  The origin of the stabilized simple-cubic (SC) structure in Po is explored by
using the first principle band calculations. We have found that the prime
origin is the inherent strong spin-orbit (SO) interaction in Po, which
suppresses the Peierls-like structural instability as usually occurs in
p-bonded systems. Based on the systematic analysis of electronic structures,
charge densities, Fermi surfaces, and susceptibilities of Se, Te, and Po, we
have proved that the stable crystal structure in VIA elements is determined by
the competition between the SO splitting and the crystal field splitting
induced by the low-symmetry structural transition. The trigonal structure is
stabilized in Se and Te by the larger crystal field splitting than the SO
splitting, whereas in Po the SC structure is stabilized by the large SO
splitting.
