Defect-mediated n-type to p-type transition and enhanced thermoelectric performance in Bi_1.8-xNi_xSb_0.2Te₃ alloys

A systematic investigation pertaining to the low temperature (10-350 K) thermoelectric properties of Ni substituted Bi_1.8-xNi_xSb_0.2Te₃ alloys (x = 0-0.08), it was synthesised employing solid state reaction method. X-ray diffraction studies verified the presence of phase pure rhombohedral structures. FESEM micrographs showed a morphological transition from hexagonal platelets to granular networks as the Ni content increased. Electrical transport measurements revealed a transition from n-type to p-type conduction for the sample x ≥ 0.06. This transition is attributed to the formation of acceptor defects, which increases the hole concentration as a majority charge carriers. The thermal conductivity decreased systematically from 1.7 to 1.0 Wm^-1 K^-1 at 350 K with substitution of Ni concentration, as a result of the increased phonon scattering due to the mass disorder and strain fields. The optimal composition (x = 0.04) unveiled a maximum [Formula: see text] and [Formula: see text] of 335 μW/mK² and 0.09 respectively at 350 K, [Formula: see text] has 125% increment over the pristine sample. These results validates that Ni doping effectively decouples thermal and electronic transport properties via controlled defect engineering. This controlled doping represents a viable strategy for advancing thermoelectric performance of Bi-Sb-Te system.