A new superlattice structure in the Al2.75Ir and Al2.63Rh 1/0 approximants

We investigate the superstructure of binary Al2.75Ir and Al2.63Rh 1/0 approximants by X-ray diffraction method and transmission electron microscopy. Al2.75Ir possesses a 2a × 2a × 2a C-centred monoclinic lattice with space group C2 or Cm or C2/m. Al2.63Rh possesses a 2a × 2a × 2a face centred lattice with space group F23 or Fm3.


Introduction
Binary Al2.75Ir and Al2.63Rh are 1/0 approximants of ternary Al-TM quasicrystals. The structure of Al2.75Ir and Al2.63Rh have been investigated by XRD, and Al2.75Ir and Al2.63Rh possess a primitive cubic lattice with space group P23 (the lattice parameter a = 7.674(1) Å and a = 7.6692(1) Å, respectively) [1]. Fig.1 shows schematic crystal structure of (a) Al2.75Ir and (b) Al63.3Cu12.3Ir24.4 phases. In the case of Al2.75Ir, the primitive cubic lattice is composed by Ir12 (green circle) icosahedron containing central Ir atom and 9~10 Al atoms (small yellow circle). It should be noted that the Al sites inside the Ir icosahedron are partially occupied with a pentagon-dodecahedral like distribution and surround central Ir atom. In contrast, the structure of ternary Al-Cu-TM 1/0 approximants are different from the binary Al-TM 1/0 approximants. The ternary Al-Cu-TM 1/0 approximants possess the double 3 ̅ lattice with the lattice constant =15.3844(2) Å (2 × 2 × 2 ) due to the ordered Cu site inside of the TM icosahedron [2,3]. Therefore, the structure of Al-Cu-TM 1/0 approximant is a 2 × 2 × 2 FCC (space group 3 ̅ ). In the case of Al-Ir, the order-disorder phase transition of Al inside TM icosahedron is predicted by theoretical calculation using pair potentials fitted to an ab initio database [4]. Moreover, Mihalkovič reported that the ordered phase of Al2.75Ir is possibly insulate phase. Similarly, for Al2.73Ir, very weak superlattice reflections were reported in the selected area electron diffraction patterns (SAED) [5]. Hence, it is necessary to investigate the superlattice structure of binary Al-TM(TM = Ir, Rh) 1/0 approximants.
In the present work, we have investigated the superstructure and the composition dependence of the Al2.75Ir and Al2.63Rh by the powder X-ray diffraction (XRD) method and transmission electron microscopy (TEM) study to understand the behaviour of atoms inside TM icosahedral clusters.

Experimental
Al-TM(TM=Ir, Rh) with composition ranging from 70 to 75 % Al were melted by arc method under an argon atmosphere (Al (99.99%), Rh (99.9%) and Ir (99.9%)). And the samples were annealed at 1373K for about 72 hours or 723K for 3 days in the quarts tube under the argon atmosphere, followed by water quenching. Powder X-ray diffraction experiments were carried out using Cu Kα to examine the phase constitution. Additionally, TEM study were performed by JEOL -TKP2 or TOPCON-002b operating at 200kV. The samples for TEM were crushed by agate mortar and dispersed on a copper micro-grid mesh.
. The Al sites (small circle) inside the Ir icosahedron are partially occupied with a pentagon-dodecahedral-like distribution. Fig. 2 shows XRD patterns of (a) Al73Ir27, (b) Al73.5Rh26.5 and (c) Al72.5Rh27.5. The most peaks are indexed by an × × lattice with reported space group P23, but weak superlattice peaks appeared. In Fig. 2, superlattice peaks are indicated by black circles and open circles. For Al73Ir27, the superlattice peaks can be indexed as 2 × 2 × 2 lattice. In addition, the peaks parallel to the (100) seem to be split into two peaks, when the Kα2 removed. Then, it is possible that the superlattice structure is monoclinic.

XRD
On the other hand, for Al-Rh, Al2.63Rh phase is observed in the composition range of Al70~74.5%. Indexed as a 2 × 2 × 2 lattice, the indexes (hFkFlF) satisfied the reflection condition (hFkFlF; hF+kF, kF+lF, lF+hF = even. It is possible that the superlattice structure possesses FCC lattice with a lattice constant aF (= 2a) = 15.340Å. Additionally, superlattice peaks disappeared in the poorer Al compositions below Al72.5%. The composition dependence on the superstructure suggests that the number of Al inside the TM icosahedron contribute to the superstructure. , there are no superlattice reflections, suggesting that the superlattice structure is cubic lattice. All the superlattice reflections can be indexed by considering a 2a×2a×2a lattice (hFkFlF), and satisfy the FCC reflection condition (hFkFlF; hF+kF, kF+lF, lF+hF = 2n). Therefore, the superlattice of Al2.63Rh can be explained as 2a×2a×2a F23 or 3 ̅ .   ). In Fig. 5(a), dark contrast line indicates domain boundary of ordered phase due to the structural phase transition. The structural phase transition might be same mechanism of order-disorder phase transition of Cd6M 1/1 approximants at about 100 K [6]. The domain structure is the rectangular shape of about a few 100 nm. Also, in the SAED taken from domains (Fig.5b), there are diffuse streaks along [001]. Additionally, many dark parallel lines generated by stacking faults can be recognized thickly at intervals of a few dozen nm parallel to (001) in the domains (Fig. 5b). It implies that the stacking of ordered clusters tend to easily disarranged to [001]. To obtain the insulate  In this work, we reported the new superlattice structure of binary Al2.75Ir and Al2.63Rh phases, which was effected by number of Al atoms in side TM(TM = Ir, Rh) icosahedron cluster. Al2.75Ir possesses a 2 × 2 × 2 monoclinic lattice with space group C2 or Cm or C2/m. Al2.63Rh possesses a 2 × 2 × 2 face centred lattice with space group F23 or 3 ̅ . The composition dependence of superlattice reflections suggest that the superlattice strucuture is determined by the number of ordered Al atoms inside TM icosahedron.