  We report on the synthesis of new nitride-based compound by using annealing
of AlN heteroepitaxial layers under a Si-atmosphere at temperatures between
1350$^\circ$C and 1550$^\circ$C. The structure and stoichiometry of this
compound are investigated by high-resolution scanning transmission electron
microscopy (HRSTEM), energy dispersive X-Ray (EDX) spectroscopy, and density
functional theory (DFT) calculations. The identified structure is a derivative
of the parent wurtzite AlN crystal where anion sublattice is fully occupied by
N atoms and the cation sublattice is the stacking of 2 different planes along
<0001>. The first one exhibits a $\times$3 periodicity along <10-10> with 1/3
of the sites being vacant. The rest of the sites in the cation sublattice are
occupied by equal number of Si and Al atoms. Assuming a semiconducting alloy,
which is expected to have a wide band gap, a range of stoichiometries is
proposed, Al$_{5+\alpha}$Si$_{5+\delta}$N$_{12}$, with $\alpha$ being between 0
and 1/3 and $\delta$ between 0 and 1/4.
