  The quantum discrete $\phi ^4$ model at finite temperature is studied in the
mean-field approximation. The phase diagrams are obtained for a wide range of
the model parameters. The domains of applicability for the classical, quantum,
and order-disorder limits are determined. It is shown that a remarkable part of
the phase diagram is reproduced by the first quantum correction to the
classical result. Parameters of Sn$_2$P$_2$S$_6$, SrTiO$_3$, and KH$_2$PO$_4$
are placed in the phase diagram of the model.
