Decagonal Model Based on Periodic Structures in the Al-Pd-Mn System
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Decagonal Model Based on Periodic Structures in the Al-Pd-Mn System Michel Boudard
L TPCM/ENSEEG,UMR5614 CNRS/INPG /UJF BP75, 38402 St Martin d'Heres Cedex, France ABSTRACT
The structure of periodic phases, in the Al-Pd-Mn system, showing the importance of chemical ordering is reviewed as well as their description in term of pentagonal chains of transition metals. These features are discussed in relation with structural models for AlPdMn decagonal quasicrystals. INTRODUCTION
Many works hav e been related to structural models of decagonal and icosahedral quasicrystals. Several of these models were derived from closely related crystalline structures[1, 2, 3], as, for instance, the cubic Al-Mn-Si[4] phase (related to icosahedral quasicrystals) or the orthorhombic Al3 Mn[5] phases (related to decagonal quasicrystals). Main advantage of working with crystals is that the structural features can precisely be described thanks to the nowadays a v ailable crystallographic tools for solving periodic structures.By example if a similar chemical order exists in the related quasicrystalline phase, then it could be used in order to build or improv e models describing the quasiperiodic structure. T oillustrate these features, we will brie y review a crystalline structure closely related to decagonal quasicrystals in the Al-Pd-Mn system. In particular we will discuss its description in term of pentagonal chains of transition metals[6] in relation with structural models for AlPdMn decagonal quasicrystals. DISCUSSION The structure of the T-Al3 (Mn,Pd) phase
Chemical order plays an important role in the formation of quasicrystalline and crystalline structures closely related to quasicrystals. We will brie y review a study[7] of the chemical order in a periodic phase isomorphic to the T-Al3 Mn phase (Ta ylor's phase[5]) and hereafter called T-Al3(Mn,Pd). The structure of T-Al3Mn has been used to derive a model for the Al70 5 Mn16 5 Pd13 decagonal phase[3] with 1.2 nm period and relations between both phases hav e been studied by several authors[3, 8, 9]. Thus the understanding of chemical order in T-Al3 Mn and T-Al3 (Mn,Pd) phases should be of great interest for a further progress on quasicrystalline models. The T-Al3(Mn,Pd) phase has a small content of Pd (about 3 at %)[10] and, with respect to the isomorphic T-Al3 Mn phase, we have investigated how Pd substitutes and yields a chemical ordering. Results of the structure determined b y single crystal X-ray di raction are summarized in Table 1. The structure of the phase TAl3 (Mn,Pd) is orthorhombic , with Al72 3 Pd3 2 Mn24 5 composition, its space group is Pnma and magnitudes of cell parameters are a=1.4717 nm, b= 1.2510 nm and c=1.2594 nm. The T-Al3(Mn,Pd) structure is built of two lay ers stack ed along the b axis: a at la y er F, and a puckered la y erP composed of two sublay ers P1 and P2. The stacking sequence :
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K4.1.1
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C+ +A
Al atoms (layer F and F’)
B+
TM atoms (layer F) TM atoms (layer F’)
c)
d)
e)
Figure 1: Flat (F) and puckered (P1 and P2) layers (fro
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