Anharmonic phonon renormalization and thermoelectric properties of CsPbX3 (X = Cl, Br, and I): first-principles calculations

Halide perovskites with ultralow thermal conductivity have emerged as promising candidates for thermoelectric materials. We study the lattice dynamics and thermoelectric properties of cubic all-inorganic lead halide perovskites CsPbX3 (X = Cl, Br, and I) through first-principles calculations. Combined with self-consistent phonon theory, we have successfully renormalized the phonon frequency using a quartic anharmonic term, allowing us to accurately reproduce the phonon dispersion of the high-temperature cubic phase of CsPbX3 without any imaginary frequencies. Cubic CsPbX3 exhibit ultralow lattice thermal conductivities (0.61-1.71 Wm-1 K-1) at room temperature. Because of the strong quartic anharmonic renormalization and hardening of the soft modes, the lattice thermal conductivities of cubic CsPbX3 all exhibit weak temperature dependence. Notably, CsPbCl3 exhibits remarkably high thermal conductivity and a long phonon lifetime. This can be attributed to the smallest atomic mean square displacement and the weakest tilting and distortions of PbCl6 octahedra, resulting from the strongest Pb-Cl covalent bonding. Furthermore, the maximum ZT value of 0.63 at 900 K is obtained for the n-type CsPbBr3.

Ultralow In-Plane Thermal Conductivity in 2D Magnetic Mosaic Superlattices for Enhanced Thermoelectric Performance

Lowering thermal conductivity via introducing heterointerfaces of heterophase fillings (HPFs) is a common strategy for optimizing thermoelectric performance, but it is always accompanied by deterioration of electrical conductivity. Here we report an ordered magnetic HPF system in a CoSe₂-SnSe mosaic heterostructure superlattice synthesized by van der Waals confined epitaxial growth (vdWCEG), which realizes a maximized filling amount to decrease in-plane thermal conductivity of SnSe layers and maintain the intact in-plane carrier transport path. The in-plane thermal conductivity of CoSe₂-SnSe superlattice reaches the lowest range among SnSe-based materials with a value of 0.27 W m^-1 K^-1 at 850 K, which can be attributed to abundant interfaces between CoSe₂ nanocrystals and SnSe layers. Moreover, the CoSe₂ nanocrystals show superparamagnetic behavior, by which the rotation of magnetic domains provides additional phonon scattering to further decrease in-plane thermal conductivity. By combination with the preserved in-plane electrical conductivity of SnSe layers, an enhanced in-plane ZT value of 0.62 is achieved at 850 K. This vdWCEG approach can also be generally applied to fabricate various other two-dimensional (2D) mosaic heterostructures, providing an avenue for artificial 2D heterostructures with desired functionalities.

Improved Thermoelectric Performance of p-Type Argyrodite Cu8GeSe6 via the Simultaneous Engineering of the Electronic and Phonon Transports

Guided by the concept of "phonon-liquid electron-crystal", many n-type argyrodite compounds have been developed as candidates for thermoelectric (TE) materials. In recent years, the p-type Cu₈GeSe₆ (CGS) compound has attracted some attention in TEs due to the presence of very strong atomic vibrational arharmonicity inside the sublattice, which is caused by the weak bonding between Cu ions and [GeSe₆]^8-. However, its TE performance is still poor, with a ZT value of only 0.2 at 623 K. Therefore, in this work, we propose to engineer both the electronic and phonon transports in CGS by incorporating the species In₂Te₃. This strategy tunes the carrier concentration and at the same time increases the phonon scattering on the point defects (In_Ge, In_interstitial, and Te_Se) and randomly distributed tetrahedra ([InSe₄]^5- and [GeTeSe₃]^4-). As a result, the phase transformation at 329 K in CGS is eliminated, and the peak ZT value is enhanced from 0.27 for CGS to ∼0.92 for (Cu₈SnSe₆)_0.9(In₂Te₃)_0.1 at 774 K; this thus proves that the incorporation of In₂Te₃ in CGS is an effective way of regulating its TE performance.

