Contents lists available at ScienceDirect Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm Mechanical, thermal and thermoelectric properties of MX 2 (M = Zr, Hf; X = S, Se) Mahmud Abdulsalam*, Elkana Rugut, D.P. Joubert Department of Physics, Umaru Musa Yar’adua University, Katsina State, Nigeria ARTICLE INFO Keywords: DFT Mechanical, thermal and thermoelectric properties Stability ABSTRACT In this paper, we used density functional theory to study the mechanical and thermal properties of MX 2 (M = Zr, Hf; X = S, Se) in P2 1 /m structure. Results from elastic constants and phonon band structure indicated that the structures were found to be stable against mechanical distortions. These MX 2 are brittle and their dominant bonding type is ionic in nature based on our computed mechanical properties. Thermal properties show that the specific heat capacity at constant volume of the selenides is greater than that of the sulphides of hafnium and zirconium. Additionally, we computed the lattice thermal conductivity as well as the transport coefficients with respect to temperature over a range of charge carrier concentrations, ranging from the Seebeck coefficient, electrical conductivity, electronic contribution to total thermal conductivity and consequently fuse them so as to calculate their respective power factors as well as the dimensionless figure of merit of the hafnium and zirconium dichalcogenides considered hereby. 1. Introduction The discovery of graphene [1] led to search for a new 2-dimensional (2-D) materials in both layered and non-layered form [2] as a result of their unique optoelectronic, physical and chemical properties [2–8]. The family of transition metal chalcogenides (TMC) belongs to the 2-D layered materials. They show a rich variety of properties ranging from semiconducting to superconducting [9], depending on the combination of the transition metal and the chalcogen. Therefore, they have a wide range of applications which include catalysis, electrochemical inter- calations, nano scale flexible field effect transistor devices [7] and photo-electrochemical solar cells [10], among others. Their high ani- sotropic electrical, mechanical and optical properties [11] and favour- able stability, catalytic properties [12] make them of great interest to researchers. Recently, TMC based fullerene and nanotubes with pos- sible use in photo-electrochemical solar cells and photo-detectors have been synthesized [10], and some members of these families are the subject of consideration as cathode intercalation compounds in lithium based batteries [13]. MX 2 (M = Zr, Hf; X = S, Se) are among the TMC's family, they crystallize in 1T-CdI 2 structure with space group P 3m1. The Hf or Zr metal atoms are located at the centre, the metals are sandwiched be- tween the two atoms of chalcogen layers, the structure consists of X-M- X trilayer atomic units, joined by covalent bonds within the layer and bonded to each other by weak van der Waal's interlayer forces [14,15]. These structures are semiconductors with potential in solar energy applications [15–17]. In line to this, the stability of the structures to be used for such applications is of paramount importance. Despite their importance, it is surprising that there are no, or very few literature that reported the stability lattice thermal conductivity as well as the transport properties of these structures. To the best of our knowledge, the only available literature on the family of these struc- tures are [18], is where the authors used Born Stability Criteria [19] to investigate the stability of MX 2 (M = Zr, Hf; X = S, Se) structures. The Born Stability Criteria are believed to be necessary, but they are not sufficient to establish the stability of number of crystals [20,21]. Singh [22] investigated the ultrasonic and thermophysical properties of HfX 2 X = S, Se). Yumnam [23] studied the high temperature thermoelectric properties of Zirconium and Hafnium based transition metal dichalco- genides. This study did not include a crucial factor in the simulation of molecular crystals; the non-covalent interaction between molecules. This vander Waal's correction is necessary for accurate description of the structural parameters of any layered structure, which in turns de- termines the accuracy of all computed properties of the materials. There is, therefore, a need to conduct a systematic investigation of the me- chanical, thermal, lattice as well as the transport properties of MX 2 https://doi.org/10.1016/j.mtcomm.2020.101434 Received 5 May 2020; Received in revised form 5 July 2020; Accepted 5 July 2020 ⁎ Corresponding author at: The National Institute for Theoretical Physics, School of Physics and Mandelstam Institute for Theoretical Physics, University of the Witwatersrand, Johannesburg, Wits 2050, South Africa. E-mail address: mahmud.abdussalam@umyu.edu.ng (M. Abdulsalam). Materials Today Communications 25 (2020) 101434 Available online 10 July 2020 2352-4928/ © 2020 Elsevier Ltd. All rights reserved. T