High-performance eco-friendly MnTe thermoelectrics through introducing SnTe nanocrystals and manipulating band structure

Physical Insights on the Lattice Softening Driven Mid‐Temperature Range Thermoelectrics of Ti/Zr‐Inserted SnTe—An Outlook Beyond the Horizons of Conventional Phonon Scattering and Excavation of Heikes’ Equation for Estimating Carrier Properties

“…Though this lattice softening effect is the main player in the reduction of κ latt in these doped SnTe compounds, the conventional phonon scattering contribution cannot be ignored entirely, especially at high-temperature ranges. To support this, a plethora of information is available in the literature (based on TEM analysis) to show that doping of elements (especially Mn) to SnTe can cause multi-scale hierarchical structural defects, [28,29,41,53] including a high density of point-defects, planar stacking faults, atomic-scale interstitials, dislocations (ordered/disordered), strain clusters, phase boundary between different scales of precipitates, all of which can effectively scatter phonons of different wavelengths or mean free paths. Due to these past well-established (and incrementally reported) findings on the nanostructural aspects of the doped-SnTe system, TEM images are not presented in this work.…”

“…[26] However, the pristine SnTe has its drawbacks with regards to its usage as a thermoelectric material such as high charge carrier concentration (≈10 21 cm -3 ) due to the intrinsic nature of SnTe to have high Sn vacancies, small bandgap often leading to bipolar conduction, and the large energy separation between the two valence band maxima (≈0.35 eV), all of which leads to a low S and high κ e , thus ultimately lower zT in pristine stochiometric SnTe. [26][27][28][29][30][31] Due to larger anharmonicity and effective phonon scattering, PbTe exhibits lower thermal conductivity than SnTe. [32][33][34][35] Various attempts and strategies have been made in the past to improve the thermoelectric performance in SnTe, which includes selfcompensation in the composition with some additional Sn content; [36,37] electronic bandstructure engineering-fostering resonance state in the vicinity of fermi-level (mostly induced by In as a dopant in SnTe), [27,38] simultaneous increase of principal bandgap and convergence of the light hole and heavy hole valence bands (realized with dopants such as Cd, Hg, Mn, Mg, Ca, and few more), [27,29,36,37,[39][40][41][42][43][44] and this also includes valence band inversion or crossing effects; and a diverse nano-structuration approaches to engineer the dense interstitials, stacking faults, point defects, nano-precipitates and semi-coherent interfaces, dislocations, strain clusters, and so forth, [45][46][47][48][49][50][51][52] (by alloying with Cu 2 Te, CdS, SrTe, ZnS, and a few more).…”

“…In this work, the strategy was that, if any of the compositions from the newly tried Ti and Zr series showed promising transport properties, then those optimized compositions are finally codoped with Mn, which is widely reported to bring in band convergence effect in SnTe. [29,41] In a nutshell, this work investigates the effect of doping the transition metals Ti and Zr and their best performing optimized codoping (Ti-Mn and Zr-Mn) on the structural, electronic, and thermoelectric properties of SnTe. More specifically, the discussion and comparison of thermoelectric transport properties of the SnTe-based materials that are presented here are kept within the realistic/practically applicable temperature ranges (T max ≈ 723 K, and ∆T ≈ 400 K), in contrary to the attractive yet practically limited thermoelectric performances reported at high-temperature ranges for several SnTe analogues.…”

“…[ 26–31 ] Due to larger anharmonicity and effective phonon scattering, PbTe exhibits lower thermal conductivity than SnTe. [ 32–35 ] Various attempts and strategies have been made in the past to improve the thermoelectric performance in SnTe, which includes self‐compensation in the composition with some additional Sn content; [ 36,37 ] electronic bandstructure engineering—fostering resonance state in the vicinity of fermi‐level (mostly induced by In as a dopant in SnTe), [ 27,38 ] simultaneous increase of principal bandgap and convergence of the light hole and heavy hole valence bands (realized with dopants such as Cd, Hg, Mn, Mg, Ca, and few more), [ 27,29,36,37,39–44 ] and this also includes valence band inversion or crossing effects; and a diverse nano‐structuration approaches to engineer the dense interstitials, stacking faults, point defects, nano‐precipitates and semi‐coherent interfaces, dislocations, strain clusters, and so forth, [ 45–52 ] (by alloying with Cu 2 Te, CdS, SrTe, ZnS, and a few more). [ 53 ] All these approaches and dopants in SnTe, though they yielded an improved thermoelectric performance ( zT max > 1), are limited for any practical applications, as their improved performance happened only at higher temperature ranges (usually between 823 and 873 K, or higher) for SnTe‐based materials.…”

Thermoelectric property enhancement by merging bands in NbFeSb-based half-Heusler mixtures

“…For example, through band engineering, the converged bands can boost the Seebeck coef-cient without detriment to the electrical conductivity. 4 Pei et al doped Na in the PbTe 1Àx Se x alloy, which can degenerate the energy bands of S and L, so that the zT of the alloy reaches 1.8 at 850 K. 5 Similarly, different effective dopants have been used to converge the S and L bands in SnTe, [6][7][8] the sister compound of PbTe. Zhang et al found that the crystal eld effect in the solid solution of chalcopyrite compounds can lead the p-band convergence around the valence band maximum (VBM), which improves the thermoelectric properties of the material.…”