Structure of ν-Al_80.61Cr_10.71Fe_8.68, a giant hexagonal approximant of a quasicrystal determined by a combination of electron microscopy and X-ray diffraction

Abstract

Nu-Al(80.61)Cr(10.71)Fe(8.68), P6(3)/m (No. 176), a = 40.68 (7), c = 12.546 (1) A, V = 17,983 (8) A3, atoms/cell = 1184.56, Dx = 3.518 g cm(-3), lambda(Mo K alpha) = 0.71069 A, mu = 5.032 mm(-1), F(000) = 18,433, T = 293 K, final R = 0.075 for 3854 reflections with Fo > 4sigma(Fo). The [001] high-resolution electron-microscopic image of the nu-AlCrFe phase clearly shows similar local characteristics to those given by the complex icosahedral cluster found in somewhat smaller hexagonal approximant structures, such as kappa-Al76Cr18Ni6 [a = 17.674 (3), c = 12.516 (3) A; Sato et al. (1997). Acta Cryst. C53, 1531-1533; Marsh (1998). Acta Cryst. B54, 925-926] and lambda-Al(4.32)Mn [a = 28.382 (9), c = 12.389 (2) A; Kreiner & Franzen (1997). J. Alloys Compd. 261, 83-104]. Using the known atomic distribution of this icosahedral cluster in the kappa and lambda phases as the starting point, the structure of the nu phase, a hexagonal intermetallic compound with probably the largest a parameter, was solved by X-ray single-crystal diffraction using direct methods. As in kappa and lambda phases, almost all TM (transition metal) atoms in the complex icosahedral cluster are icosahedrally coordinated. However, contrary to the lambda structure in which about 98% of the TM atoms have icosahedral-coordination, the TM atoms in the v structure also form capped pentagonal prisms in the region between these complex icosahedral clusters, yielding an average icosahedral coordination of about 70% for TM atoms. After rapid solidification, the v phase occurs together with a decagonal quasicrystal with a periodicity of about 12.5 A along its tenfold axis and thus also consists of six layers, two flat ones each sandwiched between two puckered layers in mirror reflection, stacked along the c axis.

On the constitution of Al4(Cr,Fe)

“…At even higher Fe concentrations, a quasiperiodic decagonal D 3 -phase was revealed [15,17]. The structure associated in [4,18] with the so-called n-phase was not observed in our experiments, nor was a C-centered monoclinic phase with lattice parameters a ¼ 3.31, b ¼ 1.23, c ¼ 2.48 nm and b ¼ 112 also reported to be formed in an Al 12 Fe 2 Cr alloy [1,5].…”

Hexagonal phases consisting of icosahedral chains/networks in (0 0 1) layers in Al–Cr–Si alloys

“…There is another family of hexagonal phases whose space group is P6 3 /m [38,39] rather than P6 3 /mmc, such as the -(AlCrCu) [40,41], -Al 4 Mn [19,20], and -(AlCrFe) [38,39]. As shown in their [0 0 1] HREM images, they have the same local structure around the 6 3 axis and it consists of a large number of interpenetrated icosahedra [38,39].…”

“…As shown in their [0 0 1] HREM images, they have the same local structure around the 6 3 axis and it consists of a large number of interpenetrated icosahedra [38,39]. For instance, in a unit cell of -Al 4 Mn 102 out of 104 Mn atoms and 36 Al atoms have icosahedral coordination [19,20].…”

Sich durchdringende polare und unpolare Untergitter in intermetallischen Phasen: die Struktur von NaCd2

“…Einige Beispiele kubischer Phasen, [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] darunter einige als Quasikristallapproximanten identifizierte Strukturen, sind in Tabelle 1 zusammengestellt. [17,19,21,22] Es wurden außerordentliche Anstrengungen unternommen, um diese Strukturen zu verstehen. Die Beschreibungen reichen von komplexen tetraedrischen dichten Kugelpackungen [23,24] bis zu Packungen konzentrischer oder verknüpfter Polyeder oder sich durchdringender Gerüste.…”

“…B. bei der partiellen Nb-Ta-Anordnung in Nb-TaSulfiden. [58] Wir sind diesem Ansatz gefolgt und haben die Bandstrukturen der MgCu 2 -und Mg 17 In Tabelle 2 sind die Gitterplätze binärer Verbindungen des a-Mn-Typs mit den beobachteten Atombesetzungen angegeben (viele Verbindungen des a-Mn-Typs sind Legierungen; eine geordnete Atomverteilung ist nicht festzustellen). Fässler und Mitarbeiter haben diese Tabelle als Teil einer Studie zur Atombesetzung in K 5 Pb 24 zusammengestellt.…”