Zeitschrift fur Kristallographie - Crystalline Materials
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Refinement of the Fe<sub>4</sub>Al<sub>13</sub> structure and its relationship to the quasihomological homeotypical structures Abstract
The crystal structure of Fe4 Al13 was refined using single crystal diffractometer data: Pearson symbol mC102, space group C 2/m ; a = 15.492(2) Å, b = 8.078(2) Å, c = 12.471(1) Å, β = 107.69(1)°; RF
= 0.053, RF
(w ) = 0.044 for 1127 reflections and 137 refined parameters. The coordination numbers of atoms are 9, 10. 11 for iron and 10, 12, 13, 14 for aluminium. The shortest interatomic distances are: Fe–Fe – 2.902 Å, Fe–Al – 2.374 Å, Al–Al – 2.533 Å. A preferred occupation of pentagonal prismatic coordinated positions by aluminium was found. The structural relationship between the Fe4 Al13 structure and chemically homologous and homeotypical structures of aluminium and gallium containing systems with the 3d transition metals is discussed. The greatest similarity was found concerning the coordination polyhedra, especially that of transition metal atoms. The main common feature of these homeotypical structures is the presence of pentagonal “channels”, which is strongly dependent on the chemical composition. With increasing atomic number of the 3d transition metal, the stability range of these structures shifts to the transition metal-rich concentration. It is concluded that there is a connection between the occurrence of aluminium and gallium-containing decagonal and icosahedral phases and the existence of the infinite one-dimensional pentagonal channels in the intermetallic compounds showing a similar chemical composition.
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 209 Số 6 - Trang 479-487 - 1994
Vanadium oxide bronzes – the double mixed valence phases Cu[unk]Cu[unk]<sub>(1−<i>t</i>)</sub>V[unk]<sub>(2−<i>t</i>)</sub>V[unk]<sub>(2−<i>t</i>)</sub>O<sub>5</sub> A joint XRD and ESCA investigation Abstract
In the scope of providing new chemical insight of the oxidation states of copper and vanadium, crystal structures of VOB's β′ -Cu
x
V2 O5 with x = 0.29, 0.47 and 0.59, have been carefully investigated. The structures are monoclinic, space group C 2/m , with a = 15.233(3) Å, 15.218(1) Å, 15.2007(7) Å, b = 3.6133(9) Å, 3.6271(6) Å, 3.6375(3) Å, c = 10.0927(9) Å, 10.087(8) Å, 10.0925(4) Å, β = 107.30(1) Å, 106.55(4) Å, 106.13(2)°, R = 0.0467, 0.0290, 0.0315.
A thorough examination of bond distances and angles allows to give a precise description of the structural evolution versus increasing values of copper insertion x in the tunnels of the [V2 O5 ]
n
framework. The three independent vanadium atoms exhibit distorted oxygen coordination polyhedra, octahedra for V(1) and V(2), square pyramid for V(3). The copper, as revealed by successive Fourier map differences, is spreaded over a sort of toroidal section inserted in a huge oxygen trigonal bipyramid; more precisely the copper coordination changes, in its asymmetric site, from CN4 for Cu(1) (in the mirror plane) to CN3 for Cu(2) (apart the mirror plane, Δy ∼± 0.12) which rarely occurs in sulphides but, up to now never in oxides. The multiplication of Cu sites as well as their various coordination let us think that copper oxidation states could be not unique; such hypothesis is confirmed by ESCA measurements, which clearly indicate that simultaneously Cu(I) and Cu(II) are present. Such results establish that β′ ′-Cux V2 O5 phases are doubly mixed valence vanadium oxide bronzes and can be formally written:
Cu
xt
+ Cu
x (1−t )
2+ V2−x (2−t )
5+ V
x (2−t )
4+ O5 (t stands for Cu(I) rate).
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 209 Số 5 - Trang 405-412 - 1994
High-pressure crystal chemistry of binary intermetallic compounds Abstract
Effects of high pressure on intermetallic compounds are reviewed with regards to structural stability and phase transitions. Changes of bonding properties and electronic structure are examplified by means of the elemental metals caesium and titanium, the latter forming an internal intermetallic compound at high pressures. After a short systematic overview regarding pressure effects, structural transformations in selected classes of intermetallic compounds like Zintl phases and AlB2 -type arrangements precedes sections concerning high-pressure synthesis of Laves phases and intermetallic clathrates.
