Metavalent Bonding in GeSe Leads to High Thermoelectric Performance
Efficient management of electrical and thermal energies has become increasingly important to achieve a sustainable energy cycle. The conversion of waste heat into useful electrical energy using thermoelectric materials is one of the most promising alternative energy solutions. Recently,GeSe has shown significant promise in the thermoelectric energy conversion. However, pristine orthorhombic GeSe exhibits covalently bonded layered structure with alow maximum experimental thermoelectric figure of merit, zT ~0.2. Earlier works of Prof. Kanishka Biswas’ group from JNCASR (India) have shown that a 10% doping of the GeSe system with AgBiTe2can lead to pure R3m rhombohedral phase of (GeSe)0.9(AgBiTe2)0.1.This material is found to show a maximum zT of ~1.35 at 627 K. This is the highest value among the GeSe based materials.
Synchrotron X-ray diffraction of (GeSe)0.9(AgBiTe2)0.1was carried out to investigate the structural evolution with pressure. It was found that a rhombohedral to cubic phase transition takes place at ~6.7 GPa in (GeSe)0.9(AgBiTe2)0.1.The occurrence of a rhombohedral to cubic phase transition at much lower pressure as ~6.7 GPa in (GeSe)0.9(AgBiTe2)0.1,compared to the absence of such transition even up to 82 GPa in GeSe, provides a clear experimental indication of metavalent bonding in the former compared to the covalent bonding in the latter. Interestingly, a moderate temperature of 563 K can also induce such a phase transition at ambient pressure.
This investigation reveals that upon AgBiTe2alloying in GeSe, the structural evolution from orthorhombic GeSe to rhombohedral (GeSe)1-x(AgBiTe2)xaccompanies a change in the nature of the chemical bonding in the system. While pristine GeSe has covalent bonding, the chemical bonding in (GeSe)1-x(AgBiTe2)xis of metavalent characteristics with large effective coordination number, moderate electrical conductivity, large anharmonicty, high Born effective charges and large optical dielectric constant. Metavalent bonding with incipient metallic character and closeness to ferroelectric instability may be thus responsible for such high thermoelectric performance in (GeSe)1-x(AgBiTe2)x(see Fig.1 and caption for phase transition evolution and thermoelectric performance).
This study, performed by Prof. Biswas’ group at Elettra (Xpress beamline) and KEK, Photon Factory synchrotrons is to evaluate the phase transition (rhombohedral to cubic) pressure and temperature in (GeSe)0.9(AgBiTe2)0.1.
Figure 1. Metavalent bonding in (GeSe)0.9(AgBiTe2)0.1 resulted in significantly reduced rhombohedral to cubic phase transition pressure (left) compared to pristine orthorhombic covalent GeSe. Moderate rise in temperature can also leads to a similar transition at ambient pressure. Rhombohedral p-type polycrystalline (GeSe)0.9(AgBiTe2)0.1 is found to show excellent thermoelectric performance (right), thanks to the occurrence of metavalent bonding and ferroelectric instability.
This research was conducted by the following research team:
Debatttam Sarkar1, Subhajit Roychowdhury1, Raagya Arora1, Tanmoy Ghosh1, Aastha Vasdev2, Boby Joseph 3, Goutam Sheet2, Umesh V. Waghmare1, and Kanishka Biswas1
1 Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru, India
2 Indian Institute of Science Education and Research Mohali, Manauli, Mohali, India
3 Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
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