scholarly journals Charge Distribution Model in Cubic Perovskite-type Compounds

2000 ◽  
Vol 55 (1-2) ◽  
pp. 261-266
Author(s):  
R. E. Alonso ◽  
A. López-García
Keyword(s):  
Oxygen Vacancies ◽  
Computational Simulation ◽  
Point Group ◽  
Group Symmetry ◽  
Distribution Model ◽  
Cubic Symmetry ◽  
Cell Parameters ◽  
Common Trend ◽  
Charge Model ◽  
Perovskite Type

A non-zero electric field gradient tensor, detected by probes that occupy sites with cubic point group symmetry, was observed in many AB03 perovskite-type compounds. This breakdown of local cubic symmetry is commonly associated with the presence of oxygen vacancies around the probe. This effect in BaTixHf1-xO3, with x = 0.7, 0.5, 0.3, 0.1, 0.05 and 0.01 is studied in this work. The cell parameters were obtained at laboratory temperature using XRD spectroscopy. The hyperfine parameters were measured at a 181Ta probe in the B site using Perturbed Angular Correla-tions (PAC) spectroscopy as a function of both temperature and composition. As a common trend, a static asymmetric and distributed quadrupolar interaction, strongly dependent on composition has been observed. The results, together with those corresponding to 1 > x ≥ 0.75, are analyzed using the point-charge model in terms of polarized oxygen vacancies, different covalence of the Ti-0 and Hf-0 bonds with computational simulation for the lattice positions of cations and oxygen vacancies.

scholarly journals Crystal structures of isotypic poly[bis(benzimidazolium) [tetra-μ-iodido-stannate(II)]] and poly[bis(5,6-difluorobenzimidazolium) [tetra-μ-iodido-stannate(II)]]

2014 ◽  
Vol 70 (10) ◽  
pp. 178-182 ◽  
Author(s):  
Iwan Zimmermann ◽  
Tony D. Keene ◽  
Jürg Hauser ◽  
Silvio Decurtins ◽  
Shi-Xia Liu
Keyword(s):  
Hydrogen Bonds ◽  
Point Group ◽  
Van Der Waals Interactions ◽  
Group Symmetry ◽  
Layered Perovskite ◽  
Out Of Plane ◽  
Related Compounds ◽  
Perovskite Type ◽  
Inorganic Layers ◽  
Perovskite Type Structure

The isostructural title compounds, {(C7H7N2)2[SnI4]}n, (1), and {(C7H5F2N2)2[SnI4]}n, (2), show a layered perovskite-type structure composed of anionic {[SnI4]2−}nsheets parallel to (100), which are decorated on both sides with templating benzimidazolium or 5,6-difluorobenzimidazolium cations, respectively. These planar organic heterocycles mainly form N—H...I hydrogen bonds to the terminal I atoms of the corner-sharing [SnI6] octahedra (point group symmetry 2) from the inorganic layer, but not to the bridging ones. This is in contrast to most of the reported structures of related compounds where ammonium cations are involved. Here hydrogen bonding to both types of iodine atoms and thereby a distortion of the inorganic layers to various extents is observed. For (1) and (2), all Sn—I—Sn angles are linear and no out-of-plane distortions of the inorganic layers occur, a fact of relevance in view of the material properties. The arrangement of the aromatic cations is mainly determined through the direction of the N—H...I hydrogen bonds. The coherence between organic bilayers along [100] is mainly achieved through van der Waals interactions.

scholarly journals Crystal structure of post-perovskite-type CaIrO3reinvestigated: new insights into atomic thermal vibration behaviors

2015 ◽  
Vol 71 (9) ◽  
pp. 1109-1113
Author(s):  
Akihiko Nakatsuka ◽  
Kazumasa Sugiyama ◽  
Akira Yoneda ◽  
Keiko Fujiwara ◽  
Akira Yoshiasa
Keyword(s):  
Crystal Structure ◽  
Atmospheric Pressure ◽  
Structure Refinement ◽  
Point Group ◽  
Group Symmetry ◽  
Point Group Symmetry ◽  
X Ray ◽  
Thermal Vibrations ◽  
Perovskite Type ◽  
Shared Edge

