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Subhajit Roychowdhury1 Tanmoy Ghosh1 Raagya Arora1 Umesh V Waghmare1 Kanishka Biswas1

1, JNCASR, Bangalore, KA, India

n-type Ge-chalcogenides with promising thermoelectric performance is urgently needed to match their p-type counterparts. However, their realization remains elusive due to intrinsic Ge vacancies which make them p-type semiconductors. GeSe crystallizes into a layered orthorhombic structure similar to the SnSe at ambient conditions. High symmetry cubic phase of GeSe is predicted to be stabilized either by applying external pressure of 7 GPa or by enhancing the entropy via increasing the temperature to 920 K. Herein, we report the stabilization of n-type cubic phase of GeSe at ambient conditions by alloying with AgBiSe2 (30-50 mol %), which enhances the entropy through solid solution mixing. The interplay of positive and negative chemical pressure anomalously changes the band gap of GeSe with increasing the AgBiSe2 concentration. The band gap of n-type cubic (GeSe)1-x(AgBiSe2)x posses a value in the 0.3-0.4 eV range, which is significantly lower than orthorhombic GeSe (1.1 eV). Cubic (GeSe)1-x(AgBiSe2)x exhibits an ultralow lattice thermal conductivity (κL ~0.43 Wm-1K-1) in the 300-723 K temperature range. The low κL is attributed to significant phonon scattering by entropy driven enhanced solid solution point defects.1

Reference:
1. S. Roychowdhury, T. Ghosh, R. Arora, U. V. Waghmare and K. Biswas, n-type Cubic GeSe Stabilized by Entropy Driven Alloying of AgBiSe2 Leads to Ultralow Thermal Conductivity and Promising Thermoelectric Performance, Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201809841.

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