POINT AND SPACE GROUPS

Symmetry ◽  
1963 ◽  
pp. 54-90
Author(s):  
R. McWEENY
Keyword(s):  
Space Groups

Microstructure and crystal symmetry of Pb2Sr2(Y/Ca)Cu3O9−δ

1990 ◽  
Vol 48 (4) ◽  
pp. 64-65
Author(s):  
Y. P. Lin ◽  
J. S. Xue ◽  
J. E. Greedan
Keyword(s):  
Electron Diffraction ◽  
High Temperature Superconductors ◽  
Carbon Film ◽  
Basic Structure ◽  
Crystal Symmetry ◽  
X Ray Diffraction ◽  
X Ray ◽  
Space Groups ◽  
New Family ◽  
Convergent Beam

A new family of high temperature superconductors based on Pb2Sr2YCu3O9−δ has recently been reported. One method of improving Tc has been to replace Y partially with Ca. Although the basic structure of this type of superconductors is known, the detailed structure is still unclear, and various space groups has been proposed. In our work, crystals of Pb2Sr2YCu3O9−δ with dimensions up to 1 × 1 × 0.25.mm and with Tc of 84 K have been grown and their superconducting properties described. The defects and crystal symmetry have been investigated using electron microscopy performed on crushed crystals supported on a holey carbon film.Electron diffraction confirmed x-ray diffraction results which showed that the crystals are primitive orthorhombic with a=0.5383, b=0.5423 and c=1.5765 nm. Convergent Beam Electron Diffraction (CBED) patterns for the and axes are shown in Figs. 1 and 2 respectively.

Symmetry breaking in convergent-beam diffraction

1989 ◽  
Vol 47 ◽  
pp. 480-481
Author(s):  
D.J. Eaglesham
Keyword(s):  
Symmetry Breaking ◽  
Point Group ◽  
X Ray Diffraction ◽  
Multiple Diffraction ◽  
Space Groups ◽  
Atomic Displacements ◽  
Small Probe ◽  
Phase Sensitive ◽  
Convergent Beam

Convergent Beam Electron Diffraction is now almost routinely used in the determination of the point- and space-groups of crystalline samples. In addition to its small-probe capability, CBED is also postulated to be more sensitive than X-ray diffraction in determining crystal symmetries. Multiple diffraction is phase-sensitive, so that the distinction between centro- and non-centro-symmetric space groups should be trivial in CBED: in addition, the stronger scattering of electrons may give a general increase in sensitivity to small atomic displacements. However, the sensitivity of CBED symmetry to the crystal point group has rarely been quantified, and CBED is also subject to symmetry-breaking due to local strains and inhomogeneities. The purpose of this paper is to classify the various types of symmetry-breaking, present calculations of the sensitivity, and illustrate symmetry-breaking by surface strains.CBED symmetry determinations usually proceed by determining the diffraction group along various zone axes, and hence finding the point group. The diffraction group can be found using either the intensity distribution in the discs

Transmission Electron Microscopy study of Bi-based superconducting phases

1990 ◽  
Vol 48 (4) ◽  
pp. 50-51
Author(s):  
J.G. Wen ◽  
K.K. Fung
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Single Crystals ◽  
Microscopy Study ◽  
Electron Microscopy Study ◽  
Space Groups ◽  
Superconducting Phases ◽  
Ceramic Samples ◽  
Transmission Electron ◽  
Modulated Structures

Bi-based superconducting phases have been found to be members of a structural series represented by Bi2Sr2Can−1Cun−1On+4, n=1,2,3, and are referred to as 2201, 2212, 2223 phases. All these phases are incommensurate modulated structures. The super space groups are P2/b, NBbmb 2201, 2212 phases respectively. Pb-doped ceramic samples and single crystals and Y-doped single crystals have been studied by transmission electron microscopy.Modulated structures of all Bi-based superconducting phases are in b-c plane, therefore, it is the best way to determine modulated structure and c parameter in diffraction pattern. FIG. 1,2,3 show diffraction patterns of three kinds of modulations in Pb-doped ceramic samples. Energy dispersive X-ray analysis (EDAX) confirms the presence of Pb in the three modulated structures. Parameters c are 3 0.06, 38.29, 30.24Å, ie 2212, 2223, 2212 phases for FIG. 1,2,3 respectively. Their average space groups are all Bbmb.

