arXiv subjects

Yi Xia

Publications and source records attributed to Yi Xia.

63 records · Page 4Linked to original sources

arXiv2018

Anharmonic stabilization and lattice heat transport in rocksalt $β$-GeTe

Peierls-Boltzmann transport equation, coupled with third-order anharmonic lattice dynamics calculations, has been widely used to model lattice thermal conductivity ($κ_{l}$) in bulk crystals. However, its application to materials with structural phase transition at relatively high temperature is fundamentally challenged by the presence of lattice instabilities (imaginary phonon modes). Additionally, its accuracy suffers from the absence of higher-than-third-order phonon scattering processes, which are important near/above the Debye temperature. In this letter, we present an effective scheme that combines temperature-induced anharmonic phonon renormalization and four-phonon scattering to resolve these two theoretical challenges. We apply this scheme to investigate the lattice dynamics and thermal transport properties of GeTe, which undergoes a second-order ferroelectric phase transition from rhombohedral $α$-GeTe to rocksalt $β$-GeTe at about 700~K. Our results on the high-temperature phase $β$-GeTe at 800~K confirm the stabilization of $β$-GeTe by temperature effects. We find that considering only three-phonon scattering leads to significantly overestimated $κ_{l}$ of 3.8~W/mK at 800~K, whereas including four-phonon scattering reduces $κ_{l}$ to 1.7~W/mK, a value comparable with experiments. To explore the possibility to further suppress $κ_{l}$, we show that alloying $β$-GeTe with heavy cations such as Pb and Bi can effectively reduce $κ_{l}$ to about 1.0~W/mK, whereas sample size needs to be around 10nm through nanostructuring to achieve a comparable reduction of $κ_{l}$.

cond-mat.mtrl-sci↗

arXiv2018

High $n$-type thermoelectric power factor and efficiency in Ba$_{2}$BiAu from a highly dispersive band

Using first-principles density-functional theory calculations, we predict the potential for unprecedented thermoelectric efficiency $zT=5$ at 800 K in $n$-type Ba$_{2}$BiAu full-Heusler compound. Such a high efficiency arises from an intrinsically ultralow lattice thermal conductivity coupled with a very high power factor reaching 7 mW m$^{-1}$ K$^{-2}$ at 500 K. The high power factor originates from a light, sixfold degenerate conduction band pocket along the $Γ$-X direction. Weak acoustic phonon scattering and sixfold multiplicity combine to yield high mobility and high Seebeck coefficient. In contrast, the flat-and-dispersive (a.k.a. low-dimensional) valence band of Ba$_{2}$BiAu fail to generate a high power factor due to strong acoustic phonon scattering. The Lorenz numbers at optimal doping are smaller than the Wiedemann-Franz value, an integral feature for $zT$ enhancement as electrons are the majority heat carriers.

cond-mat.mtrl-sci↗

arXiv2018

Thermoelectric Alchemy: Designing A Chemical Analog to PbTe with Intrinsic High Band Degeneracy and Low Lattice Thermal Conductivity

Improving the figure of merit $zT$ of thermoelectric materials requires simultaneously a high power factor and low thermal conductivity. An effective approach for increasing the power factor is to align the band extremum and achieve high band degeneracy ($\geq$ 12) near the Fermi level as realized in PbTe [\textcolor{blue}{Pei et. al. \textit{Nature} 473, 66 (2010)}], which usually relies on band structure engineering, e.g., chemical doping and strain. However, very few materials could achieve such a high band degeneracy without heavy doping or suffering impractical strain. By employing state-of-the-art first-principles methods with direct computation of phonon and carrier lifetime, we demonstrate that two new full-Heusler compounds Li$_2$TlBi and Li$_2$InBi, possessing a PbTe-like electronic structure, show exceptionally high power factors ($\sim$ 20 mWm$^{-1}$K$^{-2}$ at 300 K) and low lattice thermal conductivities (2.36 and 1.55 Wm$^{-1}$K$^{-1}$) at room temperature. The Tl$^{+}$Bi$^{3-}$ (In$^{+}$Bi$^{3-}$) sublattice forms a rock-salt structure, and the additional two valence electrons from Li atoms essentially make these compounds isovalent with Pb$^{2+}$Te$^{2-}$. The larger rock-salt sublattice of TlBi (InBi) shifts the valence band maximum from L point to the middle of the $Σ$ line, increasing the band degeneracy from fourfold to twelvefold. On the other hand, resonance bond in the PbTe-like sublattice and soft Tl-Bi (In-Bi) bonding interaction is responsible for intrinsic low lattice thermal conductivities. Our results present a novel strategy of designing high-performance thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2018