Manipulation of Rashba splitting and thermoelectric performance of MTe (M = Ge, Sn, Pb) via Te off-centering distortion

The Rashba effect is an essential phenomenon in asymmetrical ferroelectric materials with local dipole fields. Rashba spin splitting due to spin–orbit coupling in the asymmetrical 2D ferroelectricity MTe (M = Ge, Sn, Pb) has provided a promising arena for achieving an ultra-high power factor to improve their thermoelectric performance. Herein, we show that the hidden Rashba effect may exist in centrosymmetric rock-salt MTe because of the emerging macroscopic electric dipoles caused by the local Te off-centering distortion. Using first-principles calculations, we prove that Te off-centering causes a change in the atomic reciprocal displacement and induces a ferroelectric Rashba effect in rock-salt MTe. Further analyses of the energy evaluation, lattice vibration and chemical orbital confirm that the atomic off-centering behavior manipulates Rashba spin splitting and stereochemical irregularities of the s² lone electron pair of the cation, leading to the formation of strong anharmonic bonds in the original high-symmetry crystalline solids. The changes of the phonon dispersion and electronic structure also affect the electron–phonon scattering and scattering mechanism of MTe, which then manipulates the electrical transport properties. This work unravels the underlying physics on how the local Te off-centering distortion manipulates Rashba spin splitting and improves the thermoelectric performance of MTe.

The hidden Rashba effect emerges in centrosymmetric rock-salt MTe and improves thermoelectric performance due to the local Te off-centering distortion.

Mixed-Valence CsCu4Se3: Large Phonon Anharmonicity Driven by the Hierarchy of the Rigid [(Cu+)4(Se2-)2](Se-) Double Anti-CaF2 Layer and the Soft Cs+ Sublattice

Crystalline solids that exhibit inherently low lattice thermal conductivity (κ_lat) have attracted a great deal of attention because they offer the only independent control for pursuing a high thermoelectric figure of merit (ZT). Herein, we report the successful preparation of CsCu₄Q₃ (Q = S (compound 1), Se (compound 2)) with the aid of a safe and facile boron-chalcogen method. The single-crystal diffraction data confirm the P4/mmm hierarchical structures built up by the mixed-valence [(Cu⁺)₄(Q^2-)₂](Q^-) double anti-CaF₂ layer and the NaCl-type Cs⁺ sublattice involving multiple bonding interactions. The electron-poor compound CsCu₄Q₃ features Cu-Q antibonding states around E_F that facilitates a high σ value of 3100 S/cm in 2 at 323 K. Significantly, the ultralow κ_lat value of 2, 0.20 W/m/K at 650 K (70% lower than that of Cu₂Se), is mainly driven by the vibrational coupling of the rigid double anti-CaF₂ layer and the soft NaCl-type sublattice. The hierarchical structure increases the bond multiplicity, which eventually leads to a large phonon anharmonicity, as evidenced by the effective scattering of the low-lying optical phonons to the heat-carrying acoustic phonons. Consequently, the acoustic phonon frequency in 2 drops sharply from 118 cm^-1 (of Cu₂Se) to 48 cm^-1. In addition, the elastic properties indicate that the hierarchical structure largely inhibits the transverse phonon modes, leading to a sound velocity (1571 m/s) and a Debye temperature (189 K) lower than those of Cu₂Se (2320 m/s; 292 K).

Thermoelectric Properties of Strained β-Cu2Se

In recent years, the high-temperature phase of Cu₂Se, namely, β-Cu₂Se, has attracted increasing attention due to its outstanding thermoelectric efficiency. The Cu ions in β-Cu₂Se show a distinct ion-liquid behavior between the framework of a Se ion lattice. Many experimental operations may lead to a discrepancy in the lattice structure. Experimentally synthesized β-Cu₂Se was p-type, and a thermoelectric device needs both p-type and n-type materials with a close figure of merit. Studying the effect of strain and the possibility of n-type β-Cu₂Se is essential. Utilizing first-principles calculations and molecular dynamics simulations, we investigate the thermoelectric performance of strained β-Cu₂Se. The results show that the n-type β-Cu₂Se can exhibit superior zT values like the p-type one. Applying compressive strain is an effective way to promote the power factor. The tensile strain will lead to a low lattice thermal conductivity and thus boost the p-type zT values. The predicted maximum zT values for n-type and p-type β-Cu₂Se can reach 1.65 and 1.71 at 800 K, respectively. Considering the fact that applying strain is challenging in experiments, we propose a feasible strategy to manipulate the lattice structure and carrier type: doping halogen elements. Our results provide a guide for Cu₂Se-based thermoelectric devices.