The tricritical point in BaTiO3

The pressure dependence of the dielectric behaviour of BaTiO 3 , around the ferroelectric-paraelectric phase transition, has been investigated at pressures up to 45 kbar using a solid encapsulation technique and a specially designed test cell. It is found that, as the pressure increases, the line of first-order transitions goes over continuously into a line of second-order transitions at 34+or-2 kbar, 18+or-4 degrees C. The physical mechanism of the tricritical point is discussed in terms of the Landau theory and an expression is obtained which accurately describes the variation of transition temperatures in the vicinity of the tricritical point.