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 σDC 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.