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• W4211075645 abstract "The past decades have witnessed a sustained boom of thermoelectrics, among which GeTe has shown an extraordinary performance. Previous studies on GeTe thermoelectrics have mostly focused on its cubic structure where a high thermoelectric figure of merit, zT, has been realized at high temperatures. Rhombohedral GeTe has so far been revealed to enable a zT as high as that of cubic GeTe but at lower temperatures, thus greatly promoting its average zT in a broad working temperature range and as one of the highest among known thermoelectrics. This enables GeTe-based thermoelectric devices to be one of the most efficient. It is further proposed that a sufficiently high zT can be expected at near room temperature, which makes it a strong candidate for competing in efficiency with the only commercially available thermoelectric Bi2Te3. Of note, near-room-temperature thermoelectric applications are extremely important because they offer great advantages over other energy technologies. Although GeTe has been known as thermoelectrics since 1960s, it has attracted intensive renewed attention recently. GeTe undergoes a phase transition from a high-T cubic (c-GeTe) to a low-T rhombohedral structure (r-GeTe) through slightly distorting along the [111] direction at ∼720 K. Previous thermoelectric studies have mostly focused on high-symmetry c-GeTe. To date, low-symmetry r-GeTe has been revealed to show a thermoelectric figure of merit zT as high as that of c-GeTe, because the symmetry breaking electronically enables a diversified rearrangement of split bands for a high band degeneracy and thermally allows hierarchical bonds and microstructures for a low lattice thermal conductivity. This work summarizes the advancements in both r- and c-GeTe and the corresponding device-level progress, clearly demonstrating GeTe as one of the most efficient thermoelectrics for both mid-temperature (500–800 K) and near-room-temperature applications. In addition, topics that might be of interest and lead to further advancements are outlined. Although GeTe has been known as thermoelectrics since 1960s, it has attracted intensive renewed attention recently. GeTe undergoes a phase transition from a high-T cubic (c-GeTe) to a low-T rhombohedral structure (r-GeTe) through slightly distorting along the [111] direction at ∼720 K. Previous thermoelectric studies have mostly focused on high-symmetry c-GeTe. To date, low-symmetry r-GeTe has been revealed to show a thermoelectric figure of merit zT as high as that of c-GeTe, because the symmetry breaking electronically enables a diversified rearrangement of split bands for a high band degeneracy and thermally allows hierarchical bonds and microstructures for a low lattice thermal conductivity. This work summarizes the advancements in both r- and c-GeTe and the corresponding device-level progress, clearly demonstrating GeTe as one of the most efficient thermoelectrics for both mid-temperature (500–800 K) and near-room-temperature applications. In addition, topics that might be of interest and lead to further advancements are outlined. Thermoelectrics enables a direct and reversible conversion between heat and electricity without any moving parts or emissions, and thus is considered as a promising clean-energy technology.1Bell L.E. Cooling, heating, generating power, and recovering waste heat with thermoelectric systems.Science. 