Regular Article

First principles calculations of the thermoelectric properties of α-MnO₂ and β-MnO₂

¹ The National Institute for Theoretical Physics, School of Physics and Mandelstam Institute for Theoretical Physics, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa
² Department of Physics and Space Science, The Technical University of Kenya (TU-K), PO Box 52428, 00200 Nairobi, Kenya

^a e-mail: chepkoechmirriam@gmail.com

Received: 15 May 2018
Received in final form: 29 September 2018
Published online: 3 December 2018

Abstract

Oxide based thermoelectric materials have gained considerable interest due to their abundance, low toxic nature and high temperature stability. In the present work, we report the thermoelectric properties of α-MnO₂ and β-MnO₂ investigated using the Density Functional Theory with a Hubbard correction as implemented in the VASP code. Our calculated band gaps 0.27 eV and 1.28 eV for β-MnO₂ and α-MnO₂, respectively, are in excellent agreement with the experimental values and hence demonstrate the importance of including a Hubbard correction (U) in studying the physical and electronic properties of manganese oxides. The calculated elastic constants obey the Born Huang elastic stability criteria and therefore indicate that the studied materials are mechanically stable. The computed phonon band structures and the vibrational density of states do not have imaginary frequencies throughout the Brillouin zone and hence is a clear indication of the dynamic stability of these materials. Our calculated lattice thermal conductivities (κ_L) show a strong anisotropic behaviour along the a and c directions. At room temperature, the results show that acoustic phonon modes contribute ~56.0(55.8)% in α-MnO₂ and ~80.4(73.8)% in β-MnO₂ to the total κ_L along the a(c) directions respectively. In addition, the thermoelectric transport coefficients; σ and S²σ display an anisotropic behaviour between a and c directions. We obtained higher power factors, i.e., (447)(435) μW/m K² for α-MnO₂ and (134)(225) μW/m K² for β-MnO₂ with hole doping concentration of 10²⁰ cm⁻³ along the a(c) directions respectively compared to that of Bi₂Te₃ (40 μW/K²cm) at 300 K. This large thermoelectric power suggests that these materials may be potential candidates for thermoelectric applications. However, our calculated dimensionless figure of merit for β-MnO₂ and α-MnO₂ are quite small due to large values of lattice thermal conductivities. Our highest computed ZT values are 0.02 for β-MnO₂ with hole doping concentration of 10²¹ cm⁻³, and 0.14 for α-MnO₂ with electron carrier concentration of 10²¹ cm⁻³ at 800 K along the c-direction.