Intrinsically low lattice thermal conductivity ($κ_l$) is a desired requirement in many crystalline solids such as thermal barrier coatings and thermoelectrics. Here, we design an advanced machine-learning (ML) model based on crystal graph convolutional neural network that is insensitive to volumes (i.e., scale) of the input crystal structures to discover novel quaternary chalcogenides, AMM'Q$_3$ (A/M/M'=alkali, alkaline-earth, post-transition metals, lanthanides, Q=chalcogens). Upon screening the thermodynamic stability of $\sim$ 1 million compounds using the ML model iteratively and performing density functional theory (DFT) calculations for a small fraction of compounds, we discover 99 compounds that are validated to be stable in DFT. Taking several DFT-stable compounds, we calculate their $κ_l$ using phonon-Boltzmann transport equation, which reveals ultralow-$κ_l$ ($<$ 2 Wm$^{-1}$K$^{-1}$ at room-temperature) due to their soft elasticity and strong phonon anharmonicity. Our work demonstrates the high-efficiency of scale-invariant ML model in predicting novel compounds and presents experimental research opportunities with these new compounds.
Structural stability, electronic band structure, and optoelectronic properties of quaternary chalcogenide CuZn2MS4 (M =In and Ga) compounds via first principles
The development of efficient thermal energy management devices such as thermoelectrics, barrier coatings, and thermal data-storage disks often relies on compounds that possess very low lattice thermal conductivity ($κ_l$). Here, we present the computational prediction of a large family of 628 thermodynamically stable quaternary chalcogenides, AMM'Q$_3$ (A = alkali/alkaline earth/post-transition metals; M/M' = transition metals, lanthanides; Q = chalcogens) using high-throughput density functional theory (DFT) calculations. We validate the presence of low-$κ_l$ in this family of materials by ca…
The electronic and phonon transport properties of quaternary tetradymite BiSbSeTe2 are investigated using first-principles approach and Boltzmann transport theory. Unlike the binary counterpart Bi2Te3, we obtain a pair of Rashba splitting bands induced by the absence of inversion center. Such unique characteristic could lead to a large Seebeck coefficient even at relatively higher carrier concentration. Besides, we find an ultralow lattice thermal conductivity of BiSbSeTe2, especially along the interlayer direction, which can be traced to the extremely small phonon relaxation time mainly induc…
The presence of lone pair (LP) electrons is strongly associated with the disruption of lattice heat transport, which is a critical component of strategies to achieve efficient thermoelectric energy conversion. By exploiting an empirical relationship between lattice thermal conductivity $κ_L$ and the bond angles of pnictogen group LP cation coordination environments, we develop an inverse design strategy based on a materials database screening to identify chalcogenide materials with ultralow $κ_L$ for thermoelectrics. Screening the $\sim$ 635,000 real and hypothetical inorganic crystals of the …
Ternary chalcogenides, having large crystalline unit cell and van der Waal stacking of layers, are expected to be poor thermal conductors and good thermoelectric (TE) materials. We are reporting that layered Bi4GeTe7, with alternating quintuplet-septuplet layers of Bi2Te3 and Bi2GeTe4, has an ultralow thermal conductivity, \k{appa}total 0.42 Wm-1K-1 because of high degree of anharmonicity as estimated from large Gruneisen parameter (γ 4.07) and low Debye temperature (θd 135 K). The electron dominated charge transport has been realized from the Seebeck coefficient, S - 82 uV/K, at 380 K, and Ha…
Topological insulators (TIs) have been found in strained binary HgTe and ternary I-III-VI2 chalcopyrite compounds such as CuTlSe2 which have inverted band structures. However, the non-trivial band gaps of these existing binary and ternary TIs are limited to small values, usually around 10 meV or less. In this work, we reveal that a large non-trivial band gap requires the material having a large negative crystal field splitting $Δ_{CF}$ at top of the valence band and a moderately large negative $s-p$ band gap $E_g^{s-p}$. These parameters can be better tuned through chemical ordering in multina…
Two-dimensional transition metal di-chalcogenides (TMDCs) have shown great potential as good quality thermoelectric materials at high temperature since past few years due to their suitable band gap tunabilty, low dimensionality and fantastic combination of electrical conductivity and lattice thermal conductivity. Here, a first principles calculations of electronic and thermoelectric performance of two dimensional monolayer HfS 2 , HfSe 2 and their Janus monolayer HfSSe has been performed with the help of density functional theory and Boltzmann transport equation. Thermodynamical stability of a…
Quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using the cw 532 nm green laser with energy density about 0.4 J/cm2. The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for p…
Materials with ultralow thermal conductivity are of great interest for efficient energy conversion and thermal barrier coating. Copper-based semiconductors such as copper chalcogenides and copper halides are known to possess extreme low thermal conductivity, whereas the fundamental origin of the low thermal conductivity observed in the copper-based materials remains elusive. Here, we reveal that s-d coupling induced giant phonon anharmonicity is the fundamental mechanism responsible for the ultralow thermal conductivity of copper compounds. The symmetry controlled strong coupling of high-lying…
In spite of intensive studies on the chalcogenides as conventional thermoelectrics, it remains a challenge to find a proper material with high electrical but low thermal conductivities. In this work, we introduced a new class of thermoelectrics, Ge-based inorganic halide perovskites \ce{CsGeX3} (X = I, Br, Cl), which were already known as a promising candidate for photovoltaic applications. By performing the lattice-dynamics calculations and solving the Boltzmann transport equation, we revealed that these perovskites have ultralow thermal conductivities below 0.18 W m$^{-1}$ K$^{-1}$ while ver…
Tin chalcogenides (SnS, SnSe, and SnTe) are found to have improved thermoelectric properties upon the reduction of their dimensionality. Here we found the tilted AA + s stacked two-dimensional (2D) SnTe bilayer as the most stable phase among several stackings as predicted by the structural optimization and phonon transport properties. The carrier mobility and relaxation time are evaluated using the deformation potential theory, which is found to be relatively high due to the high 2D elastic modulus, low deformation potential constant, and moderate effective masses. The SnTe bilayer shows a hig…
Interest in high entropy alloy thermoelectric materials is predicated on achieving ultralow lattice thermal conductivity $κ\sub{L}$ through large compositional disorder. However, here we show that for a given mechanism, such as mass contrast phonon scattering, $κ\sub{L}$ will be minimized along the binary alloy with the highest mass contrast, such that adding an intermediate-mass atom to increase atomic disorder can increase thermal conductivity. Only when each component adds an independent scattering mechanism (such as adding strain fluctuation to an existing mass fluctuation) is there a bene…