2008; 321: 1457-1461Crossref PubMed Scopus (2894) Google Scholar High conversion efficiency for both power generation and refrigeration essentially requires materials with high dimensionless thermoelectric figure of merit, zT=S2T/ρ(κE+κL), where S, T, ρ, κE, and κL are Seebeck coefficient, absolute temperature, electrical resistivity, and electronic and lattice components of thermal conductivity, respectively.2Goldsmid H.J. Introduction to Thermoelectricity. Springer, 2009Google Scholar Core efforts have been devoted to thermoelectric materials with a high zT for making efficient devices, such as silicides,3Sadia Y. Aminov Z. Mogilyansky D. Gelbstein Y. Texture anisotropy of higher manganese silicide following arc-melting and hot-pressing.Intermetallics. 2016; 68: 71-77Crossref Scopus (45) Google Scholar PbTe4Pei Y. Shi X. LaLonde A. Wang H. Chen L. Snyder G.J. Convergence of electronic bands for high performance bulk thermoelectrics.Nature. 2011; 473: 66-69Crossref PubMed Scopus (1831) Google Scholar, 5Cohen I. Kaller M. Komisarchik G. Fuks D. Gelbstein Y. Enhancement of the thermoelectric properties of n-type PbTe by Na and Cl co-doping.J. Mater. Chem. C. 2015; 3: 9559-9564Crossref Google Scholar, 6Komisarchik G. Fuks D. Gelbstein Y. High thermoelectric potential of n-type Pb1-xTixTe alloys.J. Appl. Phys. 2016; 120 (055104)Crossref Scopus (23) Google Scholar, 7Guttmann G.M. Dadon D. Gelbstein Y. Electronic tuning of the transport properties of off-stoichiometric PbxSn1-xTe thermoelectric alloys by Bi2Te3 doping.J. Appl. Phys. 2015; 118 (065102)Crossref Scopus (33) Google Scholar, and half-Heusler8Zhou J. Zhu H. Liu T.H. Song Q. He R. Mao J. Liu Z. Ren W. Liao B. Singh D.J. et al.Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers.Nat. Commun. 2018; 9: 1721Crossref PubMed Scopus (32) Google Scholar,9Appel O. Gelbstein Y. A comparison between the effects of Sb and Bi doping on the thermoelectric properties of the Ti0.3Zr0.35Hf0.35NiSn half-Heusler alloy.J. Electron. Mater. 2014; 43: 1976-1982Crossref Scopus (40) Google Scholar for high-temperature power generation and Bi2Te310Poudel B. Hao Q. Ma Y. Lan Y.C. Minnich A. Yu B. Yan X.A. Wang D.Z. Muto A. Vashaee D. et al.High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys.Science. 2008; 320: 634-638Crossref PubMed Scopus (3566) Google Scholar,11Hao F. Qiu P. Tang Y. Bai S. Xing T. Chu H.-S. Zhang Q. Lu P. Zhang T. Ren D. et al.High efficiency Bi2Te3-based materials and devices for thermoelectric power generation between 100 and 300°C.Energy Environ. Sci. 2016; 9: 3120-3127Crossref Google Scholar for near-room-temperature refrigeration. Proven strategies for improving zT are typified either by an enhancement of power factor S2Goldsmid H.J. Introduction to Thermoelectricity. Springer, 2009Google Scholar/ρ via band engineering approaches4Pei Y. Shi X. LaLonde A. Wang H. Chen L. Snyder G.J. Convergence of electronic bands for high performance bulk thermoelectrics.Nature. 2011; 473: 66-69Crossref PubMed Scopus (1831) Google Scholar,12Tang Y. Gibbs Z.M. Agapito L.A. Li G. Kim H.S. Nardelli M.B. Curtarolo S. Snyder G.J. Convergence of multi-valley bands as the electronic origin of high thermoelectric performance in CoSb3 skutterudites.Nat. Mater. 2015; 14: 1223-1228Crossref PubMed Scopus (253) Google Scholar,13Imasato K. Kang S.D. Ohno S. Snyder G.J. Band engineering in Mg3Sb2 by alloying with Mg3Bi2 for enhanced thermoelectric performance.Mater. Horiz. 2018; 5: 59-64Crossref Google Scholar or by a suppression of lattice thermal conductivity (κL) through phonon scattering due to various sources.10Poudel B. Hao Q. Ma Y. Lan Y.C. Minnich A. Yu B. Yan X.A. Wang D.Z. Muto A. Vashaee D. et al.High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys.Science. 