Lone-Pair Electron-Driven Thermoelectrics at Room Temperature

Vacancy-Ordered Double Perovskites Cs2BI6 (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials

Vacancy-ordered double perovskites (A₂BX₆), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much, despite their potential applications. Here, we explore Cs₂BI₆ (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using state-of-the-art first-principles-based methodologies, viz., density functional theory combined with many-body perturbation theory (G₀W₀) and spin-orbit coupling. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom, leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well-captured by the Fröhlich mesoscopic model. The lattice anharmonicity is further studied using ab initio molecular dynamics and the electron localization function. The maximum anharmonicity is observed in Cs₂PtI₆, followed by Cs₂PdI₆, Cs₂TeI₆, and Cs₂SnI₆. Also, the computed average thermoelectric figure of merit (zT) for Cs₂PtI₆, Cs₂PdI₆, Cs₂TeI₆, and Cs₂SnI₆ is 0.88, 0.85, 0.95, and 0.78, respectively, which reveals their promising renewable energy applications.

The potential thermoelectric material Tl3XSe4 (X = V, Ta, Nb): a first-principles study

Searching for materials with a high thermoelectric figure of merit (ZT) has always been the goal of scientific researchers in the energy field. Here, we combine first-principles calculations to obtain the thermoelectric characteristics of Tl₃XSe₄ (X = V, Nb, or Ta). First, we compared the phonon thermal transport characteristics of Tl₃XSe₄ by solving the Boltzmann transport equation and calculated the thermal conductivity. After that, we obtained the thermoelectric properties of Tl₃XSe₄ through the relaxation time approximation theory. The results show that Tl₃XSe₄ has a high Seebeck coefficient, high electrical conductivity, high power factor (PF) and low thermal conductivity contributed by both phonons and electrons. At the same time, the ZT value of Tl₃XSe₄ shows that it is a potential thermoelectric material with excellent performance. This work demonstrates the thermoelectric transport characteristics of Tl₃XSe₄ to explore its potential applications in many other fields of thermoelectricity and energy.

Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF

Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity (κ_l) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb²⁺ 6s² lone pair, and phonon softening through the mass difference between F and Pb/X. The weak interlayer van der Waals bonding and the strong intralayer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction. Large average Grüneisen parameters (≥2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1%. The κ_l values obtained by the two channel transport model are 20-50% higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow κ_l values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low κ_l materials for thermal energy applications.

Lattice Thermal Transport in the Homogeneous Cage-Like Compounds Cu3 VSe4 and Cu3 NbSe4 : Interplay between Phonon-Phase Space, Anharmonicity, and Atomic Mass

Understanding the correlation between crystal structure and thermal conductivity in semiconductors is very important for designing heat-transport-related devices, such as high-performance thermoelectric materials and heat dissipation in micro-nano-scale devices. In this work, the lattice thermal conductivity ( κ L ) of the cage-like compounds Cu₃ VSe₄ and Cu₃ NbSe₄ was investigated by experimental measurements and first-principles calculations. The experimental κ L of Cu₃ NbSe₄ is approximately 25 % lower than that of Cu₃ VSe₄ at 300 K. The relevant important physical parameters, including the sound velocity, heat capacity, weighted phonon phase space (W), and third-order force constants along with atomic mass were theoretically analyzed. It is found that W is the dominant parameter in determining the κ L , and the other factors only play a minor role. The physical origin is the relatively "soft" lattice of Cu₃ NbSe₄ with heavier atomic mass. This research provides deep insight into the correlation between the thermal conductivity and crystal structure and paves the way for discovering high-performance thermal management device and thermoelectric materials with intrinsically low κ L .