**2. Crystal structure**

Figure 1 compares the projection of atoms (Figs. 1(a) and (b)) and the Z-contrast image of Nb4AlC3 (Fig. 1(c)), wherein the layer stacking sequence of Nb and Al atoms along the [0001] direction was directly shown. In one Nb-C slab, the number of Nb layers was four, i.e., the regular atomic arrangement was four Nb atoms layers per Al atoms layer alternately stacking along the [0001] direction. The Nb layers were separated by close packed Al atom (0001) planes. The Z-contrast image confirmed that the Nb4AlC3 crystallized in the Ti4AlN3-type crystal structure. It basically consisted of two units: nonstoichiometric NbC0.75 slab and Al atomic plane. The atom arrangement of Nb and Al was ABABACBCBC [3].

Fig. 1. (a) and (b) Atom arrangement of Nb4AlC3. (c) High-resolution Z-contrasting TEM image after FFT filtering of Nb4AlC3 [3].
