Selecting the suitable dopants: electronic structures of transition metal and rare earth doped thermoelectric sodium cobaltate

Abstract

Engineered Na_0.75CoO₂ is considered a prime candidate to achieve high efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on the electronic and crystallographic structures of Na_0.75CoO₂. Systematic ab initio density functional calculations with DMOL3 package showed that the Ni and Zn were more stable when substituting Co with formation energy −2.35 eV, 2.08 eV when Fermi level equals the valence band maximum. While Eu is more stable when it substitutes Na having a formation energy of −2.64 eV. As these results show great harmony with existing experimental data, they provide new insights into the fundamental principle of dopant selection for manipulating the physical properties in the development of high performance sodium cobaltate based multifunctional materials.