  We theoretically study the structure of a stationary soliton in the polar
phase of spin-1 Bose--Einstein condensate in the presence of quadratic Zeeman
effect at zero temperature. The phase diagram of such solitons is mapped out by
finding the states of minimal soliton energy in the defining range of polar
phase. The states are assorted into normal, anti-ferromagnetic,
broken-axisymmetry, and ferromagnetic phases according to the number and spin
densities in the core. The order of phase transitions between different
solitons and the critical behaviour of relevant continuous transitions are
proved within the mean-field theory.
