02-15U

In this paper, phase transitions in the Mitsui model without a longitudinal field but with a transverse one are investigated in the mean field approximation. The one-to-one correspondence has been established between this model and the two sublattices Ising model (or the NNNI model) with longitudinal and transverse fields. Phase diagrams and diagrams of ferroelectric phase existence are constructed. In the case $\Omega =0$, a simple analytical expression for the tricritical temperature and the condition of tricritical point existence are obtained. For $\Omega \not =0$, sets of equations for the tricritical point and for the condition of its existence are written.