Enhancement of piezoelectric property in MgTMAlN (TM = Cr, Mo, W): First-principles study

We report large enhancement in piezoelectric response (d33) of wurtzite-AlN by addition of Mg and TM (TM = Cr, Mo, W). The piezoelectric properties of wurtzite MgxTMyAl(1−x−y)N was calculated by using first-principles calculations. In the composition range of 0.25 ≦ x + y ≦ 0.5, MgMoAlN and MgWAlN exhibit higher d33 values than MgCrAlN. Among them, MgWAlN exhibited the highest d33, where the d33 value was found to be 10 times higher than that of AlN (d33 ≃ 5.3 pC/N), and 1.5 times higher than that of ScAlN (d33 ≃ 30 pC/N). We have also revealed that a higher elastic softening is the main contribution to the increase of d33 in MgMoAlN and MgWAlN. From Bader charge analysis, Mg, Mo, and W have a lower charge than that of Al which indicate that the ionic bonding character is a weaker than Al. From crystal orbital Hamilton population analysis, covalent bonding between Mg–N and Mo/W–N are also found to be weaker than Al–N. Thus, a weaker ionic and covalent bonding are considered to play an important role in promoting elastic softening for MgMoAlN and MgWAlN, which eventually promoted significant enhancement in piezoelectricity.