Eur. Phys. J. B (2015) 88: 113
https://doi.org/10.1140/epjb/e2015-50478-x

Regular Article

A density functional (PBE, PBEsol, HSE06) study of the structural, electronic and optical properties of the ternary compounds AgAlX₂ (X = S, Se, Te)

National Institute for Theoretical Physics, School of Physics and Mandelstam Institute for Theoretical Physics, University of the Witwatersrand, Private Bag 3, 2050 Johannesburg, South Africa

^a e-mail: donghomoise@gmail.com

Received: 15 July 2014
Received in final form: 27 December 2014
Published online: 7 May 2015

Abstract

First principles density functional theory (DFT) calculations for bulk structural, electronic and optical properties of ternary compounds AgAlX₂ (X = S, Se, Te) were performed with two flavours of generalised gradient approximations (PBE and PBEsol) and the hybrid functional HSE06. Using cohesive energy as a stability criterion, we found that the chalcopyrite structure is the favoured phase for these materials. PBEsol gives structural properties closer to the experimental values when compared to the results of PBE. Tetragonal distortion and anion displacement were calculated and we found them to be the cause of the crystal field splitting. Reduction of the bandgap and band splitting around the Γ in the Brillouin zone was noted when spin-orbit coupling was included in our study especially in the case of AgAlTe₂. The HSE06 bandgap and frequency dependent dielectric function were in very good agreement with experimental results. We have also shown that the maximum absorption peak lies in the ultraviolet range irrespective of the functional used. The refractive index is also discussed.