Semiconductor–Conductor Transition Analysis by Low-Frequency Impedance in Ultrasonically Synthesized Al-Doped Sodium Tantalate

An aluminum-doped NaTaO₃ perovskite sample was prepared by the ultrasonic method, employing an immersed sonotrode, followed by thermal treatment at 600 °C for 6 h in air. X-ray diffraction analysis reveals a biphasic system with relatively low crystallinity, consisting of a dominant NaTaO₃ perovskite phase and a secondary Na₂Ta₄O₁₁ phase. Optical investigations indicate a reduced band gap energy of 3.77 eV compared to undoped NaTaO₃ (4 eV), suggesting enhanced absorption toward the infrared region and improved photocatalytic potential. Fourier Transform Infrared FTIR Spectroscopy highlights the emergence of a distinct absorption band at 670 cm⁻¹, attributed to Ta–O and Al–O stretching vibrations, evidencing successful incorporation of Al dopants. Complex impedance analysis over the frequency and temperature ranges of (20 Hz–2 MHz) and (29–100) °C identifies, for the first time, the semiconductor–conductor transition temperature at 58 °C. Nyquist analysis further supports the coexistence of grain and grain boundary contributions, modeled via equivalent R and CPE parallel circuits. Conductivity studies confirm obedience to Jonscher’s universal law, with a change in σ_DC slope near 54 °C, corroborating semiconductor–conductor transition behavior. Dielectric measurements similarly indicate a relaxation process linked to interfacial polarization, with a transition temperature of (~54 °C). Overall, the ultrasonic synthesis route uniquely enables a biphasic structure that facilitates the observation of a low-temperature semiconductor-to-conductor transition, absent in analogous single-phase materials obtained via sol–gel methods.