98-25E

The study of specific features of the apex oxygens thermodynamics in the YBa$_2$Cu$_3$O$_{7-x}$--type high--T$_c$ superconductors affected by oxygen vacancies in CuO chains is made in the framework of the extended two--sublattice Mitsui model. The influence of vacancies is modelled by a random field with the bimodal distribution. Thermodynamic functions of the model are calculated in the mean field approximation for long--range interactions with an exact treatment of the intracell interaction of pseudospins. The system can have nonequivalent nonpolar phases and the polar one which correspond to different arrangements of apex oxygens. Conditions of appearance of the phase transition between nonpolar phases at $x\not =0$ which is of the first order and manifests itself as a structure bistability effect are investigated.