Abstract: (Nb5+, Si4+) co-doped TiO2 (NSTO) ceramics with the compositions (Nb0.5Si0.5)xTi1􀀀xO2,
x = 0, 0.025, 0.050 and 0.1 were prepared with a solid-state reaction technique. X-ray diffraction
(XRD) patterns and Raman spectra confirmed that the tetragonal rutile is the main phase in all the
ceramics. Additionally, XRD revealed the presence of a secondary phase of SiO2 in the co-doped
ceramics. Impedance spectroscopy analysis showed two contributions, which correspond to the
responses of grain and grain-boundary. All the (Nb, Si) co-doped TiO2 showed improved dielectric
performance in the high frequency range (>103 Hz). The sample (Nb0.5Si0.5)0.025Ti0.975O2 showed the
best dielectric performance in terms of higher relative permittivity (5.5 104) and lower dielectric
loss (0.18), at 10 kHz and 300 K, compared to pure TiO2 (1.1 103, 0.34). The colossal permittivity
of NSTO ceramics is attributed to an internal barrier layer capacitance (IBLC) effect, formed by
insulating grain-boundaries and semiconductor grains in the ceramics.