Some new structural and electronic characteristics of quasicrystals

The quasicrystals being based on quasiperiodic order other than crystal lik e periodic translational order and embodying self similarity, present uniqu e condensed matter phases. In addition to their curious structural characte ristics the paucity of translational periodicity leads to drastic deviation s in their electronic behaviour as compared to crystalline counterparts. Th is paper describes and discusses some new developments in regard to structu ral and electronic aspects of quasicrystalline materials. In regard to the structural aspects, two comparatively newer features will be described. One of them relates to the observation of variable strain approximants (VSA) f irst found in Ti68Fe26NiSi5, qc alloys; the other relates to the structure of decagonal phases. The variable strain approximants correspond to qc phas es exhibiting variable strain for the different diffraction spots for the s ame reciprocal lattice row (possessing linear shifts). The VSA is thought t o result from variable phason strain mode locking; this in contrast to RAS which results due to linear phason mode locking. The results obtained in ou r laboratory on VSA will be described and discussed, Another interesting st ructural feature emanating in the last few years relate to the development of structural models for the decagonal phases which have nearly answered th e question 'Where are the atoms?' for this qc variant. High resolution elec tron microscopy has revealed the existence of two types of atom cluster col umns with a diameter of 20 Angstrom; a pentagonal cluster column and a deca gonal one. The decagonal quasicrystals can be classified into three types o f structures according to the space groups and symmetries and arrangements of the cluster columns. These three deca structures have been typified by: deca Al-Co-Cu, deca AI-Mn and deca Al-Fe-Pd types. Some basic features on t he structures of decagonal phases as obtained recently will be outlined. Since materials owe their practical importance due to their physical behavi ours, assessment of qc materials from this point of view is of imperative i mportance. However, the real physics/science governing properties for qc is not well understood as yet. For example, the fundamental property relating to electronic conductivity and its temperature variation has been attempte d to become explicable based on (a) taking qc as disordered materials, (b) assuming qc as hierarchy of clusters and (c) bringing in new concepts gover ning the wave functions of electrons (critical wave functions) and some oth er models. However, the same results are not universally reported/reproduce d by various workers, Thus the variation of electronic conductivities with temperature of Al65Cu20Ru15, Al70Pd20Re10 and other stable quasicrystals ha ve found varied interpretations e.g. based on QIEs and power law temperatur e variation. Some results on sigma-T and related characteristics for stable qc crystals obtained by us and also by other workers will be analysed in t erms of feasible transport mechanisms.