Arrangements of Transition-Metal Atoms in Three Types of Al–Co–Ni Decagonal Quasicrystals Studied by Cs-Corrected HAADF-STEM

Abstract

Three types of AlCoNi decagonal quasicrystals in Al 72.5 Co 11 Ni 16.5 , Al 71 Co 14.5 Ni 14.5 and Al 71.5 Co 16 Ni 12.5 alloys have been examined by Cs-corrected high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM), which has an enough resolution to represent individual TM atoms as bright dots in observed images of decagonal quasicrystals. On the basis of the structure of a W-(AlNiCo) crystalline approximant, transition-metal (TM) atoms and mixed sites (MSs) of Al and TM atoms are separately distinguished in HAADF-STEM images. Most of TM atoms are arranged with pentagonal tilings of a bond-length of 0.76 nm, and MSs are located inside of pentagonal frames with definite orientations. All of the TM atoms and MSs are located at lattice points of a Penrose lattice of an edge-length of 0.25 nm, and arranged with a bond-orientational order. The present result shows that the structures of AlCoNi decagonal quasicrystals should be characterized as BOO arrangements of transition-metal atoms, instead of previous models of BOO arrangements of 2 nm atom clusters.

Structure of an Al-Fe-Ni Decagonal Quasicrystal Studied by Cs-Corrected STEM

“…3a, with an edge-length of 0.25 nm, and consequently the TM atoms and MSs are arranged with BOO. The pentagonal arrangements of MSs in pentagonal frames with denite directions can be seen in the B plane of the structure of W(AlCoNi) [1,10] and τ 2 -Al 3 Co [2] crystalline phases, which are considered to be important approximants for the interpretation of Al-TM decagonal quasicrystals and in quasiperiodic planes of several types of Al-Co-Ni decagonal quasicrystals [1,4,5]. is generated by the projection of a ve-dimensional (5D) hyper-cubic lattice through a window of Fig.…”

Structure of a crystalline approximant related to Al–Co–Ni decagonal quasicrystals studied by spherical aberration (Cs)-corrected scanning transmission electron microscopy and atomic-resolution energy dispersive X-ray spectroscopy

“…Introduction Six types of decagonal quasicrystals (DQCs) and some crystalline approximants have been found in Al-Co-Ni alloys with various compositions from 8 to 25.5 at % Co and from 20 to 5 at % Ni, respectively, with a nearly constant Al content from 70 to 72 at % [1][2][3][4][5], and so the stability of these DQCs formed in alloys with different Ni/Co ratios is considered to be affected by chemical ordering of Co and Ni. However, the study of the chemical ordering of Co and Ni in the Al-Co-Ni DQCs and their approximants has never been performed, because it has been difficult, if not impossible, to distinguish between neighbouring elements Co and Ni, which show very similar contrast to both electron and X-ray probes, although transition-metal (TM) atom arrangements have been exactly determined by recent Cs-corrected scanning transmission electron microscopy (STEM) observations [6][7][8][9][10][11].…”

“…It should be noticed that two types of the 1.2 nm clusters with opposite orientations of the pentagonal arrangements at their centres are arranged in an ordered manner in such a way that two clusters connected by a bond are always different. Figure 2b is derived from the arrangement of bright dots in Figure 2a on the basis of structures of the W-(AlCoNi) phase [6] and several Al-TM DQCs and crystalline approximants [6,7,[9][10][11]. The 1.2 nm clusters arranged with an interval of 2 nm are connected by two types of pentagonal tilings with a bondlength of 0.76 nm, as indicated by solid and broken lines in Figure 2a.…”

“…The cluster is a common structure unit in most of the Al-TM DQCs and crystalline approximants [6][7][8][9][10][11], and is called a 1.2 nm cluster with a diameter of 1.2 nm. The 1.2 nm clusters are located at vertices of a rhombic lattice formed by the periodic array of obtuse and acute rhombuses, which form the Penrose rhombic tiling, with a bond length of 2 nm.…”

“…Also, it can be seen that the 1.2 nm cluster is formed with TM atoms located at corners of a star-shaped pentagon with an edge length of 0.47 nm in a certain plane and at those of a pentagon with an edge length of 0.76 nm in a different plane, as indicated in Figure 2b. Besides the bright dots at the vertices of the 0.76 nm pentagonal tilings, relatively weak bright dots, in upward pentagonal tiles indicated by solid lines and in downward ones of broken lines, are arranged with small pentagonal distributions, and they are considered to correspond to MSs from the previous studies [6,7,[9][10][11]. The MSs with a pentagonal arrangement (hereafter call pentagonal MSs) located in pentagonal tiles with definite orientations in the 0.76 nm pentagonal tilings can be seen in structures of the W-(AlCoNi) phase [6] and many Al-TM DQCs and approximants [6,7,[9][10][11].…”