Bonding heterogeneity and lone pair induced anharmonicity resulted in ultralow thermal conductivity and promising thermoelectric properties in n-type AgPbBiSe₃

Abstract

Efficiency in generation and utilization of energy is highly dependent on materials that have the ability to amplify or hinder thermal conduction processes. A comprehensive understanding of the relationship between chemical bonding and structure impacting lattice waves (phonons) is essential to furnish compounds with ultralow lattice thermal conductivity (<i>κ</i> _lat) for important applications such as thermoelectrics. Here, we demonstrate that the n-type rock-salt AgPbBiSe₃ exhibits an ultra-low <i>κ</i> _lat of 0.5-0.4 W m^-1 K^-1 in the 290-820 K temperature range. We present detailed analysis to uncover the fundamental origin of such a low <i>κ</i> _lat. First-principles calculations augmented with low temperature heat capacity measurements and the experimentally determined synchrotron X-ray pair distribution function (PDF) reveal bonding heterogeneity within the lattice and lone pair induced lattice anharmonicity. Both of these factors enhance the phonon-phonon scattering, and are thereby responsible for the suppressed <i>κ</i> _lat. Further optimization of the thermoelectric properties was performed by aliovalent halide doping, and a thermoelectric figure of merit (<i>zT</i>) of 0.8 at 814 K was achieved for AgPbBiSe_2.97I_0.03 which is remarkable among n-type Te free thermoelectrics.