  The phase transitions in the Bose-Hubbard model are investigated. A
single-particle Green's function is calculated in the random phase
approximation and the formalism of the Hubbard operators is used. The regions
of existence of the superfluid and Mott insulator phases are established and
the $(\mu,t)$ (the chemical potential -- transfer parameter) phase diagrams are
built. The influence of temperature change on this transition is analyzed and
the phase diagram in the $(T,\mu)$ plane is constructed. The role of thermal
activation of the ion hopping is investigated by taking into account the
temperature dependence of the transfer parameter. The reconstruction of the
Mott-insulator lobes due to this effect is analyzed.