2008; 320: 634-638Crossref PubMed Scopus (3566) Google Scholar,14Chen Z. Ge B. Li W. Lin S. Shen J. Chang Y. Hanus R. Snyder G.J. Pei Y. Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics.Nat. Commun. 2017; 8: 13828Crossref PubMed Scopus (177) Google Scholar, 15Joshi G. Lee H. Lan Y. Wang X. Zhu G. Wang D. Gould R.W. Cuff D.C. Tang M.Y. Dresselhaus M.S. et al.Enhanced thermoelectric figure-of-merit in nanostructured p-type silicon germanium bulk alloys.Nano Lett. 2008; 8: 4670-4674Crossref PubMed Scopus (701) Google Scholar, 16Biswas K. He J. Blum I.D. Wu C.I. Hogan T.P. Seidman D.N. Dravid V.P. Kanatzidis M.G. High-performance bulk thermoelectrics with all-scale hierarchical architectures.Nature. 2012; 489: 414-418Crossref PubMed Scopus (2244) Google Scholar Moreover, complex crystal structure,17Bux S.K. Zevalkink A. Janka O. Uhl D. Kauzlarich S. Snyder J.G. Fleurial J.-P. Glass-like lattice thermal conductivity and high thermoelectric efficiency in Yb9Mn4.2Sb9.J. Mater. Chem. A. 2014; 2: 215-220Crossref Google Scholar low sound velocity,18Li W. Lin S. Ge B. Yang J. Zhang W. Pei Y. Low sound velocity contributing to the high thermoelectric performance of Ag8SnSe6.Adv Sci. 2016; 3: 1600196Crossref Scopus (112) Google Scholar and strong lattice anharmonicity19Morelli D.T. Jovovic V. Heremans J.P. Intrinsically minimal thermal conductivity in cubic I-V-VI2 semiconductors.Phys. Rev. Lett. 2008; 101 (035901)Crossref PubMed Scopus (474) Google Scholar always result in a low κL. These strategies guarantee the highest possible zT only when the carrier concentration is optimized.20Bu Z. Li W. Li J. Zhang X. Mao J. Chen Y. Pei Y. Dilute Cu2Te-alloying enables extraordinary performance of r-GeTe thermoelectrics.Mater. Today Phys. 2019; 9: 100096Crossref Scopus (14) Google Scholar Semiconducting group IV–VI chalcogenides, particularly PbTe, SnTe, and GeTe, are well-known thermoelectrics for their outstanding performance at mid-temperatures (500–800 K). Among them, PbTe-based thermoelectrics have been most extensively studied from both material and device aspects. Although GeTe-based materials have been known as promising thermoelectrics since the 1960s,21Rosi F.D. Dismukes J.P. Hockings E.F. Semiconductor materials for thermoelectric power generation up to 700 C.Electr. Eng. 1960; 79: 450-459Crossref Google Scholar it is only recently that they have attracted emergent attention, which in turn has led to significant improvements. GeTe alone undergoes a phase transition from high-temperature cubic (c-GeTe) to low-temperature rhombohedral crystal structure (r-GeTe) at ∼720 K. The rhombohedral structure can be approximated as a slightly distorted rock-salt structure along its [111] direction.22Chattopadhyay T. Boucherle J.X. Schnering H.G. Neutron diffraction study on the structural phase transition in GeTe.J. Phys. C Solid State Phys. 1987; 20: 1431-1440Crossref Scopus (223) Google Scholar Such a symmetry breaking has recently been revealed to cause a significant difference in band structures between r-GeTe and c-GeTe.23Li J. Zhang X. Chen Z. Lin S. Li W. Shen J. Witting I.T. Faghaninia A. Chen Y. Jain A. et al.Low-symmetry rhombohedral GeTe thermoelectrics.Joule. 2018; 2: 976-987Abstract Full Text Full Text PDF Scopus (102) Google Scholar The band structure of c-GeTe is very similar to that of PbTe and SnTe, where the valence band maximum locates at L (band degeneracy Nv=4) and the secondary valence band locates at Σ (Nv=12) with a small energy offset.24Li J. Chen Z. Zhang X. Sun Y. Yang J. Pei Y. Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides.NPG Asia Mater. 2017; 9: e353Crossref Scopus (83) Google Scholar In the rhombohedral GeTe, having a lower symmetry than that of the cubic structure, the most distinctive feature is the switch of the energy levels of L and Σ valence bands. This enables an additional degree of freedom of band manipulation to achieve a high overall band degeneracy and electronic enhancements.23Li J. Zhang X. Chen Z. Lin S. Li W. Shen J. Witting I.T. Faghaninia A. Chen Y. Jain A. et al.Low-symmetry rhombohedral GeTe thermoelectrics.Joule. 2018; 2: 976-987Abstract Full Text Full Text PDF Scopus (102) Google Scholar Most of the literature reported thermoelectric properties of GeTe in a temperature range actually covering both rhombohedral and cubic phase (i.e., from 300 to 800 K), because the phase transition happens at ∼700 K. This leads literature reporting on the thermoelectric performance of r-GeTe to be about as frequent as that of c-GeTe (Figure 1A). However, historical research on GeTe-based thermoelectrics (such as TAGS21Rosi F.D. Dismukes J.P. Hockings E.F. Semiconductor materials for thermoelectric power generation up to 700 C.Electr. Eng. 1960; 79: 450-459Crossref Google Scholar,25Davidow J. Gelbstein Y. A comparison between the mechanical and thermoelectric properties of three highly efficient p-Type GeTe-rich compositions: TAGS-80, TAGS-85, and 3% Bi2Te3-doped Ge0.87Pb0.13Te.J. Electron. Mater. 2013; 42: 1542-1549Crossref Scopus (0) Google Scholar,26Yang S.H. Zhu T.J. Sun T. Zhang S.N. Zhao X.B. He J. Nanostructures in high-performance (GeTe)x(AgSbTe2)100-x thermoelectric materials.Nanotechnology. 2008; 19: 245707Crossref PubMed Scopus (164) Google Scholar) mainly focused on the performance of the high-temperature cubic phase (Figure 1A).20Bu Z. Li W. Li J. Zhang X. Mao J. Chen Y. Pei Y. Dilute Cu2Te-alloying enables extraordinary performance of r-GeTe thermoelectrics.Mater. Today Phys. 2019; 9: 100096Crossref Scopus (14) Google Scholar,21Rosi F.D. Dismukes J.P. Hockings E.F. Semiconductor materials for thermoelectric power generation up to 700 C.Electr. Eng. 1960; 79: 450-459Crossref Google Scholar,23Li J. Zhang X. Chen Z. Lin S. Li W. Shen J. Witting I.T. Faghaninia A. Chen Y. Jain A. et al.Low-symmetry rhombohedral GeTe thermoelectrics.Joule. 2018; 2: 976-987Abstract Full Text Full Text PDF Scopus (102) Google Scholar,24Li J. Chen Z. Zhang X. Sun Y. Yang J. Pei Y. Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides.NPG Asia Mater. 2017; 9: e353Crossref Scopus (83) Google Scholar,26Yang S.H. Zhu T.J. Sun T. Zhang S.N. Zhao X.B. He J. Nanostructures in high-performance (GeTe)x(AgSbTe2)100-x thermoelectric materials.Nanotechnology. 2008; 19: 245707Crossref PubMed Scopus (164) Google Scholar, 27Gelbstein Y. 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Controlling metallurgical phase separation reactions of the Ge0.87Pb0.13Te alloy for high thermoelectric performance.Adv. Energy Mater. 2013; 3: 815-820Crossref Scopus (117) Google Scholar,28Hong M. Chen Z.G. Yang L. Zou Y.C. Dargusch M.S. Wang H. Zou J. Realizing zT of 2.3 in Ge1-x-ySbxInyTe via reducing the phase-transition temperature and introducing resonant energy doping.Adv. Mater. 2018; 30: 29349887Crossref Scopus (99) Google Scholar which is superior to most of p-type thermoelectrics in the mid-temperature range. A significant enhancement in zT in r-GeTe has been realized since 2010, as can be seen from Figure 1A, largely contributing to the rapid increase in average zT (zTavg) (Figure 1B), which is imperative for efficient thermoelectric applications since a thermoelectric device works under a temperature gradient requiring a large zT over the entire working temperature range. In fact, zTavg achieved in GeTe thermoelectrics is higher than that in most thermoelectrics (Figure 1C).10Poudel B. Hao Q. Ma Y. Lan Y.C. Minnich A. Yu B. Yan X.A. Wang D.Z. Muto A. Vashaee D. et al.High-thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys.Science. 