scholarly journals The unit cells and space groups of SeCl4and TeCl4

1964 ◽  
Vol 17 (6) ◽  
pp. 756-756 ◽  
Author(s):  
A. W. Cordes ◽  
R. F. Kruh ◽  
E. K. Gordon ◽  
M. K. Kemp
Keyword(s):  
Space Groups ◽  
Unit Cells

Crystal chemistry of transition metal diarsenates M 2As2O7 (M = Mn, Co, Ni, Zn): variants of the thortveitite structure

2010 ◽  
Vol 66 (6) ◽  
pp. 603-614 ◽  
Author(s):  
Matthias Weil ◽  
Berthold Stöger
Keyword(s):  
Crystal Structure ◽  
Low Temperature ◽  
Transition Metal ◽  
Basic Structure ◽  
Structure Type ◽  
Ambient Conditions ◽  
Modulated Structure ◽  
Triclinic Crystal System ◽  
Space Groups ◽  
Unit Cells

The structures of the 3d divalent transition-metal diarsenates M 2As2O7 (M = Mn, Co, Ni, Zn) can be considered as variants of the monoclinic (C2/m) thortveitite [Sc2Si2O7] structure type with a ≃ 6.7, b ≃ 8.5, c ≃ 4.7 Å, α ≃ 90, β ≃ 102, γ ≃ 90° and Z = 2. Co2As2O7 and Ni2As2O7 are dimorphic. Their high-temperature (β) polymorphs adopt the thortveitite aristotype structure in C2/m, whereas their low-temperature (α) polymorphs are hettotypes and crystallize with larger unit cells in the triclinic crystal system in space groups P\bar 1 and P1, respectively. Mn2As2O7 undergoes no phase transition and likewise adopts the thortveitite structure type in C2/m. Zn2As2O7 has an incommensurately modulated crystal structure [C2/m(α,0,γ)0s] with q = [0.3190 (1), 0, 0.3717 (1)] at ambient conditions and transforms reversibly to a commensurately modulated structure with Z = 12 (I2/c) below 273 K. The Zn phase resembles the structures and phase transitions of Cr2P2O7. Besides descriptions of the low-temperature Co2As2O7, Ni2As2O7 and Zn2As2O7 structures as five-, three- and sixfold superstructures of the thortveitite-type basic structure, the superspace approach can also be applied to descriptions of all the commensurate structures. In addition to the ternary M 2As2O7 phases, the quaternary phase (Ni,Co)2As2O7 was prepared and structurally characterized. In contrast to the previously published crystal structure of the mineral petewilliamsite, which has the same idealized formula and has been described as a 15-fold superstructure of the thortveitite-type basic structure in space group C2, synthetic (Ni,Co)2As2O7 can be considered as a solid solution adopting the α-Ni2As2O7 structure type. Differences of the two structure models for (Ni,Co)2As2O7 are discussed.

TheComputational Crystallography Toolbox: crystallographic algorithms in a reusable software framework

2002 ◽  
Vol 35 (1) ◽  
pp. 126-136 ◽  
Author(s):  
Ralf W. Grosse-Kunstleve ◽  
Nicholas K. Sauter ◽  
Nigel W. Moriarty ◽  
Paul D. Adams
Keyword(s):  
Structural Genomics ◽  
Macromolecular Structure ◽  
Software Framework ◽  
Software Components ◽  
Software Developers ◽  
Space Groups ◽  
Reusable Software ◽  
New Methods ◽  
Structure Solution ◽  
Unit Cells

The advent of structural genomics initiatives has led to a pressing need for high-throughput macromolecular structure determination. To accomplish this, new methods and inevitably new software must be developed to accelerate the process of structure solution. To minimize duplication of effort and to generate maintainable code efficiently, a toolbox of basic crystallographic software components is required. The development of theComputational Crystallography Toolbox(cctbx) has been undertaken for this purpose. In this paper, the fundamental requirements for thecctbxare outlined and the decisions that have lead to its implementation are explained. Thecctbxcurrently contains algorithms for the handling of unit cells, space groups and atomic scatterers, and is released under an Open Source license to allow unrestricted use and continued development. It will be developed further to become a comprehensive library of crystallographic tools useful to the entire community of software developers.

Fundamentals of crystallography

2013 ◽  
Author(s):  
Carmelo Giacovazzo
Keyword(s):  
Unit Cell ◽  
Generic Point ◽  
Cell Content ◽  
Daily Work ◽  
Space Groups ◽  
Crystallographic Symmetry ◽  
Unit Cells ◽  
Basic Concepts ◽  
Periodic Repetition ◽  
Unit Vectors

In this chapter we summarize the basic concepts, formulas and tables which constitute the essence of general crystallography. In Sections 1.2 to 1.5 we recall, without examples, definitions for unit cells, lattices, crystals, space groups, diffraction conditions, etc. and their main properties: reading these may constitute a useful reminder and support for daily work. In Section 1.6 we establish and discuss the basic postulate of structural crystallography: this was never formulated, but during any practical phasing process it is simply assumed to be true by default. We will also consider the consequences of such a postulate and the caution necessary in its use. We recall the main concepts and definitions concerning crystals and crystallographic symmetry. Crystal. This is the periodic repetition of a motif (e.g. a collection of molecules, see Fig. 1.1). An equivalent mathematical definition is: the crystal is the convolution between a lattice and the unit cell content (for this definition see (1.4) below in this section). Unit cell. This is the parallelepiped containing the motif periodically repeated in the crystal. It is defined by the unit vectors a, b, c, or, by the six scalar parameters a, b, c, α, β, γ (see Fig. 1.1). The generic point into the unit cell is defined by the vector . . . r = x a + y b + z c, . . . where x, y, z are fractional coordinates (dimensionless and lying between 0 and 1). The volume of the unit cell is given by (see Fig. 1.2) . . . V = a ∧ b · c = b ∧ c · a = c ∧ a · b. (1.1). . .

