Broadband phonon scattering in PbTe-based materials driven near ferroelectric phase transition by strain or alloying

Abstract

The major obstacle in the design of materials with low lattice thermal conductivity is the difficulty in efficiently scattering phonons across the entire frequency spectrum. Using first principles calculations, we show that driving PbTe materials to the brink of the ferroelectric phase transition could be a powerful strategy to solve this problem. We illustrate this concept by applying tensile [001] strain to PbTe and its alloys with another rock-salt IV-VI material, PbSe; and by alloying PbTe with a rhombohedral IV-VI material, GeTe. This induces extremely soft optical modes at the zone center, which increase anharmonic acoustic-optical coupling and decrease phonon lifetimes at all frequencies. We predict that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven close to the phase transition in the described manner will have considerably lower lattice thermal conductivity than that of PbTe (by a factor of 2 − 3). The proposed concept may open new opportunities for the development of more efficient thermoelectric materials.

Ferroelectric phase transition and the lattice thermal conductivity of Pb1−xGexTe alloys

“…The anharmonic κ decreases smoothly with the increased proximity to the phase transition mostly due to the reduced anharmonic τ , which results in a factor of ∼1.5 reduction in both the rocksalt and rhombohedral phases with respect to x = 0 and 1. Therefore, the minimization of anharmonic κ at the phase transition would be more pronounced for alloys with soft optical modes whose overall mass difference is smaller than that of Pb 1−x Ge x Te alloys, or for bulk materials driven to the phase transition via pressure or strain [17]. However, the κ of such materials may not be as low as reported here since mass disorder would be irrelevant or weaker than in Pb 1−x Ge x Te.…”

“…Recent studies reported extremely low κ values in marginally stable IV-VI and I-V-VI 2 rocksalt structures [14][15][16]. We showed previously that driving PbTe to the verge of the phase transition to the rhombohedral phase via strain or alloying reduces the κ substantially [17]. In several I-V-VI 2 materials with slightly distorted rocksalt-like structures, κ decreases as their bond angle approaches that of the rocksalt phase [18].…”

“…We argue that the structure and its degree of resonant bonding are less critical factors for the low thermal conductivity than the proximity to the phase transition, average atomic mass, and mass disorder. Due to the symmetryforbidden electron-phonon coupling between the TO mode at and the valence band maximum and conduction band minimum in PbTe [17,85], the electronic properties beneficial for a high figure of merit may not be hindered by the phase transition. Consequently, combining soft optical modes with alloying is a promising strategy in the design of materials with low thermal conductivity and high thermoelectric figure of merit.…”

“…We computed the acoustic-TO contribution to the lifetimes at all frequencies by accounting for the triplets of interacting states that contain at least one acoustic and one TO mode as described in Ref. [17]. We distinguished between TO and longitudinal acoustic (LA) modes whose order changes throughout the Brillouin zone by projecting their eigenvectors onto the corresponding wave vector, and then determined which state is more longitudinal.…”

Thermally induced band gap increase and high thermoelectric figure of merit of n-type PbTe

“…[48]. The lattice thermal conductivity values are taken from our previous first principles calculations [50]. For the doping concentrations above 5.8×10 19 cm −3 , the electronic contribution to the thermal conductivity becomes higher than the lattice contribution.…”