  The spontaneous polarization of wurtzite III-V nitrides XN (X=Al, Ga, In) and
II-VI oxides YO (Y=Be, Zn) is investigated via first-principles computational
methods. The modern treatment defines this quantity as the polarization
difference between the investigated system and an appropriate reference state.
We demonstrate that the trigonal structure can be used as a reference to
determine the spontaneous polarization of wurtzite materials. We compare the
current values with the widely-known zincblende results reported in the
literature and find a very good agreement. It is shown that the electronic
contribution of polarization is greater than the ionic one. Furthermore, we
reproduce the experimental value of the spontaneous polarization of wurtzite
GaN reported in a previous study. In order to do so, we calculate the
spontaneous polarization for each type of stacking faults using periodic
supercells and the Berry-phase method. This theoretical analysis leads to a
value nearly identical to the experimental measurement.
