Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping

Abstract

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit () of ∼1.5 at 773 K and an average of ∼1.1 from 300 to 773 K in cubic GeMnBiTe. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

Positive Effect of Ge Vacancies on Facilitating Band Convergence and Suppressing Bipolar Transport in GeTe‐Based Alloys for High Thermoelectric Performance

“…The average ZT of the Ge 0.8 Pb 0.1 Bi 0.1 Te 1+ x within 300–773 K shows an increase trend with the amount of extra Te x (Figure 8b), increased by ≈94% from 0.69 for x = 0 to 1.34 for x = 0.06. Compared with previous reported GeTe‐based alloys, [ 21,24,26,33,40,42,43 ] our GeTe‐based alloys demonstrate superior thermoelectric ZT in both low‐ and high‐temperature region (Figure 8c), thus displaying outstanding average ZT within 300–773 K (Figure 8d).…”

“…The Bi and Sb have been widely used to reduce the hole concentration of GeTe due to their donor dopant nature. [ 29–33 ] Other doping or alloying with Pb, [ 25,34–36 ] Se, [ 30,37–39 ] Bi‐Sb, [ 40 ] Bi‐Cu, [ 41 ] Mn‐Bi, [ 42 ] Mn‐Sb, [ 43 ] Pb‐Sb, [ 28,44 ] Pb‐Bi, [ 45 ] Cd‐Bi, [ 46,47 ] Sb‐Zn, [ 48 ] Sb‐In, [ 49 ] Bi 2 Te 3 , [ 23,26 ] Sb 2 Te 3 , [ 50 ] and AgSbTe 2 [ 51,52 ] have also been widely applied to optimize the carrier density and to reduce κ lat for enhancing the ZT of GeTe‐based alloys. Combined with the synergic effects of carrier‐density optimization, band engineering and phonon engineering strategies, many GeTe‐based alloys with peak ZT of around 2 have been reported recently.…”

“…Many studies have considered Ge vacancies as negative effects for enhancing ZT of GeTe‐based alloys, [ 24,42,44,53–56 ] and have also proposed various approaches for suppressing Ge vacancies. Dong et al have reported that excess Ge can be effective in eliminating the Ge vacancies in pristine GeTe for reducing carrier density and increasing carrier mobility, [ 24 ] resulting in a peak ZT of ≈1.6 around 650 K. As reported by Li et al, [ 53 ] alloying with PbSe can elevate the formation energy of Ge vacancies by increasing the mean size of cations and decreasing the average size of anions, resulting in suppressed Ge vacancies.…”

Manipulating the Ge Vacancies and Ge Precipitates through Cr Doping for Realizing the High‐Performance GeTe Thermoelectric Material

“…a) Room temperature carrier concentration‐dependent Seebeck coefficient for Cr doping and Cr‐Bi/Sb codoping samples, with a comparison to those of GeTe reported in literatures. [ 39,40,59,61–63 ] Temperature‐dependent electrical transport properties for all samples: b) Seebeck coefficient, c) electrical resistivity, and d) power factor.…”

“…Temperature‐dependent thermoelectric transport properties for Cr doping and Cr‐Bi/Sb codoping samples: a) total thermal conductivity, b) lattice thermal conductivity, c) figure of merit (ZT), and c) average ZT with a comparison to the literatures. [ 39–41,47,63–65 ] …”