Lone-pair engineering: Achieving ultralow lattice thermal conductivity and enhanced thermoelectric performance in Al-doped GeTe-based alloys

Highlights

• Al-induced lone-pair distortion is demonstrated.

• Al-doing leads to the reduction of phonon velocity and increase of anharmonicity.

• An ultralow ${\kappa }_{lat}$ of 0.21 Wm^-1 K⁻¹ at 773 K can be obtained.

• A peak ZT of 2.21 at 773 K and average ZT of 1.51 can be achieved.

Abstract

The lone pair in Ⅳ-Ⅵ compounds plays a significant role on tuning the phonon and electron transport for excellent thermoelectric performance. Here, we theoretically reveal a unique lone-pair distortion feature in Al-doped GeTe and further experimentally demonstrate the reduction of lattice thermal conductivity by Al doping. Due to the reduction of phonon velocity and enhanced anharmonicity, together with other phonon scattering mechanisms of point defects and stacking faults, an ultralow lattice thermal conductivity of ∼0.21Wm^-1 K⁻¹ at 773 K can be obtained in Al-doped GeTe. Besides, owing to the distorted distribution of Al-3s lone-pair electrons, the Al also show unusual p-type doping behavior in GeTe. By systemically combining the doping effects of Al, Sb and Pb, we can obtain a high figure of merit (ZT) of 2.21 at 773 K and a high average ZT of 1.51 within 300–773 K in the Ge_0.74Al_0.02Pb_0.1Sb_0.1Te. This study demonstrates the impact of lone-pair distortion on enhancing the ZT of GeTe, which also paves a new path way for developing other high-ZT thermoelectric materials.

Introduction

Thermoelectricity provides a sustainable and ecofriendly energy resource by directly converting heat to electricity [1,2]. The performance of thermoelectric (TE) materials at a given temperature (T) is usually evaluated by a dimensionless figure of merit (ZT) [3], ZT = S²Tσ/κ, where S, σ and κ are the Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. The κ generally consists of the lattice thermal conductivity (${\kappa }_{lat}$) and electronic thermal conductivity (${\kappa }_e$) [3], where the ${\kappa }_e$ is related to the σ, T and Lorentz number $L$, ${\kappa }_e=L\sigma T$. Because the S, σ and ${\kappa }_e$ all show dependence on the carrier density (n), optimizing the n by doping or alloying external elements is a routine way to improve the power factor PF (S²σ) and ZT [3,4]. Besides, many band-engineering and phonon-engineering strategies have been introduced to enhance the ZT by traditional doping and alloying methods [[5], [6], [7], [8], [9], [10]]. However, it is still a very tough work to explore novel mechanisms of dopants for enhancing ZT.

Among the big family of TE materials [3,5,6], GeTe-based alloys are one kind of the best mid-temperature TE materials so far [[11], [12], [13], [14], [15], [16], [17]], demonstrating peak ZT > 2 by doping or alloying with appropriate elements [[18], [19], [20], [21], [22], [23]]. The pristine GeTe usually have high Ge vacancy (V_Ge) concentration due to the low formation energy of V_Ge [11,12], and the VA-group elements of Sb and Bi are the mostly used dopants for reducing the high hole-carrier density in GeTe [21,[23], [24], [25], [26], [27], [28], [29]]. The Sb-doped and Bi-doped samples are the best single-dopant GeTe-based TE materials having peak ZT > 1.5, such as single-crystal Ge_0.92Sb_0.08Te [30], polycrystalline Ge_0.9Sb_0.1Te [31,32], (GeTe)₁₇Sb₂Te₃ [22], Ge_0.93Bi_0.07Te [33], Ge_0.95Bi_0.05Te_1.025 [34] and Ge_0.99Bi_0.05Te [23]. To further improve the ZT values, other doping elements together with Sb or Bi are usually co-doped into GeTe for tuning the band structures and phonon scatterings, such as Pb-Sb [35,36], Pb-Bi [37,38], Mg-Bi [39], In-Sb [21], Cd-Bi [20], Cu-Sb [40], Cr-Bi [41], Cr-Sb [42] and Zn-Sb [43], Se-Sb [44], achieving enhanced peak ZT > 2. Exploring other doping elements with novel effects is still significant for further improving the ZT of GeTe-based alloys.

As the origin of the structure-property relationship, chemical bonding has been frequently used to understand the transport of phonons and electrons in TE materials [[45], [46], [47], [48]]. The so-called lone pair is formally from the s-valence electron pair (s²), which shows great promise for reducing ${\kappa }_{lat}$ [[48], [49], [50], [51], [52], [53], [54], [55]]. The s² lone pair behaves differently depending on the local coordination number [45]. It can be stereochemically “quenched” in the octahedral environment with high structural symmetry, or be stereochemically active to lead strong lattice distortion [45]. The stereochemically active lone-pair electrons usually can amplify the anharmonicity [45,[48], [49], [50], [51],[53], [54], [55]], which is an important factor for low ${\kappa }_{lat}$. For the IV-VI compounds (i.e. PbTe, GeTe, SnSe), the lone pairs have been widely used to explain their unique band structures and low ${\kappa }_{lat}$ for excellent TE properties [45,56]. Waghmare et al. considered the active cation s² lone pair as the driving force for the structural distortion in those IV-VI compounds [57], such as the Peierls distortion in rhombohedral GeTe [56]. Because the valence shell of Ge-4s²4p² has a pair of s electrons, the lone pair has also been considered as a key feature of bonding in GeTe [43,56], although Waghmare et al. earlier revealed the poor mixing of s and p states on the cation site of GeTe [57]. Therefore, adjusting the lone-pair features in GeTe can provide a new path way to enhance the ZT of GeTe, which however has rarely been considered before.

Herein, we introduce the Al dopant to tune the lone-pair features of GeTe for enhancing their TE performance. The Al-induced distortion of lone pair can reduce the phonon velocity and enhance the anharmonicity, which together with other phonon scattering mechanisms leads to an ultralow ${\kappa }_{lat}$ of ∼0.21 Wm^-1 K⁻¹ at 773 K in our samples. Besides, the Al shows unusual p-type doping behavior in GeTe due to the localized lone-pair electrons of Al-3s, which can be used to optimize the carrier density for higher PF. Thus, owing to the obtained ultralow ${\kappa }_{lat}$ and optimized PF by Al doping and Pb-Sb co-alloying, a high peak ZT of ∼2.21 at 773 K and high average ZT of ∼1.51 within 300–773 K can be obtained in Ge_0.74Al_0.02Pb_0.1Sb_0.1Te.