Abstract
Recently several hexagonal polytypes of III-V compounds have been discovered during growth of nanowires. They exhibit a spontaneous polarization. We calculate the polarization using the ab initio density functional theory within the Berry phase and the electrostatic method. We demonstrate its clear relationship to the crystal field characterized by internal-cell parameters and polytype hexagonality. Sign and magnitude of the polarization field in wurtzite follow the deviation of the internal-cell parameter from its ideal value, thereby explaining the opposite behavior of III-nitrides and the corresponding phosphides, arsenides, and antimonides.