  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. The first principle calculations reproduce
experimental results well, and predict a maximum of 270% increase in
electromechanical coupling coefficient caused by 1) enhanced piezoelectric
response induced by the local strain of Yb ions and 2) structural flexibility
of the YbAlN alloy. Additionally, our calculations reveal the existence of
repulsion among tetrahedral Yb$^{3+}$ sites, which suppresses further
mechanical softening for YbAlN at high Yb concentrations.
