  Our experiments demonstrate that alloying the cubic-phase YbN into the
wurtzite-phase AlN results in clear mechanical softening and enhanced
electromechanical coupling of AlN. First-principle calculations reproduce
experimental results well, and predict a maximum 270% increase in
electromechanical coupling coefficient caused by (1) an enhanced piezoelectric
response induced by the local strain of Yb ions and (2) a structural
flexibility of the (Yb,Al)N alloy. Extensive calculations suggest that the
substitutional neighbor Yb-Yb pairs in wurtzite AlN are energetically stable
along $c$ axis, and avoid forming on the basal plane of wurtzite structure due
to the repulsion between them, which explains that (Yb,Al)N films with high Yb
concentrations are difficult to fabricate in our sputtering experiments.
Moreover, the neighbor Yb-Yb pair interactions also promote structural
flexibility of (Yb,Al)N, and are considered a cause for mechanical softening of
(Yb,Al)N.