Minimal supergroups and magnetic space groups (III)

1977 ◽  
Vol 2 ◽  
pp. 169-177
Author(s):  
E. F. Bertaut
Keyword(s):  
Space Groups

Equivalency relations in magnetic space groups (II)

1975 ◽  
Vol 14 (9) ◽  
pp. 109-124 ◽  
Author(s):  
E. F. Bertaut
Keyword(s):  
Space Groups

Solid State Analysis and Theoretical Explorations on Polymorphic and Hydrate Forms of p-Hydroxybenzaldehyde Isonicotinichydrazone: Effect of Additives on Polymorphic Crystallization

2018 ◽  
Author(s):  
Lincy Tom ◽  
Victoria A. Smolenski ◽  
Jerry P. Jasinski ◽  
M.R. Prathapachandra Kurup
Keyword(s):  
Hirshfeld Surface ◽  
Framework Analysis ◽  
Reaction Conditions ◽  
Space Groups ◽  
Effect Of Additives ◽  
Ft Ir ◽  
Packing Arrangement ◽  
The Stability ◽  
Ir And Raman ◽  
Isonicotinic Hydrazide

The reaction of p-hydroxybenzaldehyde with an equimolar amount of isonicotinic hydrazide afforded two polymorphic and hydrate forms of p-hydroxybenzaldehyde isonicotinichydrazone (HBIH) by varying the experimental reaction conditions. The compounds are fully characterized by means of single crystal and powder diffraction methods, vibrational spectroscopy (FT-IR and Raman), thermal and elemental analysis. The compound crystallizes in three different forms in two different space groups, P21/c (form PA and PB) and Pbca (PC). The Hirshfeld surface analysis shows the differences in the relative contributions of intermolecular interactions to the total Hirshfeld surface area for the HBIH molecules. The calculated pairwise interaction energies (104-116 kJ/mol) can be related to the stability of the crystals. Energy framework analysis identifies the interaction hierarchy and their topology. The geometry and conformation of the three forms are essentially similar which differ only by packing arrangement.

Solid State Analysis and Theoretical Explorations on Polymorphic and Hydrate Forms of p-Hydroxybenzaldehyde Isonicotinichydrazone: Effect of Additives on Polymorphic Crystallization

2018 ◽  
Author(s):  
Lincy Tom ◽  
Victoria A. Smolenski ◽  
Jerry P. Jasinski ◽  
M.R. Prathapachandra Kurup
Keyword(s):  
Hirshfeld Surface ◽  
Framework Analysis ◽  
Reaction Conditions ◽  
Space Groups ◽  
Effect Of Additives ◽  
Ft Ir ◽  
Packing Arrangement ◽  
The Stability ◽  
Ir And Raman ◽  
Isonicotinic Hydrazide

The reaction of p-hydroxybenzaldehyde with an equimolar amount of isonicotinic hydrazide afforded two polymorphic and hydrate forms of p-hydroxybenzaldehyde isonicotinichydrazone (HBIH) by varying the experimental reaction conditions. The compounds are fully characterized by means of single crystal and powder diffraction methods, vibrational spectroscopy (FT-IR and Raman), thermal and elemental analysis. The compound crystallizes in three different forms in two different space groups, P21/c (form PA and PB) and Pbca (PC). The Hirshfeld surface analysis shows the differences in the relative contributions of intermolecular interactions to the total Hirshfeld surface area for the HBIH molecules. The calculated pairwise interaction energies (104-116 kJ/mol) can be related to the stability of the crystals. Energy framework analysis identifies the interaction hierarchy and their topology. The geometry and conformation of the three forms are essentially similar which differ only by packing arrangement.

Packing Effect on the Transfer Integrals and Mobility in α,α′-bis(dithieno[3,2-b:2′,3′-d]thiophene) (BDT) and its Heteroatom-Substituted Analogues

2011 ◽  
Vol 64 (12) ◽  
pp. 1587 ◽  
Author(s):  
Ahmad Irfan ◽  
Abdullah G. Al-Sehemi ◽  
Shabbir Muhammad ◽  
Jingping Zhang
Keyword(s):  
Electron Transfer ◽  
Electron Mobility ◽  
Hole Mobility ◽  
Experimental Investigations ◽  
Hole Transfer ◽  
Space Group ◽  
Space Groups ◽  
Transfer Integrals ◽  
Packing Effect ◽  
Good Agreement