Pd$_{2}$Se$_{3}$ Monolayer: A Promising Two Dimensional Thermoelectric Material with Ultralow Lattice Thermal Conductivity and High Power Factor

A high power factor and low lattice thermal conductivity are two essential ingredients of highly efficient thermoelectric materials. Although monolayers of transition metal dichalcogenides possess high power factors, high lattice thermal conductivities significantly impede their practical applications. Our first-principles calculations show that these two ingredients are well fulfilled in the recently synthesized Pd$_{2}$Se$_{3}$ monolayer, whose crystal structure is composed of [Se$_{2}$]$^{2-}$ dimers, Se$^{2-}$ anions, and Pd$^{2+}$ cations coordinated in a square planar manner. Our detailed analysis of third-order interatomic force constants reveals that the anharmonicity and soft phonon modes associated with [Se$_2$]$^{2-}$ dimers lead to ultra-low lattice thermal conductivities in Pd$_{2}$Se$_{3}$ monolayers (1.5 and 2.9 Wm$^{-1}$K$^{-1}$ along the $a$- and $b$-axes at 300\,K respectively), which are comparable to those of high-performance bulk thermoelectric materials such as PbTe. Moreover, the "pudding-mold" type band structure, caused by Pd$^{2+}$ ($d^{8}$) cations coordinated in a square planar crystal field, leads to high power factors in Pd$_{2}$Se$_{3}$ monolayers. Consequently, both electron and hole doped thermoelectric materials with a considerably high $zT$ can be achieved at moderate carrier concentrations, suggesting that Pd$_{2}$Se$_{3}$ is a promising two-dimensional thermoelectric material.

cond-mat.mtrl-sci↗

arXiv2017

Lattice thermal transport in group II-alloyed PbTe

PbTe, one of the most promising thermoelectric materials, has recently demonstrated thermoelectric figure of merit ($ZT$) of above 2.0 when alloyed with group II elements. The improvements are due mainly to significant reduction of lattice thermal conductivity ($κ_{l}$), which was in turn attributed to nanoparticle precipitates. However, a fundamental understanding of various phonon scattering mechanisms within the bulk alloy is still lacking. In this work, we apply the newly-developed density-functional-theory (DFT)-based compressive sensing lattice dynamics (CSLD) approach to model lattice heat transport in PbTe, MTe, and Pb$_{0.94}$M$_{0.06}$Te (M=Mg, Ca, Sr and Ba), compare our results with experimental measurements, with focus on strain effect and mass disorder scattering. We find that (1) CaTe, SrTe and BaTe in the rock-salt structure exhibit much higher $κ_{l}$ than PbTe, while MgTe in the same structure shows anomalously low $κ_{l}$; (2) lattice heat transport of PbTe is extremely sensitive to static strain induced by alloying atoms in solid solution form; (3) mass disorder scattering plays a major role in reducing $κ_{l}$ for Mg/Ca/Sr-alloyed PbTe through strongly suppressing the lifetimes of intermediate- and high-frequency phonons, while for Ba-alloyed PbTe, precipitated nanoparticles are also important.

