Date Published: March 01, 2019
Publisher: International Union of Crystallography
Author(s): Anastasiya I. Vinokur, Katerina P. Hilleke, Daniel C. Fredrickson.
The Fe2Al5 structure is remarkable among intermetallic phases for its channels of weakly ordered Al atoms. This article traces the origins of these channels to the cooperative effects of soft atomic motions dictated by chemical pressure quadrupoles and preferred electron concentrations.
The structural chemistry of intermetallic phases is rich in phenomena that go beyond simple notions of translational symmetry in 3D space, such as disorder, incommensurability and quasicrystalline arrangements. In each of these scenarios, weakly specified atomic positions are often distributed within frameworks of rigidly positioned atoms with a variety of morphologies. In some cases, such as the disordered tetrahedra at the centers of the icosahedral building units of Tsai-type quasicrystals and their approximants (Gómez & Lidin, 2003 ▸; Piao et al., 2006 ▸; Takakura et al., 2007 ▸), the less organized regions can be isolated to small pockets of the structure. Disordered or incommensurately spaced atoms can also run along features of higher dimensionality: 1D columns (Mählpfordt, 1997 ▸; Rohrer et al., 2001 ▸; Piao & Lidin, 2007 ▸; Kanno et al., 2017 ▸) or 2D planes (Latturner & Kanatzidis, 2002 ▸; Gray et al., 2008 ▸; Kilduff & Fredrickson, 2016 ▸). In the most extreme examples, as in the classic β-Mg2Al3 structure (Samson, 1965 ▸; Feuerbacher et al., 2007 ▸), the scrambled atoms can form 3D continuous sublattices that permeate the whole crystal. Such a duality of strict and loose order within the same structure offers immense opportunities for material properties, e.g. for the realization of the phonon-glass electron crystal concept (Nolas et al., 1999 ▸), or the creation of well defined paths for atomic diffusion (Mehrer, 1996 ▸). However, the absence of periodic order in these situations makes their design from first-principles calculations challenging. In this article, we will work towards deriving chemical principles for the emergence of such features in intermetallics, using the channels of weakly ordered atoms in the Fe2Al5 structure as a model system.
This work was inspired by the curious disordered channels of Al atoms that appear in the high-temperature structure of Fe2Al5 and contribute to its promise as a thermoelectric material. After confirming an experimental model in which the Al atoms are disordered in an undulating pattern throughout the compound, we applied a series of theoretical methods to elucidate the role of this channel in stabilizing the structure. Using the raMO analysis, we traced the nonstoichiometry of the Al content to the desire by the Fe bonding network to achieve 18-electron configurations, following the 18 − n rule. DFT-CP schemes then revealed how the channels in the structure accommodate the Al2 disorder: the placement of these sites between zigzag chains of Fe atoms leads to CP quadrupoles that allow for soft motions threading Al atoms along the path defined by the Fe.
The following references are cited in the supporting information: Bruker (2016 ▸), Rigaku (2015 ▸), von Goldbeck (1982 ▸), Grin et al. (1994 ▸), Petříček et al. (2014 ▸), Sheldrick (1996 ▸, 2015a ▸,b ▸).