Pressure dependence of free-energy expansion coefficients inPbTiO3andBaTiO3and tricritical-point behavior

Previous data for the pressure dependence of the order parameter for ${\mathrm{BaTiO}}_3$ at room temperature (RT) as well as new data for the pressure dependence of the order parameter for ${\mathrm{PbTiO}}_3$ at RT have been analyzed within the framework of a phenomenological theory for ferroelectrics. It is concluded that in ${\mathrm{BaTiO}}_3$ a tricritical point is close but not accessible at RT, while in ${\mathrm{PbTiO}}_3$ it is accessible at p=1.75×${10}^{10}$ dyn/${\mathrm{cm}}^2$, allowing a full characterization of the pressure dependence of the free-energy expansion coefficients.