Publications by authors named "Debattam Sarkar"

3 Publications

  • Page 1 of 1

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance.

Angew Chem Int Ed Engl 2021 Feb 22. Epub 2021 Feb 22.

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, 560064, India.

Orthorhombic GeSe is a promising thermoelectric material. However, large band gap and strong covalent bonding result in a low thermoelectric figure of merit, zT≈0.2. Here, we demonstrate a maximum zT≈1.35 at 627 K in p-type polycrystalline rhombohedral (GeSe) (AgBiTe )  , which is the highest value reported among GeSe based materials. The rhombohedral phase is stable in ambient conditions for x=0.8-0.29 in (GeSe) (AgBiTe )  . The structural transformation accompanies change from covalent bonding in orthorhombic GeSe to metavalent bonding in rhombohedral (GeSe) (AgBiTe )  . (GeSe) (AgBiTe ) has closely lying primary and secondary valence bands (within 0.25-0.30 eV), which results in high power factor 12.8 μW cm  K at 627 K. It also exhibits intrinsically low lattice thermal conductivity (0.38 Wm  K at 578 K). Theoretical phonon dispersion calculations reveal vicinity of a ferroelectric instability, with large anomalous Born effective charges and high optical dielectric constant, which, in concurrence with high effective coordination number, low band gap and moderate electrical conductivity, corroborate metavalent bonding in (GeSe) (AgBiTe ) . We confirmed the presence of low energy phonon modes and local ferroelectric domains using heat capacity measurement (3-30 K) and switching spectroscopy in piezoresponse force microscopy, respectively.
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http://dx.doi.org/10.1002/anie.202101283DOI Listing
February 2021

Ferroelectric Instability Induced Ultralow Thermal Conductivity and High Thermoelectric Performance in Rhombohedral -Type GeSe Crystal.

J Am Chem Soc 2020 Jul 30;142(28):12237-12244. Epub 2020 Jun 30.

Department of Physical Sciences, Indian Institute of Science Education and Research Mohali, Sector 81, S. A. S. Nagar, Manauli 140306, India.

The orthorhombic phase of GeSe, a structural analogue of layered SnSe (space group: ), has recently attracted attention after a theoretical prediction of high thermoelectric figure of merit, zT > 2. The experimental realization of such high performance in orthorhombic GeSe, however, is still elusive (zT ≈ 0.2). The rhombohedral phase of GeSe, a structural analogue of GeTe (space group: 3), previously stabilized at high pressure (2 GPa) and high temperature (1600 K), is promising due to its theoretically predicted ferroelectric instability and the higher earth abundance of Se compared to Te. Here, we demonstrate high thermoelectric performance in the rhombohedral crystals of GeSe, which is stabilized at ambient conditions by alloying with 10 mol % AgBiSe. We show ultralow lattice thermal conductivity (κ) of 0.74-0.47 W/mK in the 300-723 K range and high zT ≈ 1.25 at 723 K in the -type rhombohedral (GeSe)(AgBiSe) crystals grown using Bridgman method. First-principles density functional theoretical analysis reveals its vicinity to a ferroelectric instability which generates large anomalous Born effective charges and strong coupling of low energy polar optical phonons with acoustic phonons. The presence of soft optical phonons and incipient ferroelectric instability in (GeSe)(AgBiSe) are directly evident in the low temperature heat capacity () and switching spectroscopy piezoresponse force microscopy (SS-PFM) experiments, respectively. Effective scattering of heat carrying acoustic phonons by ferroelectric instability induced soft transverse optical phonons significantly reduces the κ and enhances the thermoelectric performance in rhombohedral (GeSe)(AgBiSe) crystals.
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http://dx.doi.org/10.1021/jacs.0c03696DOI Listing
July 2020

Highly Converged Valence Bands and Ultralow Lattice Thermal Conductivity for High-Performance SnTe Thermoelectrics.

Angew Chem Int Ed Engl 2020 Jun 28;59(27):11115-11122. Epub 2020 Apr 28.

New Chemistry Unit, International Centre for Materials Science and School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore, 560064, India.

A two-step optimization strategy is used to improve the thermoelectric performance of SnTe via modulating the electronic structure and phonon transport. The electrical transport of self-compensated SnTe (that is, Sn Te) was first optimized by Ag doping, which resulted in an optimized carrier concentration. Subsequently, Mn doping in Sn Ag Te resulted in highly converged valence bands, which improved the Seebeck coefficient. The energy gap between the light and heavy hole bands, i.e. ΔE decreases to 0.10 eV in Sn Ag Mn Te compared to the value of 0.35 eV in pristine SnTe. As a result, a high power factor of ca. 24.8 μW cm  K at 816 K in Sn Ag Mn Te was attained. The lattice thermal conductivity of Sn Ag Mn Te reached to an ultralow value (ca. 0.3 W m  K ) at 865 K, owing to the formation of Ag Te nanoprecipitates in SnTe matrix. A high thermoelectric figure of merit (z T≈1.45 at 865 K) was obtained in Sn Ag Mn Te.
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http://dx.doi.org/10.1002/anie.202003946DOI Listing
June 2020