cond-mat.mtrl-sci↗

arXiv2017

Renormalized Lattice Dynamics and Thermal Transport in VO$_{2}$

Vanadium dioxide (VO$_{2}$) undergoes a first-order metal-insulator transition (MIT) upon cooling near room temperature, concomitant with structural change from rutile to monoclinic. Accurate characterization of lattice vibrations is vital for elucidating the transition mechanism. To investigate the lattice dynamics and thermal transport properties of VO$_{2}$ across the MIT, we present a phonon renormalization scheme based on self-consistent phonon theory through iteratively refining vibrational free energy. Using this technique, we compute temperature-dependent phonon dispersion and lifetimes, and point out the importance of both magnetic and vibrational entropy in driving the MIT. The predicted phonon dispersion and lifetimes show quantitative agreement with experimental measurements. We demonstrate that lattice thermal conductivity of rutile VO$_{2}$ is nearly temperature independent as a result of strong intrinsic anharmonicity, while that of monoclinic VO$_{2}$ varies according to $1/T$. Due to phonon softening and enhanced scattering rates, the lattice thermal conductivity is deduced to be substantially lower in the rutile phase, suggesting that Wiedemann-Franz law might not be strongly violated in rutile VO$_{2}$.

cond-mat.mtrl-sci↗

arXiv2016

Ultralow Thermal Conductivity in Full-Heusler Semiconductors

Semiconducting half- and, to a lesser extent, full-Heusler compounds are promising thermoelectric materials due to their compelling electronic properties with large power factors. However, intrinsically high thermal conductivity resulting in a limited thermoelectric efficiency has so far impeded their widespread use in practical applications. Here, we report the computational discovery of a class of hitherto unknown stable semiconducting full-Heusler compounds with ten valence electrons ($X_2YZ$, $X$=Ca, Sr, and Ba; $Y$= Au and Hg; $Z$=Sn, Pb, As, Sb, and Bi) through high-throughput $ab-initio$ screening. These new compounds exhibit ultralow lattice thermal conductivity $κ_{\text{L}}$ close to the theoretical minimum due to strong anharmonic rattling of the heavy noble metals, while preserving high power factors, thus resulting in excellent phonon-glass electron-crystal materials.

cond-mat.mtrl-sci↗

arXiv2014

Lattice anharmonicity and thermal conductivity from compressive sensing of first-principles calculations

First-principles prediction of lattice thermal conductivity $κ_L$ of strongly anharmonic crystals is a long-standing challenge in solid state physics. Making use of recent advances in information science, we propose a systematic and rigorous approach to this problem, compressive sensing lattice dynamics (CSLD). Compressive sensing is used to select the physically important terms in the lattice dynamics model and determine their values in one shot. Non-intuitively, high accuracy is achieved when the model is trained on first-principles forces in {\it quasi-random\/} atomic configurations. The method is demonstrated for Si, NaCl, and Cu$_{12}$Sb$_4$S$_{13}$, an earth-abundant thermoelectric with strong phonon-phonon interactions that limit the room-temperature $κ_L$ to values near the amorphous limit.

cond-mat.mtrl-sci↗

arXiv2010

Equivalent Circuit Description of Non-compensated n-p Codoped TiO2 as Intermediate Band Solar Cells

The novel concept of non-compensated n-p codoping has made it possible to create tunable intermediate bands in the intrinsic band gap of TiO2, making the codoped TiO2 a promising material for developing intermediate band solar cells (IBSCs). Here we investigate the quantum efficiency of such IBSCs within two scenarios - with and without current extracted from the extended intermediate band. Using the ideal equivalent circuit model, we find that the maximum efficiency of 57% in the first scenario and 53% in the second are both much higher than the Shockley-Queisser limit from single gap solar cells. We also obtain various key quantities of the circuits, a useful step in realistic development of TiO2 based solar cells invoking device integration. These equivalent circuit results are also compared with the efficiencies obtained directly from consideration of electron transition between the energy bands, and both approaches reveal the intriguing existence of double peaks in the maximum quantum efficiency as a function of the relative location of IBs.

cond-mat.mtrl-sci↗