Single crystals of the title compound, the post-perovskite-type CaIrO3[calcium iridium(IV) trioxide], have been grown from a CaCl2flux at atmospheric pressure. The crystal structure consists of an alternate stacking of IrO6octahedral layers and CaO8hendecahedral layers along [010]. Chains formed by edge-sharing of IrO6octahedra (point-group symmetry 2/m..) run along [100] and are interconnected along [001] by sharing apical O atoms to build up the IrO6octahedral layers. Chains formed by face-sharing of CaO8hendecahedra (point-group symmetrym2m) run along [100] and are interconnected along [001] by edge-sharing to build up the CaO8hendecahedral layers. The IrO6octahedral layers and CaO8hendecahedral layers are interconnected by sharing edges. The present structure refinement using a high-power X-ray source confirms the atomic positions determined by Hiraiet al.(2009) [Z. Kristallogr.224, 345–350], who had revised our previous report [Sugaharaet al.(2008).Am. Mineral.93, 1148–1152]. However, the displacement ellipsoids of the Ir and Ca atoms based on the present refinement can be approximated as uniaxial ellipsoids elongating along [100], unlike those reported by Hiraiet al.(2009). This suggests that the thermal vibrations of the Ir and Ca atoms are mutually suppressed towards the Ir...Ca direction across the shared edge because of the dominant repulsion between the two atoms.

scholarly journals Crystal structure of SrGeO3in the high-pressure perovskite-type phase

2015 ◽  
Vol 71 (5) ◽  
pp. 502-504 ◽  
Author(s):  
Akihiko Nakatsuka ◽  
Hiroshi Arima ◽  
Osamu Ohtaka ◽  
Keiko Fujiwara ◽  
Akira Yoshiasa
Keyword(s):  
Crystal Structure ◽  
High Pressure ◽  
Point Group ◽  
Group Symmetry ◽  
Point Group Symmetry ◽  
Thermal Vibrations ◽  
Perovskite Type ◽  
Pressure Phase ◽  
Perovskite Type Structure ◽  
The Ideal

Single crystals of the SrGeO3(strontium germanium trioxide) high-pressure phase have been synthesized successfully at 6 GPa and 1223 K. The compound crystallizes with the ideal cubic perovskite-type structure (space groupPm-3m), which consists of a network of corner-linked regular GeO6octahedra (point-group symmetrym-3m), with the larger Sr atoms located at the centers of cavities in the form of SrO12cuboctahedra (point-group symmetrym-3m) in the network. The degrees of covalencies included in the Sr—O and the Ge—O bonds calculated from bond valences are 20.4 and 48.9%, respectively. Thus, the Ge—O bond of the GeO6octahedron in the SrGeO3perovskite has a strong covalency, comparable to those of the Si—O bonds of the SiO4tetrahedra in silicates with about 50% covalency. The thermal vibrations of the O atoms in the title compound are remarkably suppressed in the directions of the Ge—O bonds. This anisotropy ranks among the largest observed in stoichiometric cubic perovskites.

Purification, crystallization and preliminary X-ray analysis of catabolic ornithine carbamoyltransferase from Pseudomonas aeruginosa

1999 ◽  
Vol 55 (9) ◽  
pp. 1591-1593 ◽  
Author(s):  
G. Sainz ◽  
J. Vicat ◽  
R. Kahn ◽  
C. Tricot ◽  
V. Stalon ◽  
...  
Keyword(s):  
Pseudomonas Aeruginosa ◽  
Point Group ◽  
Unit Cell ◽  
Group Symmetry ◽  
Ornithine Carbamoyltransferase ◽  
Unit Cell Parameters ◽  
Crystal Forms ◽  
Space Group ◽  
Cell Parameters ◽  
Hexagonal Crystals

The catabolic ornithine carbamoyltransferase (OTCase) from Pseudomonas aeruginosa exhibits allosteric behaviour, with two conformational states of the molecule: an active R form and an inactive T form. The enzyme is a dodecamer with a molecular mass of 455700 Da. Three crystal forms have been obtained. Crystals of allosteric state T are rhombohedral, belonging to the R3 space group, with hexagonal unit-cell parameters a = b = 180.6, c = 122.0 Å. They diffract to a resolution of 4.5 Å. Two crystal forms for allosteric state R have been obtained, with hexagonal and cubic symmetries. Hexagonal crystals, which diffract to a resolution of 3.4 Å, belong to the space group P63 with unit-cell parameters a = b = 140.8, c = 145.6 Å. The cubic crystals belong to space group I23, with unit-cell parameter a = 134.32 Å and diffract to a resolution better than 2.5 Å. In all crystal forms, the dodecamer exhibits a 23 point-group symmetry.

