  Aluminum nitride (AlN) is an important piezoelectric material for a wide
range of applications, many efforts are devoted to improving its piezoelectric
response by alloying with transition metals (TMs). In this paper, the influence
of the type and distribution of TM on the piezoelectric response is discussed
for the first time. TM0.0625Al0.9375N with twenty-eight different TMs are
investigated, and most show higher values of piezoelectric strain modulus d33
than that of AlN. This is because the TM introduces weaker TM-N bonds and
locates closer to the centre of three neighbouring N atoms. The location of TM
is determined to be significantly correlated with its group number. Alloys of
TMxAl1-xN (TM=Sc, Cr, Sr, Mo, Ru and Rh) with varying x are further studied. On
basis of the cost of the TMs and piezoelectric performances, the alloy with Mo
is more effective in enhancing d33. A high d33 of 12.3 times that of pure AlN
is realized in a metastable configuration of Mo0.167Al0.833N. The distribution
of Mo plays a key role in the piezoelectric performance. A higher d33 is more
likely to appear in MoxAl1-xN with more Al sublayers containing Mo atoms and
with fewer dimers of Mo atoms along the z-axis.
