Chalcogenides SrPbSe<sub>2</sub>: A potential thermoelectric material

<p indent="0mm">Thermoelectric (TE) materials enable the direct conversion of thermal energy into electrical energy, offering a promising avenue for clean energy generation and addressing the global energy crisis. This technological advancement has significant potential to mitigate the adverse environmental impacts associated with carbon-based energy consumption. However, the widespread application of TE materials has been substantially constrained by their relatively low efficiency, often quantified by the dimensionless figure of merit (<italic>ZT</italic>). The <italic>ZT</italic> value is determined by the interplay of several TE parameters, including Seebeck coefficient, electrical conductivity, electronic thermal conductivity and lattice thermal conductivity. Overcoming the competitive relationship between Seebeck coefficient and electrical conductivity to achieve a higher power factor (PF) can enable the optimization of TE performance in semiconductors with inherently low thermal conductivity. Over the past decades, extensive theoretical and experimental studies have proposed strategies to enhance PF and reduce thermal conductivity, including band and strain engineering, defect and alloying engineering, and materials genome engineering. Recently, the computational materials science has highlighted the fundamental characteristics of high-performance TE materials, emphasizing the wide bandgap, complex structure, anisotropic multi-band structure, and ultralow thermal conductivity. TE materials represented by chalcogenides have been shown to have broad application prospects in recent years. PbSe is a TE material renowned for its exceptional performance in mid-to-high temperature applications. With the increasing demand for high-performance, low-cost TE materials, PbSe-based compounds have made significant progress in recent years, with peak<italic> ZT</italic> of ~1.8 and room <italic>ZT </italic>of ~0.7 for n-type compounds and with peak <italic>ZT</italic> of ~2.0 and room <italic>ZT</italic> of ~0.8 for p-type compounds. Studies have shown that alkaline earth metals significantly impact the TE properties of PbSe-based compounds. Experimentally, trace amounts of alkaline earth metals are usually doped to enhance the electrical transport properties of TE materials, such as Pb_0.95Ca_0.04Na_0.01Se, Pb_0.96Ba_0.03Na_0.01Se, and Pb_0.97Sr_0.02Na_0.01Se. However, research on the large-scale solid solution of alkaline earth metals to form stable IV-VI compounds remains relatively limited. In this work, we have designed a crystal structure model of SrPbSe₂ based on PbSe. Using first-principles calculations and semi-classical Boltzmann transport theory, we comprehensively investigate the crystal structure, mechanical properties, phonon and electronic transport, and TE properties of SrPbSe₂. The results show that the SrPbSe₂ is a direct band gap semiconductor with a narrow band gap (~<sc>0.26 eV).</sc> The lone pair electron 6s² of Pb²⁺ in SrPbSe₂ exhibit stereochemical activity, causing lattice distortion around Pb atom and then enhancing lattice anharmonicity. Moreover, the calculated elastic constants satisfy the Born-Huang stability criteria, proving that SrPbSe₂ is mechanically stable. By the phonon spectrum, phonon density of states, second-order force constants and phonon scattering analysis, we have demonstrated that the low thermal conductivity of SrPbSe₂ is mainly due to the weak Pb–Se bonding caused by the local lattice distortions of PbSe6 octahedron and more phonon scattering centers. In addition, based on the multiple carrier scattering mechanisms, we have predicted a maximum <italic>ZT </italic>value of 1.31 (p-type) and 0.95 (n-type) at <sc>900 K</sc> in SrPbSe₂. Our results not only provide a promising PbSe-based TE material, but also uncover the underlying transport properties in SrPbSe₂ compound.