Crystallographic studies of cobalt arsenates II. Crystal structure of Co8As3O16

1970 ◽  
Vol 48 (20) ◽  
pp. 3124-3131 ◽  
Author(s):  
N. Krishnamachari ◽  
C. Calvo
Keyword(s):  
Bond Length ◽  
Mirror Symmetry ◽  
Point Group ◽  
Group Symmetry ◽  
Cell Parameters ◽  
Octahedral Environment ◽  
Deviation From Stoichiometry ◽  
Closest Packing ◽  
The Mean ◽  
Oxygen Atoms

CO8AS3O16, the Co analogue of the mineral aerugite, crystallizes in the rhombohedral space group [Formula: see text] with hexagonal unit cell parameters a = 6.046(1) and c = 28.062(8) Å, and Z = 3. The structure has been refined by a full matrix least squares technique using 613 reflections to a final R value of 0.080. The cations are in tetrahedral and octahedral interstices between layers of oxygen atoms nearly showing cubic closest packing. The structure has one third of the As ions in sites of point group symmetry [Formula: see text] with a nearly perfect octahedral environment of oxygen atoms. The remaining As ions lie in sites having the point group 3m and showing tetrahedral coordination to oxygen atoms. In the former case the mean As—O bond length is 1.84 Å while in the latter case it is 1.68 Å. The Co ions lie in positions having either 2/m or m symmetry with a mean Co—O bond length of 2.13 Å in each case. All the atoms have at least mirror symmetry about them. Some deviation from stoichiometry has been found and may indicate that some of the cobalt has been oxidized.

Crystallization of the N-terminal domain of human sex hormone-binding globulin, the major sex steroid carrier in blood

1999 ◽  
Vol 55 (12) ◽  
pp. 2053-2055 ◽  
Author(s):  
Irina Grishkovskaya ◽  
Gisela Sklenar ◽  
George V. Avvakumov ◽  
David Dales ◽  
Joachim Behlke ◽  
...  
Keyword(s):  
Rotational Symmetry ◽  
Point Group ◽  
Sex Hormone ◽  
Sex Hormone Binding Globulin ◽  
Group Symmetry ◽  
Data Sets ◽  
Hormone Binding ◽  
Dimerization Domain ◽  
Crystal Forms ◽  
Cell Parameters

The amino-teminal laminin G-like domain of human sex hormone-binding globulin (SHBG), which contains the steroid-binding site and the dimerization domain, has been produced in Escherichia coli, purified to homogeneity and crystallized in complex with 5α-dihydrotestosterone (DHT) in two different crystal forms. Native data sets have been collected for tetragonal crystals (space group P4122 or P4322; unit-cell parameters a = 52.2, c = 148.4 Å) diffracting to 3.3 Å and trigonal crystals (R32; a = 104.0, c = 84.4 Å) diffracting to better than 1.6 Å. Since both crystal forms can only accommodate a single monomer in the asymmetric unit and share twofold rotational symmetry, it is proposed that the homodimer of this truncated form of SHBG, as observed in ultracentrifugation experiments, displays C 2 point-group symmetry.

Hydrothermal Synthesis and Solid-State Laser Refrigeration of Ytterbium-Doped Potassium Lutetium Fluoride (KLF) Microcrystals

2020 ◽  
Author(s):  
Xiaojing Xia ◽  
Anupum Pant ◽  
Xuezhe Zhou ◽  
Elena Dobretsova ◽  
Alex Bard ◽  
...  
Keyword(s):  
Hydrothermal Synthesis ◽  
Solid State ◽  
Information Science ◽  
Point Group ◽  
Solid State Laser ◽  
Lanthanide Ions ◽  
Group Symmetry ◽  
Luminescence Spectra ◽  
Crystalline Phases ◽  
State Laser

