We report high-pressure Raman measurements of the zone-center phonons in two ferroelectric crystals that closely resemble the ABO3 perovskite crystal PbTiO3. These crystals are (Pb0.22Ba0.78)TiO3, i.e., Ba replacing Pb on the A site, and Pb(Ti0.81Sn0.19)O3, i.e., Sn replacing Ti on the B site. In both cases, at room temperature, we follow the modes and determine Pc, the transition pressure from the ferroelectric tetragonal phase to the cubic phase, to be 4.3 and 9.0 GPa, respectively. By observing the coalescence to the same frequency of the appropriate high-energy A1(TO)+E(TO) pairs of phonons, we determine the second-order character of the phase transitions at Pc. The tendency towards a second-order phase transition seems to be the rule at Pc as long as one makes the measurements at a temperature well below Tc; this is in agreement with theory. Thus, these systems exhibit tricritical points in the (P,T) phase diagram. The soft-E(TO)-phonon frequency (0) and damping constant () can be measured to pressures reasonably close to Pc while the mode remains underdamped. These results are discussed in terms of a frequency-independent damping constant for the behavior of 0 and near Pc. In the (Pb,Ba)TiO3 crystal, the hydrostatic pressure increases the intensity of the soft A1(TO) mode making it observable. This seems to happen in general in the perovskites. In the Pb(Ti,Sn)O3 crystal we observe the coupling of the soft E(TO) mode with an extra mode at 59 cm-1; this also has been studied as a function of temperature. © 1984 The American Physical Society.