Citation: Ramoshaba, M.; Mosuang, T. Correlations of the Electronic, Elastic and Thermo-Electric Properties of Alpha Copper Sulphide and Selenide. Computation 2023, 11, 233. https://doi.org/10.3390/ computation11110233 Academic Editors: Cuiying Jian and Aleksander Czekanski Received: 1 September 2023 Revised: 5 October 2023 Accepted: 20 October 2023 Published: 17 November 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). computation Article Correlations of the Electronic, Elastic and Thermo-Electric Properties of Alpha Copper Sulphide and Selenide Moshibudi Ramoshaba and Thuto Mosuang * Department of Physics, University of Limpopo, University Road, Mankweng, Polokwane 0727, South Africa * Correspondence: thuto.mosuang@ul.ac.za; Tel.: +27-(0)15-268-3576 Abstract: A full potential all-electron density functional method within generalized gradient ap- proximation is used herein to investigate correlations of the electronic, elastic and thermo-electric transport properties of cubic copper sulphide and copper selenide. The electronic band structure and density of states suggest a metallic behaviour with a zero-energy band gap for both materials. Elastic property calculations suggest stiff materials, with bulk to shear modulus ratios of 0.35 and 0.44 for Cu 2 S and Cu 2 Se, respectively. Thermo-electric transport properties were estimated using the Boltzmann transport approach. The Seebeck coefficient, electrical conductivity, thermal conductivity and power factor all suggest a potential p-type conductivity for α-Cu 2 S and n-type conductivity for α-Cu 2 Se. Keywords: CuS; CuSe; density functional theory; electronic structure; elastic constants; transport properties; power factor 1. Introduction Copper-based chalcogenides, especially copper sulphides (CuS) and selenides (CuSe), have the potential to replace some of the leading silicon families of energy-harvesting materials, which are becoming extinct at present. These binary compounds are generated from group IB transitional metals and group VIA non-metals. The three elements, copper, sulphur and selenium, are readily available from the Earth’s crust. Uniquely designed CuS and CuSe chalcogenides allow the development of cost-effective energy compounds with minor environmental hazards [1]. Studies show that both CuS and CuSe can exist in a variety of stoichiometries, with crystal forms ranging from the cubic to the hexagonal phase [1–7]. Heating and cooling processes within the materials mostly lead to a transition from one form to the other [2,3]. A CuSe configuration is a blended conductor that displays diverse phase transitions from stable to metastable forms and from low- to high-temperature forms. In particular, Cu 2 Se undergoes a low-temperature monoclinic to high-temperature face-centred cubic (fcc) phase transition at 410 K [3]. On the other hand, Cu 2 S undergoes two phase transitions: one at around 370 K and another at 700 K [4,5]. At 370 K, a transition from the monoclinic to hexagonal phase takes place, coupled with the hexagonal to cubic phase at 700 K. The intermediate phases, which include monoclinic and orthorhombic, can also be categorized as superionic due to them having fast mobile fluidic Cu 1+ or Cu 2+ ions within the focal Se 2- ions lattice. Namsani et al. [2] and Kim et al. [6] revealed that at room temperature CuSe is not well defined, but at high temperatures, the cubic phase is dominant. The prevailing lattice originates from Se ions, with Cu ions arbitrarily dispersed at different sites within this lattice matrix. Upon the cubic-faced Se ion lattice, Cu ions exhibit fluidic behaviour, which results in the good thermo-electric character of the compound. Such a performance shows cubic CuSe as a favourable material when it comes to thermal and electronic parameter regulation. Even though the Se atom lattice is well defined, the cubic CuSe still demonstrates Computation 2023, 11, 233. https://doi.org/10.3390/computation11110233 https://www.mdpi.com/journal/computation