The quasicrystal approximant Zn6Sc: a surprising dynamical flexibility
Quasiscrystals are materials in which the atoms are arranged in such a manner that long range order is maintained, however, without periodicity, and thus in contrast to conventional crystals. To better understand the structure and the way atoms move inside a quasicrystal, an international team of European and Japanese scientists (European C-MAC network, www.eucmac.eu) has studied the periodic approximant Zn6Sc, using an approach that combines X-ray (beamline D2AM at the ESRF) and quasielastic neutron scattering experiments (IN5 at the ILL) with atomistic computer simulations. The structure of this periodic approximant(*) is described by a close-packing of atomic cluster motives, consisting of successive shells (like a Matryoshka), arranged on a body centered cubic lattice (lattice parameter 1.38 nm). The cluster actually consists of a close-packing of « small » Zn atom and « large » Sc atoms (see figure) and is also found in the quasicrystalline phase. In a surprising manner all cluster shells are close to icosahedral symmetry(**), except for the innermost one, which is a tetrahedron.
When the temperature decreases below 160 K, these tetrahedra order in an antiparallel fashion along the  direction (see figure). Above 160 K, X-ray scattering experiments clearly evidence the existence of short-range order, meaning that the tetrahedra already begin to organize themselves with respect to each other. Quasielastic neutron scattering experiments have shown that the occurring disorder is dynamic in nature. The central zinc tetrahedron behaves as a single « molecule » and reorients by flipping between equivalent positions. These jumps occur very frequently, approximately every few picoseconds(***). Atomistic simulations, in perfect accord with the experimental results, have allowed showing that the rotation of the zinc tetrahedron is accompanied by a strong deformation of the successive cluster shells, as depicted in the lower part of the figure.
This results in a completely new picture of this quasicrystal approximant, with a dynamical flexibility that is absolutely exceptional for an intermetallic alloy. The structure gets permanently deformed at an elevated frequency for each rotation of the tetrahedron. These deformations enable an understanding of the interaction mechanism between the distant (~1.4 nm apart) tetrahedra. Moreover, the unique flexibility certainly plays an important role in the stabilization mechanisms of these complex phases and the corresponding quasicrystal, as well as for thermal transport properties of these materials
(*) A quasicrystal approximant is the periodic version of a quasicrystal: The same atomic clusters are packed periodically (in case of the approximant) or quasiperiodically, meaning ordered but not periodic (in case of the quasicrystal).
(**) Having the shape of an icosahedron, the surface of which consists of 20 equilateral triangles
(***) 1 picosecond (ps) = 10-12 seconds or one trillionth of a second.
Euchner, H., Yamada, T., Schober, H., Rols, S., Mihalkovic, M., Tamura, R., Ishimasa, T. & de Boissieu, M.
Ordering and dynamics of the central tetrahedron in the 1/1 Zn6Sc periodic approximant to quasicrystal
J. Phys. : Condens. Matter 2012, 24, 415403.
Marc de Boissieu, Sciences de l'Ingénierie, des Matériaux et des Procédés – Grenoble
Email : Marc.de-Boissieu@simap.inpg.fr
Phone : 04 76 82 67 03