Fluoride crystals, due to their low phonon energies, are attractive hosts of trivalent lanthanide ions for applications in upconverting phosphors, quantum information science, and solid-state laser refrigeration. In this article, we report the rapid, low-cost hydrothermal synthesis of potassium lutetium fluoride (KLF) microcrystals for applications in solid-state laser refrigeration. Four crystalline phases were synthesized, namely orthorhombic K<sub>2</sub>LuF<sub>5</sub> (Pnma), trigonal KLuF<sub>4</sub> (P3<sub>1</sub>21), orthorhombic KLu<sub>2</sub>F<sub>7</sub> (Pna2<sub>1</sub>), and cubic KLu<sub>3</sub>F<sub>10</sub> (Fm3m), with each phase exhibiting unique microcrystalline morphologies. Luminescence spectra and emission lifetimes of the four crystalline phases were characterized based on the point-group symmetry of trivalent cations. Laser refrigeration was measured by observing both the optomechanical eigenfrequencies of microcrystals on cantilevers in vacuum, and also the Brownian dynamics of optically trapped microcrystals in water. Among all four crystalline phases, the most significant cooling was observed for 10%Yb:KLuF<sub>4</sub> with cooling of 8.6 $\pm$ 2.1 K below room temperature. Reduced heating was observed with 10%Yb:K<sub>2</sub>LuF<sub>5</sub>

scholarly journals Multiplicity, Parity and Angular Momentum of a Cooper Pair in Unconventional Superconductors of D4h Symmetry: Sr2RuO4 and Fe-Pnictide Materials

Symmetry ◽  
2021 ◽  
Vol 13 (8) ◽  
pp. 1435
Author(s):  
Victor G. Yarzhemsky
Keyword(s):  
Angular Momentum ◽  
Cooper Pair ◽  
Point Group ◽  
Group Symmetry ◽  
Cooper Pairs ◽  
Group Approach ◽  
Angular Momentum Projection ◽  
Space Group ◽  
Unconventional Superconductors ◽  
Even Pairs

Sr2RuO4 and Fe-pnictide superconductors belong to the same point group symmetry D4h. Many experimental data confirm odd pairs in Sr2RuO4 and even pairs in Fe-pnictides, but opposite conclusions also exist. Recent NMR results of Pustogow et al., which revealed even Cooper pairs in Sr2RuO4, require reconsideration of symmetry treatment of its SOP (superconducting order parameter). In the present work making use of the Mackey–Bradley theorem on symmetrized squares, a group theoretical investigation of possible pairing states in D4h symmetry is performed. It is obtained for I4/mmm , i.e., space group of Sr2RuO4, that triplet pairs with even spatial parts are possible in kz direction and in points M and Y. For the two latter cases pairing of equivalent electrons with nonzero total momentum is proposed. In P4/nmm space group of Fe- pnictides in point M, even and odd pairs are possible for singlet and triplet cases. It it shown that even and odd chiral states with angular momentum projection m=±1 have nodes in vertical planes, but Eg is nodal , whereas Eu is nodeless in the basal plane. It is also shown that the widely accepted assertion that the parity of angular momentum value is directly connected with the spatial parity of a pair is not valid in a space-group approach to the wavefunction of a Cooper pair.

Raman-Spektren von SnJ4, GeJ4, TiBr4 und TiJ4 in flüssiger und kristalliner Phase / Raman Spectra of SnJ4, GeJ4, TiBr4, and TiJ4 in the Liquid and Crystalline Phase

1970 ◽  
Vol 25 (12) ◽  
pp. 1374-1381 ◽  
Author(s):  
W. Kiefer ◽  
H. W. Schrötter
Keyword(s):  
Raman Spectra ◽  
Ruby Laser ◽  
Detection System ◽  
Point Group ◽  
Visible Region ◽  
Group Symmetry ◽  
Pure Liquid ◽  
Electronic Detection ◽  
Polarization Studies ◽  
First Time

The Raman spectra of four molecules absorbing in the visible region (SnJ4, GeJ4, TiBr4, and TiJ4) are presented. They were excited with a quasi-continuous ruby laser and recorded with a special electronic detection system. Except for TiJ4, complete Raman spectra of crystal powder pellets could be obtained for the first time. The assignment reported by previous authors was confirmed by accurate polarization studies of solutions or pure liquid. The assignment is also in the solid state possible on the basis of Td point group symmetry. The fundamental vibrations of TiJ4 in solutions are: ν1 (A1) =162, ν2 (E) =51, ν3 (F2) =319 and ν4 (F2) Y = 67 cm-1

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