First-Principles Study On Transport Properties Of Sn-S-Based Thermoelectric Materials

Thermoelectrics（TEs）,as one of the most noticed new energy materials,can recycle the waste heat by converting it to electricity,which are expected to help China reach“carbon emission peak”and“carbon neutrality”in the next four decades.Recently,Sn S-based compounds,as one of the new TEs,are winning favors from researchers due to the low-cost,earth-abundant,and eco-friendly composition elements,which presents a promising outlook for large-scale commercial use.The research of Sn S-based TE materials has achieved impressive progress.However,some problems still remain,including:（1）the ongoing experimental research of Sn S optimization is only focused on improving the electrical properties by doping,whereas limited progress were made in further suppressing the lattice thermal conductivity of Sn S.Reasons for this phenomenon need to be analyzed.Meanwhile,there is no specific guideline for choosing dopants,synthetic chemical environment,and temperature for Sn S samples.（2）There are few systematic theoretical studies of the underlying heat and charge transport mechanism in the two prototype tin chalcogenides,Sn S and Sn S₂.Several theoretical papers used the over-simplified theories such as deformation potential theory,whose accuracy remains to be examined.（3）Research is only focused on optimizing Sn S and Sn S₂.None of new materials with Sn–S framework are proposed.To solve the aforementioned problems,we investigate Sn–S-based compounds with the state-of-the-art first-principles methods,reveal the underlying mechanisms of the TE transport process in Sn S and Sn S₂ systematically,and come up with corresponding optimization strategies.Meanwhile,we also investigate the TE transport features of several new Sn–S-based compounds,predict their TE properties,and provide routes and candidates for the following search of new Sn–S-based TE compounds with high performance.The core content of this paper is as follows:（1）Optical phonon dominated heat and charge transport and defects calculations of Sn S.Using the ab initio phonon and electron lifetime respectively based on the three-phonon interaction and Fermi’s golden rule,we thoroughly investigate the heat and charge transport process of Sn S.We find that lattice thermal conductivity（κ_L）of Sn S is hard to be further suppressed due to the dominated contribution from optical phonons,which have mean free path shorter than 7 nm and cannot be scattered by larger nanostructure.The major heat-carrying optical phonons correspond to the unique“antiphase”movement of adjacent Sn–S sublayers,which is originated from the interlaced Sn S₃ tetrahedra resulting from the lone-pair electrons of Sn（II）.As for the charge transport,our calculations suggest that carrier mobility of Sn S is dominantly limited by polar optical phonon scattering instead of acoustic phonon deformation scattering proposed by previous studies,which makes our predicted electrical properties more accurate.We predict the p-type Sn S will achieve the maximum figure of merit ZT of 1.68 at 850 K and hole concentration of 5.5×10¹⁹ cm^-3.To achieve the optimal hole concentration,we screen the four most effective dopants（Na,K,Tl,and Ag）out of eleven candidates and propose that p-type doped Sn S samples should be synthesized under the sulfur-excess circumstance and at temperatures higher than 1353 K.（2）“antiphase”movement disturbance leads to ultralow lattice thermal conductivity of Ba Sn S₂.By analyzing theκ_L of Ba Sn S₂,we find that Ba disturbs the high-κ_L contribution“antiphase”movements in the Sn–S framework.The existence of Ba also leads to a softer lattice and higher lattice anharmonicity of Ba Sn S₂,making theκ_L of Ba Sn S₂ only half of that in Sn S,which is 0.34 W m^-1 K^-1 at 850 K.Due to the strong lattice anharmonicity of Ba Sn S₂,extraκ_L from wave-like tunneling effect based on Allen–Feldman model is considered,resulting in milder decay ofκ_L with respect to temperature as T^-0.76.Although the existence of Ba deteriorates the carrier mobility,p-type and n-type Ba Sn S₂ can still achieve high ZT of 1.1 and 2.2 at 850 K and optimal carrier concentration,respectively.（3）Weak-bonding elements lead to synergic improvement of carrier mobility and thermal conductivity in the highly symmetric polarized system.We discover that high in-planeκ_L and low carrier mobility of Sn S₂ are due to the highly symmetric crystal structure and polarized Sn–S ionic covalent bonding.We choose ASn S₃（A=Ba,Sr）with Sn–S framework as subjects and find the two compounds both present softened and distorted lattice owing to the existence of weak-bonding element A.The local vibrations of A contribute to low-frequency optical phonons,which couple strongly with acoustic phonons and further enhance the phonon scattering rates.All the factors above eventually result in the ultralowκ_L of ASn S₃.As for the electrical transport process,A to some extent neutralize the polarity of Sn–S bonding,boosting the carrier mobility.Eventually,ultralowκ_L combined with moderate power factor gives an excellent ZT of 2.89 and 2.77 in p-type Ba Sn S₃ and Sr Sn S₃ at 900 K and optimal hole concentration,respectively.Our findings are further developed by studying the solid solutions of the two compounds and identifying a stable new compound Ba_0.5Sr_0.5Sn S₃.The compound experiences order-disorder phase transition at220 K.It is found that A has minor influence on the electrical transport of the system,thus the benefit of phonon scattering induced by disordered A sites would overweigh the loss of electron scattering,suggesting the disordered phase Ba_0.5Sr_0.5Sn S₃ has a potentially high ZT approaching 3.0.（4）Study of the stability and electrical and thermal transport properties in Sn SSe Janus monolayer.We study two-dimensional materials,including Sn S₂,Sn Se₂,and Sn SSe Janus monolayer.After molecular dynamics and thermodynamics simulation,we find that Sn SSe is kinetically stable but thermodynamically metastable.Although Sn SSe has similar phonon dispersion to Sn S₂ and Sn Se₂,it has much higher lattice anharmonicity than its parents owing to the loss of mirror symmetry about the Sn-layer.Even if we only take into account three-phonon interaction,Sn SSe already shows ultralowκ_L of 2.5 W m^-1 K^-1 at 900 K.We further elaborate that four-phonon interaction and three-phonon interaction both play an important role inκ_L by comparing their phase spaces,which suggests experimentalκ_L would be lower than 2.5W m^-1 K^-1 due to the four-phonon scattering.Regarding the electrical transport,Sn SSe shows lower electrical conductivity because of weaker chemical bonding.Its valence band maximum moving to the center of Brillouin zone and the loss of degeneracy are both condemned as the reasons for the lower Seebeck coefficient.Nevertheless,Sn SSe still achieves a high ZT of 1.8 at 900 K with its ultralowκ_L,showing promising application as a new two-dimensional thermoelectric material.Through systematically calculating and analyzing,this study reveals that the central optimization routes of Sn S-based thermoelectrics are suppressing the optical phonon heat transport（decreaseκ_L）and tuning the carrier concentration（increase the electrical conductivity）,whereas the Sn S₂-based thermoelectrics focus on neutralizing the highly polarized ionic covalent bonding（boost the carrier mobility）and breaking the structural symmetry（decreaseκ_L）.Based on the routes above,this work predicts the properties of a series of new thermoelectrics,including Ba Sn S₂,Ba Sn S₃,Sr Sn S₃,Ba_0.5Sr_0.5Sn S₃,and Sn SSe Janus monolayer,etc.These materials are expected to have a maximum ZT over 1.0,which is worthy of further experimental synthesis study.