;DR: In this paper, transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis.
Abstract: Transmission electron microscopy on natural calcium metatitanate perovskite (dysanalyte) reveals the following twin laws in the orthorhombic (space group Pbnm) phase: reflection twins on the {110} and {112} planes, and 90° rotation twins about the [001] axis (referred to as [001]90° twin). Single crystals that were heattreated and quenched from above 1585 K exhibit a dramatic change in domain structure compared with the starting material and specimens quenched from T < 1470 K. Mutually perpendicular {110} and [001]90° twins are observed throughout the crystal, forming a cross-hatched domain texture. 1/2[001] antiphase domains, which are very rarely observed in the starting material, also become dominant in the crystal. This change in domain structure is interpreted as due to a structural phase transition in perovskite at a temperature below 1585 K. From the point symmetry elements that describe the twin laws and the translational elements that relate the antiphase domains, the most likely phase near 1585 K is tetragonal with space group P4/mbm. These results are consistent with high-temperature powder X-ray diffraction study. On the other hand, density of the {112} twins is increased significantly in the crystal quenched from 1673 K. Twin domains are either bound by mutually perpendicular {110} and (001) walls, or by {112} walls with {110} twin domains within the polygonal {112} domains. Both twin density variation and domain morphology suggest that the crystal may be cubic at this temperature. Microstructure of a single crystal deformed at 1273 K and 3.5 GPa (within the orthorhombic stability field) is morphologically quite distinct from that of the heat-treated specimens. Dislocations dominate the microstructure and often interact with twin domain boundaries.