Microwave and infrared dielectric properties of Sr_1−3x/2Ce_xTiO₃ (x = 0.154–0.400) incipient ferroelectrics at cryogenic temperatures

Sr_1−3x/2Ce_xTiO₃ (x = 0.154–0.400) or Sr_2+nCe₂Ti_5+nO_15+3n (n ⩽ 8) ceramics were prepared by the mixed oxide route. The microwave (MW) dielectric properties of the compounds were investigated in the temperature range from 8 to 295 K. The permittivity increases for decreasing temperatures and saturates below 30 K, following Barrett's equation, demonstrating the incipient ferroelectric nature of the investigated materials. The dielectric loss tangent decreases for decreasing temperatures, reaching a minimum at about 80–120 K, and again increases with further cooling due to the rotations of TiO₆ octahedra. Infrared-reflectivity data show that the dielectric response of the system is driven by the lowest-frequency polar (soft) mode, particularly at lower temperatures, where the phonons become practically uncoupled. The results help us to understand why Sr_1−3x/2Ce_xTiO₃ materials present more appropriate dielectric properties for MW tunable applications, compared with pure SrTiO₃.