Philosophical Magazine, Vol. 86, Nos. 6–8, 21 February–11 March 2006, 709–716 Atomic structure and electron transport properties of the Al–Pd–Mn–Re–Si 1/1-cubic approximant T. TAKEUCHI* EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan (Received 15 May 2005; in final form 19 July 2005) The formation area of the Al–Pd–Mn–Re–Si 1/1-cubic approximant (1/1-AC) was determined by partially substituting Re for Mn in the Al–Pd–Mn–Si 1/1-AC. It was found that the Al–Pd–Mn–Re–Si 1/1-AC is stabilized at compositions expressed by the formula Al 670.125x Pd 11þ0.375x Mn 141.25x Re x Si 8 with Re concen- tration x < 5 at. %. Synchrotron radiation Rietveld analysis was performed on the approximants to investigate the mechanism determining the formation range of the 1/1-AC. Re atoms were found not to be substituting for all Mn atoms but only those in limited sites. This selection rule for the Re substitution is caused by the 10% larger atomic radius of Re than that of Mn. The importance of the size effect of the constituent elements is strongly suggested for stabilizing quasicrystals and approximants. 1. Introduction Thermodynamically stable Al–TM icosahedral quasicrystals (TM-transition metal elements) are known to exhibit a high electrical resistivity exceeding 10 000  cm. The electrical resistivity of the Al–Pd–Re quasicrystal (QC), which has the largest magnitude for any QC, increases above 1  cm at low temperatures and its tempe- rature dependence is well accounted for by the Mott variable range hopping (VRH) conduction inherent in insulators under Anderson localization [1–5]. This behaviour of the electrical resistivity has been frequently discussed in terms of the critical states caused by the quasiperiodicity. However, rather low electrical resistivity (100–200  cm) in some QCs of high structure quality [6, 7] and the high electrical resistivity in the corresponding rational approximants (ACs) [8–11] led us to believe that factors other than the quasiperiodicity would play more an important role in their possession of the high electrical resitivity. Analyses using the corresponding ACs are highly useful to reveal the nature of QCs because periodicity in ACs allows us to employ band calculations and structure analyses well-developed for crystalline materials [8, 10, 11]. It is, therefore, of crucial importance to find approximants of the quasicrystal of interest. In order to find the 1/1-cubic AC (1/1-AC) of the Al 70 Pd 20 Re 10 QC possessing highest electrical resistivity, we substitute Re for Mn in the Al 67 Pd 11 Mn 14 Si 8 1/1-AC [12] because Re *Email: takeuchi@nuap.nagoya-u.ac.jp Philosophical Magazine ISSN 1478–6435 print/ISSN 1478–6443 online ß 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/14786430500263983