2008; 320: 634-638Crossref PubMed Scopus (3566) Google Scholar,16Biswas K. He J. Blum I.D. Wu C.I. Hogan T.P. Seidman D.N. Dravid V.P. Kanatzidis M.G. High-performance bulk thermoelectrics with all-scale hierarchical architectures.Nature. 2012; 489: 414-418Crossref PubMed Scopus (2244) Google Scholar,51Roychowdhury S. Panigrahi R. Perumal S. Biswas K. Ultrahigh Thermoelectric figure of merit and enhanced mechanical stability of p-type AgSb1–xZnxTe2.ACS Energy Lett. 2017; 2: 349-356Crossref Scopus (0) Google Scholar, 52Wu Y. Chen Z. Nan P. Xiong F. Lin S. Zhang X. Chen Y. Chen L. Ge B. Pei Y. 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Jain A. et al.Low-symmetry rhombohedral GeTe thermoelectrics.Joule. 2018; 2: 976-987Abstract Full Text Full Text PDF Scopus (102) Google Scholar very recently, is even higher than that in c-GeTe, which largely renews the interests in this class of materials.34Perumal S. Samanta M. Ghosh T. Shenoy U.S. Bohra A.K. Bhattacharya S. Singh A. Waghmare U.V. Biswas K. Realization of high thermoelectric figure of merit in GeTe by complementary co-doping of Bi and in.Joule. 2019; 3: 2565-2580Abstract Full Text Full Text PDF Scopus (9) Google Scholar,35Wu D. Xie L. Xu X. He J. High thermoelectric performance achieved in GeTe–Bi2Te3 pseudo-binary via van der waals gap-induced hierarchical ferroelectric domain structure.Adv. Funct. Mater. 2019; 29: 1806613Crossref Scopus (16) Google Scholar,56Dong J. Sun F.-H. Tang H. Pei J. Zhuang H.-L. Hu H.-H. Zhang B.-P. Pan Y. Li J.-F. 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SPIE. 2014; 9115: 911507Crossref Scopus (6) Google Scholar More recently, GeTe-alloys with diverse compositions received great attentions for thermoelectric power generation, and the experimental efficiency of 12.3% realized in single-leg-based Ge0.93In0.01Bi0.06Te device is actually the highest reported so far for single-stage devices under the temperature difference of <600 K.34Perumal S. Samanta M. Ghosh T. Shenoy U.S. Bohra A.K. Bhattacharya S. Singh A. Waghmare U.V. Biswas K. Realization of high thermoelectric figure of merit in GeTe by complementary co-doping of Bi and in.Joule. 2019; 3: 2565-2580Abstract Full Text Full Text PDF Scopus (9) Google Scholar These recent developments, particularly on low-temperature rhombohedral GeTe (r-GeTe), are important mainly owing to the following factors: (1) the significantly advanced knowledge on the fundamentals of bonding features induced both atomic and electronic structure variations, and (2) the enhancement of zT in r-GeTe promoting the average zT (zTavg) to be the highest among known thermoelectrics (Figure 1), thereby enabling this class of materials to be potentially used for near-room-temperature applications. Therefore, this perspective focuses on these important advancements, particularly in r-GeTe thermoelectrics. In this paper, the electronic origins of high performance in thermoelectric GeTe are discussed in relation to its crystal-structure-induced change in the band structure. The rearrangement of the split valence bands in r-GeTe provides additional degree of freedom for band manipulation, and thus, comparably high (or even higher) power factor can be realized in r-GeTe, compared with the c-GeTe. Thermally, the lattice thermal conductivity (κL) of r-GeTe is intrinsically lower than that of c-GeTe and SnTe.24Li J. Chen Z. Zhang X. Sun Y. Yang J. Pei Y. Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides.NPG Asia Mater. 2017; 9: e353Crossref Scopus (83) Google Scholar Possible reasons are discussed in detail, together with the successful approaches for strengthening phonon scattering by hierarchical microstructures. In addition to the superior zT, GeTe-alloys have shown better mechanical properties over conventional Bi2Te3 and other IV–VI thermoelectrics,61Perumal S. Roychowdhury S. Biswas K. High performance thermoelectric materials and devices based on GeTe.J. Mater. Chem. C. 2016; 4: 7520-7536Crossref Google Scholar making this class of materials show great potentials as p-type legs for efficient thermoelectric devices. In conclusion, GeTe materials are promising candidates for thermoelectric applications at both mid-temperatures and near room temperatures with possible further advancements. Unlike the other two high-performance IV-VI thermoelectrics, PbTe and SnTe, both of which crystallize in a face-centered cubic structure and undergo no solid-state phase transitions at room temperature and above, GeTe exhibits a rock-salt structure (space group Fm_3m) only at T>720 K, with a lattice constant of a=6.00 Å.22Chattopadhyay T. Boucherle J.X. Schnering H.G. Neutron diffraction study on the structural phase transition in GeTe.J. Phys. C Solid State Phys. 1987; 20: 1431-1440Crossref Scopus (223) Google Scholar At T<720 K, GeTe undergoes a phase transition to a rhombohedral structure (space group of R3m) (Figure 2A).22Chattopadhyay T. Boucherle J.X. Schnering H.G. Neutron diffraction study on the structural phase transition in GeTe.J. Phys. C Solid State Phys. 1987; 20: 1431-1440Crossref Scopus (223) Google Scholar Rhombohedral GeTe (r-GeTe) can be approximated as a result of a directional distortion of the high-symmetry cubic one (c-GeTe) along its [111] direction, with a lattice constant of a=5.98 Å and a rhombohedral angle of α=88.2° at 300 K.22Chattopadhyay T. Boucherle J.X. Schnering H.G. Neutron diffraction study on the structural phase transition in GeTe.J. Phys. C Solid State Phys. 1987; 20: 1431-1440Crossref Scopus (223) Google Scholar The inset of Figure 2C shows the primitive cell of r-GeTe; in the rhombohedral structure, Ge and Te atoms are displaced from each other, leading to a movement of Ge atoms from (0.5, 0.5, 0.5) to (0.5-δ, 0.5-δ, 0.5-δ). Thus, Ge is 6-fold coordinated with Te having three shorter and three longer bonds. Neutron-diffraction results suggest that the ferroelectric structural transition in GeTe is displacive, and when the rhombohedral lattice transits to cubic, the shorter and longer bond lengths show a symmetrical deviation to its average.62Chatterji T. Kumar C.M.N. Wdowik U.D. Anomalous temperature-induced volume contraction in GeTe.Phys. Rev. B. 2015; 91 (054110)Crossref Scopus (19) Google Scholar Among group IV–VI chalcogenides, the rock-salt structure becomes increasingly stable as one moves to the heavier cation-anion pairs (i.e., cation changes from Ge to Sn and then Pb), which can be reasonably understood by the variations in orbital hybridization and bonding ionicity.63Lencer D. Salinga M. Grabowski B. Hickel T. Neugebauer J. Wuttig M. A map for phase-change materials.Nat. Mater. 2008; 7: 972-977Crossref PubMed Scopus (446) Google Scholar,64Waghmare U.V. Spaldin N.A. Kandpal H.C. Seshadri R. Firs" @default.
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