Abstract
The environmental burden of fossil fuels and the rising impact of global warming have created an urgent need for sustainable clean energy sources. This has led to widespread interest in thermoelectric (TE) materials to recover part of the ∼60% of global energy currently wasted as heat as usable electricity. Oxides are particularly attractive as they are thermally stable, chemically inert, and formed of earth-abundant elements, but despite intensive efforts there have been no reports of oxide TEs matching the performance of flagship chalcogenide materials such as PbTe, Bi2Te3 and SnSe. A number of ternary X4Y2Z mixed-anion systems, including oxides, have predicted band gaps in the useful range for several renewable-energy applications, including as TEs, and some also show the complex crystal structures indicative of low lattice thermal conductivity. In this study, we use ab initio calculations to investigate the TE performance of two structurally-similar mixed-anion oxypnictides, Ca4Sb2O and Ca4Bi2O. Electronic-structure and band-alignment calculations using hybrid density-functional theory (DFT), including spin–orbit coupling, suggest that both materials are likely to be p-type dopable with large charge-carrier mobilities. Lattice-dynamics calculations using third-order perturbation theory predict ultra-low lattice thermal conductivities of ∼0.8 and ∼0.5 W m−1 K−1 above 750 K. Nanostructuring to a crystal grain size of 20 nm is predicted to further reduce the room temperature thermal conductivity by around 40%. Finally, we use the electronic- and thermal-transport calculations to estimate the thermoelectric figure of merit ZT, and show that with p-type doping both oxides could potentially serve as promising earth-abundant oxide TEs for high-temperature applications.
Highlights
Recovering the $60% of global energy currently wasted as heat is critical to tackling the environmental and economic issues emerging from climate change
The environmental burden of fossil fuels and the rising impact of global warming have created an urgent need for sustainable clean energy sources. This has led to widespread interest in thermoelectric (TE) materials to recover part of the $60% of global energy currently wasted as heat as usable electricity
We use the electronic- and thermal-transport calculations to estimate the thermoelectric figure of merit ZT, and show that with p-type doping both oxides could potentially serve as promising earthabundant oxide TEs for high-temperature applications
Summary
A viable mass-market thermoelectric needs to be costeffective, stable under the temperature gradients in operating devices, should maintain a high average ZT over a wide operating temperature range, and should be chemically inert to avoid the. We nd good agreement between our electronic density of states (DoS) calculations and previous work by Xia et al.[36] with the valence band maxima and conduction band minima consisting predominantly of Sb/Bi p-states and Ca 3d states respectively Both materials are found to have favourable electrical properties for thermoelectric applications, with low carrier effective masses and suitable band gaps, and an assessment of the valence band energies indicate native ptype semiconducting behaviour. Thermal-conductivity calculations show that the acoustic modes and low-frequency optic modes primarily responsible for the bulk of thermal transport in both structures are short-lived, leading to ultra-low kl These short lifetimes are found to arise from both a high density of lowfrequency optic phonon modes, providing a large number of energy-conserving scattering channels, together with strong phonon–phonon interactions. To the best of our knowledge this represents the rst comprehensive investigation of the electronic and thermal transport properties of Ca4Sb2O and Ca4Bi2O, and the fundamental understanding from this modelling study will help to provide guiding principles to identify and improve oxidebased TEs suitable for widespread use
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