The ceramic system with composition (1-x)[Bi12In0.5O18.75 + *g-Bi2O3]:(x)PbTiO3 with x = 0.0, 0.3, 0.5 and 0.8 was synthesized and subsequently characterized for its crystal structure and dielectric properties. X-ray diffraction showed that the ceramic with nominal composition Bi12In0.5O18.75 + *g-Bi2O3 has sillenite structure, which is an uncommon metastable mineral phase with composition Bi12In0.5O18.75. The dielectric constants and loss tangents were measured at different frequencies and temperatures. The dielectric properties of the sample with x = 0.0, Bi12In0.5O18.75+*g-Bi2O3, showed strong frequency and temperature dependence of dielectric constant and loss tangent. The high value of both the dielectric constant and loss tangent showed a conductivity value similar to a typical semiconductor. The addition of PbTiO3 significantly reduced the temperature and frequency dependence at the low frequency range. For the sample x = 0.8 (80% PbTiO3) the dielectric properties are similar to a typical dielectric material and the conductivity was not revealed in the loss tangent. The results indicate that the electrical properties of (1-x)[Bi12In0.5O18.75 + *g-Bi2O3]:(x)PbTiO3 ceramics can be tuned from a conducting oxide to an insulator and to dielectric, depending on the amount of PbTiO3
