Multivalley Band Structure and Phonon-Glass Behavior of TlAgTe

Abstract

We show that the extraordinary crystal structure of TlAgTe results in a phonon-glass electron-crystal behavior. The material's electronic transport properties are evaluated by first-principles calculations and Boltzmann transport theory. We find a multivalley conduction band (ndoping) and low dispersion at the valence band edge (p-doping), which results in a high power factor. Since the mean free path of a large number of phonon modes is found to fall below the Ioffe−Regel limit, semiclassical Boltzmann transport theory cannot describe the phonon transport, but a twochannel model must be applied. The lattice thermal conductivity turns out to be as low as 0.43 W m −1 K −1 because of strong lattice anharmonicity (originating from Tl 6s 2 lone pairs) and low group velocities (originating from loose bonding of the Tl atoms), which renders TlAgTe to be a highly promising thermoelectric material.

Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF

“…TlInTe 2 , 65 and TlAgTe. 66 At 300 K, the calculated κ l values are 1.66, 1.14, and 0.90 Wm −1 K −1 for PbClF, PbBrF, and PbIF, respectively, exceeding those obtained using Sheng-BTE by iteratively solving the BTE by 20-50% (Figures 4 & 10b), which demonstrates that the two channel model is essential to calculate κ l accurately for materials with ultralow κ l , consistent with previous reports. [63][64][65][66] For instance, the κ l value calculated by the BTE is 0.158 63 (0.17 66 ) Wm −1 K −1 and that calculated by the two channel model is 0.368 63 (0.43 66 )…”

“…The Cahill–Watson–Pohl model provides where v i and θ i are the phonon velocity, and Debye temperature of the acoustic branch i , respectively (calculated using eq ). The two channel transport model reproduces the experimental κ l values for various ultralow κ l materials such as Tl 3 VSe 4 , Tl 3 VS 4 , Tl 3 TaS 4 , TlInTe 2 , and TlAgTe . At 300 K, the calculated κ l values are 1.66, 1.14, and 0.90 Wm –1 K –1 for PbClF, PbBrF, and PbIF, respectively, exceeding those obtained using ShengBTE by iteratively solving the BTE by 20–50% (Figures and b), which demonstrate that the two channel model is essential to calculate κ l accurately for materials with ultralow κ l , consistent with previous reports. − For instance, the κ l value calculated by the BTE is 0.158 (0.17 66 ) Wm –1 K –1 and that calculated by the two channel model is 0.368 (0.43 66 ) Wm –1 K –1 for Tl 3 VSe 4 (TlAgTe), , being comparable with the experimental value of 0.3 ± 0.05 (0.44) Wm –1 K –1 .…”

“…two channel model is essential to calculate κ l accurately for materials with ultralow κ l , consistent with previous reports. 63−66 For instance, the κ l value calculated by the BTE is 0.158 63 (0.17 66 ) Wm −1 K −1 and that calculated by the two channel model is 0.368 63 (0.43 66 ) Wm −1 K −1 for Tl 3 VSe 4 (TlAgTe), 63,67 being comparable with the experimental value of 0.3 ± 0.05 (0.44) Wm −1 K −1 .…”

Theoretical Prediction and Thermal Transport Properties of Novel Monolayer TlPt2Se3

“…[8,9] Pt-based alkali metal chalcogenides are superior thermoelectric materials because of low 𝜅 l and high figure of merit. [10,11] Thallium-based materials show excellent thermal transport properties due to stereoactive lone pair, contributing to diverse structures, [12,13] and low thermal conductivity due to the high scattering of charge carriers, which is primarily absent in alkali metals. Thallium chalcogenides such as TlAgTe, [13] TlInTe, [12] Tl 2 O [14] show excellent thermoelectric performance with low 𝜅 l because 𝜅 l decreases as heavy atomic mass and suppressed sound velocity.…”

“…[10,11] Thallium-based materials show excellent thermal transport properties due to stereoactive lone pair, contributing to diverse structures, [12,13] and low thermal conductivity due to the high scattering of charge carriers, which is primarily absent in alkali metals. Thallium chalcogenides such as TlAgTe, [13] TlInTe, [12] Tl 2 O [14] show excellent thermoelectric performance with low 𝜅 l because 𝜅 l decreases as heavy atomic mass and suppressed sound velocity. TlPt 2 Se 3 is synthesized experimentally in the late 20th century via a melting reaction of thallium carbonate with Pt and Se powders at temperature range from 400 to 950 °C, [15] and crystalizes in a stacked atomic arrangement similar to Pt-based alkali metal chalcogenides.…”