  Remarkable feature of the phase diagram of sulfur hydrides is a sharp
increase in critical temperature from 120K up to200K. This increase is a
signature of the structural transition. The study described below is concerned
with the nature of this phase transition. One can demonstrate that the symmetry
analysis along with an analysis of the impact of lattice deformations lead to
the conclusion that we are dealing with the first order transition. It leads to
an abrupt appearance of small pockets on the Fermi surface and to the two-gap
energy spectrum.
