Unusual Lattice Dynamics and Anisotropic Thermal Conductivity in In₂Te₅ Due to a Layered Structure and Planar-Coordinated Te-Chains

Abstract

Searching for novel materials with intrinsically low thermal conductivity and uncovering their origin is an essential way to realize high performance in various energy-related applications, including thermoelectrics. Understanding the role of different structural features in materials can assist us in designing thermal transport properties. Herein, we report thermal transport behavior of In 2 Te 5 , which is closely related to its layered zigzag structure and bonding in planar-coordinated Te-chains. The experimental thermal conductivity shows significant anisotropy for the measured two directions and an extremely low value in the interlayer direction of about 0.3 W m −1 K −1 at 673 K. The theoretical calculations of lattice thermal conductivity based on density functional theory support this behavior, and furthermore, they even reveal unprecedented anisotropy in the in-layer direction. We demonstrate that these thermal transport behaviors can be attributed to resonant bonding and lattice dynamics observed at the Te atoms in the planar-coordinated environment. Particularly strikingly, we discover a counterintuitive large positive and negative split of the Born effective charge (+8.7 and −5.1) at the adjacent Te atoms bonded covalently within the same plane. They induce a significant dipole interaction in the x-direction. Optical phonons are significantly softened in the specific direction by these characteristics in the Te-plane, which leads to large anisotropy in thermal transport. Our findings should encourage further excavation of novel features in lattice dynamics out of unexplored materials with complex structures.

Physical Insights on the Lattice Softening Driven Mid‐Temperature Range Thermoelectrics of Ti/Zr‐Inserted SnTe—An Outlook Beyond the Horizons of Conventional Phonon Scattering and Excavation of Heikes’ Equation for Estimating Carrier Properties

“…The transducer receives and measures the echoed ultrasound reflections by acting as a detector when it initially sends a stress-wave pulse into the sample. By maximizing the cross-correlation of the two reflections, the time delay (t d ) was estimated, and the sound velocity in a specific direction (ν) is calculated using the following relation, [24,94] 2d t d ν = (22)…”

“…[9][10][11][12][13][14][15][16][17][18] The thermal conductivity is known to be suppressed by nanostructuring, rattling impurities, phonon mode softening, resonant bonding, and so forth. [10,[19][20][21][22][23][24][25] The lead-free IV-VI chalcogenide semiconductor, SnTe with the same rock-salt crystal structure as PbTe and with better mechanical properties, is considered a viable eco-friendly option for thermoelectrics. [26] However, the pristine SnTe has its drawbacks with regards to its usage as a thermoelectric material such as high charge carrier concentration (≈10 21 cm -3 ) due to the intrinsic nature of SnTe to have high Sn vacancies, small bandgap often leading to bipolar conduction, and the large energy separation between the two valence band maxima (≈0.35 eV), all of which leads to a low S and high κ e , thus ultimately lower zT in pristine stochiometric SnTe.…”

“…[ 9–18 ] The thermal conductivity is known to be suppressed by nanostructuring, rattling impurities, phonon mode softening, resonant bonding, and so forth. [ 10,19–25 ]…”

N-type thermoelectric Ag8SnSe6 with extremely low lattice thermal conductivity by replacing Ag with Cu

“…[13][14][15] The former aims at optimizing the electronic structure and the latter strengthening phonon scattering by introducing lattice distortion and nanostructures. Additionally, some materials with very low intrinsic thermal conductivity has been attracting much attention, due to either resonant bonding, 16,17 the loosely bonded atoms in atomic voids with rattling 18 or "liquid-like behavior". 19,20 The aforementioned strategies can generally be realized by off-stoichiometry in composition and element doping.…”

A material catalogue with glass-like thermal conductivity mediated by crystallographic occupancy for thermoelectric application

“…It should be emphasized that in boride compounds, in the context of disorder, partial occupancy of rare earth sites is assumed to contribute to the observed glass-like thermal property, 19,22,23 but the related mechanisms have not been explicitly analyzed in detail. Recently, the large suppression of thermal conductivity from lone pair coordination, 24,25 resonant bonding, [26][27][28] bonding heterogeneity, 29,30 rattling atom, [31][32][33] strong anharmonicity, 34,35 and liquid-like behavior [36][37][38][39][40] are identified. The microscopic understanding of those phenomena requires time-consuming theoretical investigation on lattice dynamics, making it difficult to use them as the guide for the search of low thermal conductivity materials in advance.…”