  We identify a new scenario for dynamical phase transitions associated with
time-integrated observables occurring in diffusive systems described by the
macroscopic fluctuation theory. It is characterized by the pairwise meeting of
first- and second-order bias-induced phase transition curves at two tricritical
points. We formulate a simple, general criterion for its appearance and derive
an exact Landau theory for the tricritical behavior. The scenario is
demonstrated in three examples: the simple symmetric exclusion process biased
by an activity-related structural observable; the Katz-Lebowitz-Spohn lattice
gas model biased by its current; and in an active lattice gas biased by its
entropy production.
