Complex dielectric behaviours in $BiFeO_{3}/Bi_{2}Fe_{4}O_{9}$ ceramics

The complex dielectric permittivity of a sintered ceramic tablet consisting of 70.5 % $BiFeO_{3}$ and 27.7% $Bi_{2}Fe_{4}O_{9}$ was analyzed as a function of temperature from -120 {\deg}C to 230 {\deg}C. The results reveal a complicated dielectric response with temperature activated relaxation features. They also reveal a ferroelectric phase transition that decayed with repeated heating cycles of the tablet. The source of the behaviour is assigned to relaxation processes happening along the grain boundaries of differing compositions in the tablet. The origin of the phase transition is traced to locally induced strains on grain boundaries because of unit cell size mismatch between $BiFeO_{3}$ and $Bi_{2}Fe_{4}O_{9}$.