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.
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