Theoretically calculated mobility has revealed that BDT is a hole transfer material, which is in good agreement with experimental investigations. The BDT, NHBDT, and OBDT are predicted to be hole transfer materials in the C2/c space group. Comparatively, hole mobility of BHBDT is 7 times while electron mobility is 20 times higher than the BDT. The packing effect for BDT and designed crystals was investigated by various space groups. Generally, mobility increases in BDT and its analogues by changing the packing from space group C2/c to space groups P1 or . In the designed ambipolar material, BHBDT hole mobility has been predicted 0.774 and 3.460 cm2 Vs–1 in space groups P1 and , which is 10 times and 48 times higher than BDT (0.075 and 0.072 cm2 Vs–1 in space groups P1 and ), respectively. Moreover, the BDT behaves as an electron transfer material by changing the packing from the C2/c space group to P1 and .

scholarly journals Crystal Structure of Escherichia coli Agmatinase: Catalytic Mechanism and Residues Relevant for Substrate Specificity

2021 ◽  
Vol 22 (9) ◽  
pp. 4769
Author(s):  
Pablo Maturana ◽  
María S. Orellana ◽  
Sixto M. Herrera ◽  
Ignacio Martínez ◽  
Maximiliano Figueroa ◽  
...  
Keyword(s):  
Escherichia Coli ◽  
Active Site ◽  
Catalytic Mechanism ◽  
Conformational Dynamics ◽  
High Specificity ◽  
Arginine Decarboxylase ◽  
Space Groups ◽  
X Ray Crystallography ◽  
High Dynamics ◽  
Dynamics Simulations

Agmatine is the product of the decarboxylation of L-arginine by the enzyme arginine decarboxylase. This amine has been attributed to neurotransmitter functions, anticonvulsant, anti-neurotoxic, and antidepressant in mammals and is a potential therapeutic agent for diseases such as Alzheimer’s, Parkinson’s, and cancer. Agmatinase enzyme hydrolyze agmatine into urea and putrescine, which belong to one of the pathways producing polyamines, essential for cell proliferation. Agmatinase from Escherichia coli (EcAGM) has been widely studied and kinetically characterized, described as highly specific for agmatine. In this study, we analyze the amino acids involved in the high specificity of EcAGM, performing a series of mutations in two loops critical to the active-site entrance. Two structures in different space groups were solved by X-ray crystallography, one at low resolution (3.2 Å), including a guanidine group; and other at high resolution (1.8 Å) which presents urea and agmatine in the active site. These structures made it possible to understand the interface interactions between subunits that allow the hexameric state and postulate a catalytic mechanism according to the Mn2+ and urea/guanidine binding site. Molecular dynamics simulations evaluated the conformational dynamics of EcAGM and residues participating in non-binding interactions. Simulations showed the high dynamics of loops of the active site entrance and evidenced the relevance of Trp68, located in the adjacent subunit, to stabilize the amino group of agmatine by cation-pi interaction. These results allow to have a structural view of the best-kinetic characterized agmatinase in literature up to now.

The effect of partial methylation of the glycine amino group on crystal structure inN,N-dimethylglycine and its hemihydrate

2012 ◽  
Vol 68 (8) ◽  
pp. o283-o287 ◽  
Author(s):  
Vasily S. Minkov ◽  
Elena V. Boldyreva
Keyword(s):  
Crystal Structure ◽  
Hydrogen Bonds ◽  
Water Molecules ◽  
Amino Group ◽  
Asymmetric Unit ◽  
Partial Methylation ◽  
Space Groups ◽  
Carboxylate Groups ◽  
Anhydrous Compound ◽  
Additional Hydrogen

N,N-Dimethylglycine, C4H9NO2, and its hemihydrate, C4H9NO2·0.5H2O, are discussed in order to follow the effect of the methylation of the glycine amino group (and thus its ability to form several hydrogen bonds) on crystal structure, in particular on the possibility of the formation of hydrogen-bonded `head-to-tail' chains, which are typical for the crystal structures of amino acids and essential for considering amino acid crystals as mimics of peptide chains. Both compounds crystallize in centrosymmetric space groups (PbcaandC2/c, respectively) and have twoN,N-dimethylglycine zwitterions in the asymmetric unit. In the anhydrous compound, there are no head-to-tail chains but the zwitterions formR44(20) ring motifs, which are not bonded to each other by any hydrogen bonds. In contrast, in the crystal structure ofN,N-dimethylglycinium hemihydrate, the zwitterions are linked to each other by N—H...O hydrogen bonds into infiniteC22(10) head-to-tail chains, while the water molecules outside the chains provide additional hydrogen bonds to the carboxylate groups.

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