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 221 Số 5-7 - Trang 420-434 - 2006
Structural phase transition of GdGa<sub>2</sub> at high pressure Abstract
Structural properties of the intermetallic compound GdGa2 were investigated under hydrostatic pressures up to 20 GPa using angle dispersive x-ray powder diffraction in combination with the diamond anvil cell technique. At 7.7(5) GPa the onset of a first order phase transition from the AlB2 type structure into a hexagonal high-pressure phase is observed. The transformation is associated with a 2.5 % elongation of the lattice parameter a and an 8 % shortening of c . TB-LMTO bandstructure calculations reveal that the two-dimensional character of the gallium partial structure is preserved in the high-pressure modification.
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 216 Số 6 - Trang 331-336 - 2001
Clinoptilolite: the distribution of potassium atoms and its role in thermal stability Abstract
The crystal structure of clinoptilolite has been worked out, based on C2/m , using two crystals from two different localities: one from Kuruma Pass, Japan, a = 17.660(4) Å, b = 17.963(5) Å, c = 7.400(3) Å, β = 116.47(3)°, and the other from Agoura, U. S. A., a = 17.662(4) Å, b = 17.911(5) Å, c = 7.407(3) Å, β = 116.40(3)°. In the tetrahedral framework of the heulandite type there are recognized four kinds of main cation positions, M(l), M(2), M(3), and M(4). At M(l) and M(2), which have so far been known as cation positions in the heulandite structure, Na and C'a are located ; M(l) tends to accommodate more Na than M(2). The new position, M(3), which is specific for K, is located almost at the center of the eight-membered ring of the channel parallel to a . This position is coordinated by six framework oxygen atoms and three water molecules. At M(4), which is octahedrally coordinated by six water molecules, Mg is located. It is very likely that these four positions are characteristic of all members of the heulandite-group zeolites. The difference between clinoptilolite and heulandite is primarily in the difference of site content: for clinoptilolite, M(l) and M(2) are rich in Na, while for heulandite, they are rich in Ca. The occupancy of K at M(3) in clinoptilolite is, in general, higher than that of heulandite. The location and coordination of K explain the evidence that the presence of K in the structure is one of the controlling factors of the thermal behaviour of the heulandite-group zeolites.
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 145 Số 3-4 - Trang 216-239 - 1977
First principles methods using CASTEP Abstract
The CASTEP code for first principles electronic structure calculations will be described. A brief, non-technical overview will be given and some of the features and capabilities highlighted. Some features which are unique to CASTEP will be described and near-future development plans outlined.
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 220 Số 5-6 - Trang 567-570 - 2005
Thermal analysis of the phases of HMX using X-ray diffraction Abstract
The 4 phases of HMX and its transitions were investigated using X-ray diffraction. The phases were heated stepwise in a temperature range from 170 K to 510 K, measuring a diffraction pattern after each step.
The thermal expansion of the different phases and the volume changes during the phase transitions were obtained. Anomalies and strongly anisotropic expansions were observed with β - and γ -HMX.
The highest thermal expansion was found with γ -HMX, followed by δ -, β - and α -HMX. The highest volume change was observed with the β → δ transition, followed by the α → δ and the γ → δ transition.
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 204 Số 1 - Trang 121-128 - 1993
CRYSTAL: a computational tool for the <i>ab initio</i> study of the electronic properties of crystals Abstract
CRYSTAL [1] computes the electronic structure and properties of periodic systems (crystals, surfaces, polymers) within Hartree-Fock [2], Density Functional and various hybrid approximations.
CRYSTAL was developed during nearly 30 years (since 1976) [3] by researchers of the Theoretical Chemistry Group in Torino (Italy), and the Computational Materials Science group in CLRC (Daresbury, UK), with important contributions from visiting researchers, as documented by the main authors list and the bibliography.
The basic features of the program CRYSTAL are presented, with two examples of application in the field of crystallography [4, 5].
Zeitschrift fur Kristallographie - Crystalline Materials - Tập 220 Số 5-6 - Trang 571-573 - 2005
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