Large, larger, largest – a family of cluster-based tantalum copper aluminides with giant unit cells. II. The cluster structure

Abstract

This is the second of two papers, where we discuss the cluster structures of a novel family of cluster-based intermetallic phases of unprecedented complexity: cF444-Al(63.6)Ta(36.4) (AT-19), a = 19.1663 (1) A, V = 7040 A3, cF(5928-x)-Al(56.6)Cu(3.9)Ta(39.5), x = 20 (ACT-45), a = 45.376 (1) A, V = 93,428 A3 and cF(23,256-x)-Al(55.4)Cu(5.4)Ta(39.1), x = 122 (ACT-71), a = 71.490 (4) A, V = 365,372 A3. The space group is F43m in all three cases. The structures can be described as packings of clusters such as fullerenes, dodecahedra, pentagonal bifrusta and Friauf polyhedra. A characteristic feature of the two larger structures are nets of hexagonal bipyramidal Ta clusters (h.b.p.). The extremely short distance of 2.536-2.562 A between their apical Ta atoms indicates unusually strong bonding. The large h.b.p. nets are sandwiched between slabs of Friauf polyhedra resembling the structure of the mu phase.

Mg11Cu6Al12, A New Link in the Structural Chemistry of MgCu2‐Type Clusters

“…NaCd 2 has been joined by not only the closely related structures of β-Mg 2 Al 3 [4] and Cd 3 Cu 4 , [5] but also by the exponentially more complex structures of Al 56.5 Cu 3.9 Ta 39.5 (cF5908) and Al 55.4 Cu 5.4 Ta 39.1 (cF23134). [6,7] Our ability to solve the complex crystal structures of these phases has greatly improved since Pauling's original experiments on NaCd 2 , but the chemical bonding factors underlying the formation of these incredible structures remain mysterious.…”

“…This attempt is based on the observa- [ tion of a shared feature of these phases: they all contain large blocks of the cubic Laves phase structure type, the MgCu 2 type. [8][9][10]6,7] We wondered how tunable these blocks might be, and whether it is possible to enlarge or shrink them through, say, elemental substitution. A simple way to approach this would be to examine ternary systems which contain both Laves phases and one of these complex cubic structures.…”

Synthesis, Crystal Structure, and Magnetic Properties of Giant Unit Cell Intermetallics R117Co52+δSn112+γ (R = Y, La, Pr, Nd, Ho)

“…The realm of these materials can be expanded by slight variation of the compositions that can lead not only to the expansion of the existing isostructural systems but also to the formation of new crystal structures. For instance, in the Al-Cu-Ta system [11,12], Al 56.6 Cu 3.9 Ta 39.5 crystallizes in the F43m space group with the unit cell volume around 93,000 Å3, but a slight compositional change leads to another phase, Al 55. 4 Cu 5.4 Ta 39.1 , which exhibits a more complicated crystal structure and has the largest intermetallic unit cell reported to date, with the volume around 365,000 Å3.…”

More statistics on intermetallic compounds – ternary phases

“…One example of such a ternary system is Al-Cu-Ta, where Cu and Ta are immiscible. Three complex intermetallic compounds have been discovered in this ternary system so far Conrad et al, 2009;Dshemuchadse et al, 2013). In the remaining 15 ternary systems already studied, ternary compounds have been observed, although two of the three binary subsystems do not form any intermetallic phase.…”