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Hydrogen Ordering in Hexagonal Intermetallic AB_5 Type Compounds

100%
Sikora W., Kuna A.
Acta Physica Polonica A
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2008
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vol. 113
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issue 4
1211-1224
EN
Intermetallic compounds AB_5 type (A = rare-earth atoms, B = transition metal) are known to store reversibly large amounts of hydrogen and as that are discussed in this work. It was shown that the alloy cycling stability can be significantly improved by employing the so-called non-stoichiometric compounds AB_5+x and that is why analysis of change of structure turned out to be interesting. A tendency for ordering of hydrogen atoms is one of the most intriguing problems for the unsaturated hydrides. The symmetry analysis method in the frame of the theory of space group and their representation gives opportunity to find all possible transformations of the parent structure. In this work symmetry analysis method was applied for AB_5+x structure type (P6/mmm parent symmetry space group). There were investigated all possible ordering types and accompanying atom displacements in positions 1a, 2c, 3g (fully occupied in stoichiometric compounds AB_5), in positions 2e, 6l (where atom B could appear in non-stoichiometric compounds) and also 4h, 6m, 6k, 12n, 12o, which could be partly occupied by hydrogen as a result of hydrides. An analysis was carried out of all possible structures of lower symmetry, following from P6/mmm for we k=(0, 0, 0). Also the way of getting the structure described by the P6_3mc space group with double cell along the z-axiswe k=(0, 0, 0.5), as it is suggested in the work of Latroche et al. is discussed by the symmetry analysis. The analysis was obtained by computer program MODY. The program calculates the so-called basis vectors of irreducible representations of a given symmetry group, which can be used for calculation of possible ordering modes.
2
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Crowd Behavior as an Example of the Evolution of a Complex System - Evacuation Models Proposal Based on the Symmetry Analysis Approach

100%
Sikora W., Malinowski J.
Acta Physica Polonica A
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2013
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vol. 124
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issue 6
1005-1012
EN
The evacuation of football stadium scenarios as examples of evolution of complex system are discussed. The models are presented as movements of individuals according to fields of displacements, calculated correspondingly to the given scenario. The assumption has been made that the most efficient evacuation is left based on the accordance of symmetry of allowed space, and this symmetry is taken into account while calculating the displacements field. The displacements related to every point of this space are calculated by the symmetry analysis method and fulfill the symmetry conditions of allowed space. The speed of each individual at every point in the presented model has the same quantity. Consequently, the times of evacuation and average presses acting on individuals during the evacuation are given. Both parameters are compared with and without symmetry considerations. They are calculated in the simulation procedure. For the realization of the simulation tasks the new program (using modified Helbing model) has been elaborated.
3
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Magnetic Structures of Ho_5Rh_4Ge_{10}

81%
Szytuła A., Penc B., Hofmann M., Sikora W.
Acta Physica Polonica A
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2013
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vol. 124
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issue 6
1002-1004
EN
A powder diffraction measurement of Ho_5Rh_4Ge_{10} is reported. This compound crystallizes in the tetragonal Sc_5Co_4Si_{10}-type structure (space group P4/mbm) in which the Ho atoms occupy three different sites. The neutron diffraction measurements indicate antiferromagnetic order with the Néel temperature T_{N}=7 K. Below T_{N} an additional phase transition at 4.5 K connected with the change of the magnetic structure is observed. The Ho moments in 4(h) site form collinear order up to T_{N} while moments at 2(a) site form sine modulated structure. Determined experimentally magnetic structures are compared to the results of symmetry analysis.
4
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Magnetic Structure of RNiGe and RPdSn Compounds

52%
André G., Bourée F., Bombik A., Oleś A., Sikora W., Kolenda M., Szytuła A., Pacyna A., Zygmunt A.
Acta Physica Polonica A
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1994
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vol. 85
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issue 2
275-278
EN
Magnetic properties including magnetic structure of ternary RNiGe (R = Tb-Er) and RPdSn (R = Ce, Tb-Ho) compounds are determined. In low temperatures an antiferromagnetic ordering is observed. For compounds with R = Tb-Er the magnetic structure changes with temperature.
5
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Strategies in Crowd and Crowd Structure

52%
Gawroński P., Malarz K., Krawczyk M., Malinowski J., Kupczak A., Sikora W., Kułakowski K., Wąs J., Kantelhardt J.
Acta Physica Polonica A
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2013
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vol. 123
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issue 3
522-525
EN
In an emergency situation, imitation of strategies of neighbours can lead to an order-disorder phase transition, where spatial clusters of pedestrians adopt the same strategy. We assume that there are two strategies, cooperating and competitive, which correspond to a smaller or larger desired velocity. The results of our simulations within the Social Force Model indicate that the ordered phase can be detected as an increase of spatial order of positions of the pedestrians in the crowd.
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