Structure and spectroscopic properties of (AA′)(BB′)O3 mixed-perovskite crystals

2005 ◽  
Vol 20 (12) ◽  
pp. 3329-3337 ◽  
Author(s):  
Dorota A. Pawlak ◽  
Masahiko Ito ◽  
Lukasz Dobrzycki ◽  
Krzysztof Wozniak ◽  
Masaoki Oku ◽  
...  
Keyword(s):  
Spectroscopic Properties ◽  
Regular System ◽  
Interstitial Oxygen ◽  
X Ray Diffraction ◽  
Absorption Bands ◽  
Space Groups ◽  
Oxygen Ions ◽  
Reducing Atmosphere ◽  
Lanthanum Strontium ◽  
Unit Cells

Five different mixed-perovskite (AA′)(BB′)O3 single crystals were grown, where A = La, Nd; A′ = Sr; B = Al, Ga; and B′ = Ta, Nb. The as-grown crystals were yellowish/brownish. After annealing in air, the coloration was more intense. Annealing in a reducing atmosphere decreased coloration. The crystals were investigated by transmission spectroscopy, electron spectroscopy for chemical analysis (ESCA), and single-crystal x-ray diffraction. Additional broad absorption bands in the transmission spectra were observed for the as-grown samples. They are in line with the changes of the shape of O(1s) ESCA peaks. Redundant interstitial oxygen ions were recognized as the reason for the crystal coloration. All structures were solved and refined in different space groups of the regular system. Some of the unit cells have a doubled lattice constant: (i) lanthanum strontium gallium niobate, Pm3m, 3.9323(5) Å, at 100 K, 3.9270(5) Å; (ii) neodymium strontium aluminum tantalate, Pm3m, 3.8353(4) Å; (iii) lanthanum strontium aluminum tantalate, Pn3m, 7.720(1) Å, annealed in reducing atmosphere, 7.708(1) Å; (iv) neodymium strontium aluminum niobate, Fm3m, 7.744(4) Å.

MCE2005– a new version of a program for fast interactive visualization of electron and similar density maps optimized for small molecules

2007 ◽  
Vol 40 (3) ◽  
pp. 600-601 ◽  
Author(s):  
Jan Rohlíček ◽  
Michal Hušák
Keyword(s):  
Small Molecules ◽  
Interactive Visualization ◽  
Unit Cell ◽  
Basic Unit ◽  
Space Groups ◽  
Density Maps ◽  
Unit Cells ◽  
Similar Density ◽  
Atom Position

A new version ofMCEis described. This new version supports atom-position generation based on space groups and can display more unit cells around the basic unit cell. The display settings have been improved to allow the colors of the background, atoms and maps to be changed.

scholarly journals reciprocalspaceship: a Python library for crystallographic data analysis

2021 ◽  
Vol 54 (5) ◽  
pp. 1521-1529
Author(s):  
Jack B. Greisman ◽  
Kevin M. Dalton ◽  
Doeke R. Hekstra
Keyword(s):  
Data Analysis ◽  
X Rays ◽  
Space Groups ◽  
Reflection Data ◽  
Software Packages ◽  
Unit Cells ◽  
Automated Processing ◽  
Exploratory Data ◽  
Closed Source ◽  
Structure Of Matter

Crystallography uses the diffraction of X-rays, electrons or neutrons by crystals to provide invaluable data on the atomic structure of matter, from single atoms to ribosomes. Much of crystallography's success is due to the software packages developed to enable automated processing of diffraction data. However, the analysis of unconventional diffraction experiments can still pose significant challenges – many existing programs are closed source, sparsely documented, or challenging to integrate with modern libraries for scientific computing and machine learning. Described here is reciprocalspaceship, a Python library for exploring reciprocal space. It provides a tabular representation for reflection data from diffraction experiments that extends the widely used pandas library with built-in methods for handling space groups, unit cells and symmetry-based operations. As is illustrated, this library facilitates new modes of exploratory data analysis while supporting the prototyping, development and release of new methods.

Distribution of molecular centres in unit cells with respect to packing patterns

2004 ◽  
Vol 60 (5) ◽  
pp. 539-546 ◽  
Author(s):  
Elna Pidcock ◽  
W. D. Sam Motherwell
Keyword(s):  
Building Blocks ◽  
Unit Cell ◽  
Space Group ◽  
Space Groups ◽  
Molecular Building Blocks ◽  
Unit Cells ◽  
Symmetry Operators

Packing patterns, a new description of the limited number of possible arrangements of molecular building blocks in a unit cell, were assigned to many thousands of structures belonging to the space groups P21/c, P\bar 1, P212121, P21 and C2/c [Pidcock & Motherwell (2004). Cryst. Growth. Des. 4, 611–620]. The position of the molecular centre (in fractional coordinates) in the unit cell for these structures has been surveyed, with respect to the space group and the packing pattern. The results clearly show that the position at which the molecular centre is found in the unit cell is correlated with the packing pattern. The relationships between the orientation of the packing pattern in the unit cell and the symmetry operators of the space group are explored. Popular orientations of packing patterns within the unit cell are given.

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