Electronic Band Structure Engineering and Enhanced Thermoelectric Transport Properties in Pb-Doped BiCuOS Oxysulfide
Résumé
In this paper, Bi1-xPbxCuOS samples (0 <= x <= 0.05) have been synthesized with a simple and scalable ball-milling process, followed by a reactive Spark Plasma Sintering. Our results highlight that, Pb for Bi substitution increases the charge carriers concentration by more than 2 orders of magnitude from 1.4 x 10(17) cm(-3) to 2.6 x 10(19) cm(-3) for x = 0 and x = 0.05, respectively. As a result, the electrical resistivity is divided by more than 50 at room temperature and the Seebeck coefficient drops from 707 mu V K-1 to 265 mu V K-1 where our experimental results are supported with density functional theory (DFT) calculations. Electronic structure calculations show that, just below the top of the valence band, several other bands are present and may contribute to the transport properties with appropriate tuning of the heavy-light valence band and the position of the Fermi level. Pb doping increases the number of holes pockets and several band degeneracies appear around the Fermi level, leading to a drastic enhancement of the power factor up to 0.2 mW m(-1) K-2 at 700 K. This is 5 times higher than the value of the pristine compound. The intrinsically low thermal conductivity of 0.7 W m(-1) K-1 at 700 K is interpreted on the basis of vibrational properties calculations within the Density Functional Perturbation Theory (DFPT) approach. It indicates that soft acoustic modes along the Gamma-Z direction suggest weak interatomic bonding between the layers and possible strong anharmonicity. The power factor being enhanced with a minimal impact on the thermal conductivity, the figure of merit ZT reaches 0.2 at 700 K for x = 0.05. To the best of our knowledge, it is considered the best reported value among the family of oxysulfides.