Metastable Al-Fe intermetallic stabilised by epitaxial relationship

We report the adsorption of Fe for several dosing conditions on Al(1 0 0) surface held at different temperatures. For 2 monolayer equivalent of Fe deposited on the substrate held at 593 K, we observe two rotational domains of a (root 5 x root 5)R26.6 degrees structure by low energy electron diffraction and scanning tunneling microscopy. We identify this structure as the formation of the metastable Al9Fe2 intermetallic phase. The segregation energy and the structural stability of this intermetallic has been analysed using density functional theory based calculations. We propose that the formation of the Al9Fe2(0 0 1) metastable phase at the expense of other thermodynamically stable Al-Fe compounds, relies on a specific structural similarity between pure Al (0 0 1) planes of the intermetallic and Al(1 0 0) layers. These 'common' planes lead to the epitaxial relationship (1 0 0)(Al) [0 0 1](Al) parallel to (0 0 1)(Al9Fe2) [1 (3) over bar 0](Al9Fe2). The Al9Fe2-Al matrix interface is sharp as revealed by transmission electron microscopy measurements.