Mahlaga P. Molepo

Bio: Mahlaga P. Molepo is an academic researcher from University of the Witwatersrand. The author has contributed to research in topics: Density functional theory & Materials science. The author has an hindex of 4, co-authored 4 publications receiving 56 citations.

Computational study of the structural phases of ZnO

Abstract: We use first-principles calculations based on density functional theory to study the structural properties and pressure-induced solid-solid phase transitions of ZnO. Both the local-density and the generalized gradient approximations are employed together with the projector augmented wave potentials to mimic the electron-ion interaction. We consider the wurtzite (B4), rocksalt (B1), zinc blende (B3), CsCl (B2), NaTl (B32), WC (B${}_{h}$), BN (B${}_{k}$), NiAs (B8${}_{1}$), and AsTi (B${}_{i}$) modifications of ZnO. The calculated structural properties in the B4, B3, B1, and B2 phases are in excellent agreement with earlier ab initiopredictions, as is the transition pressure between them. We find that the B4 phase is the most preferred low-pressure candidate in ZnO while the B2 phase is favorable at high pressures. Apart from the previously reported $B4\ensuremath{\rightarrow}B1\ensuremath{\rightarrow}B2$ phase transition, our study reveals other possible paths for a transition from the B4 to the B2 phase.

First-principles study of structural stability, electronic properties and lattice thermal conductivity of KAgX (X = S, Se, Te)

Abstract: The present study is the first attempt towards establishing computational insights into the structural, electronic, mechanical, dynamical and thermal properties of the tetragonal phases of potassium chalcoargentates (KAgX). We find that the lattice thermal conductivity of KAgX is anisotropic, with values of 0.553 (0.279), 0.509 (0.369) and 0.221 (0.125) Wm−1K−1 at room temperature (300 K) along the a-axis (c-axis) for KAgS, KAgSe and KAgTe, respectively. The calculated values of the lattice thermal conductivity are very small, especially along the c-axis. This highlights the potential of using KAgX in designing thermoelectric materials, since low lattice thermal conductivity is a requisite for maximizing the dimensionless figure of merit which defines the efficiency of a system in converting thermal to electrical energy and vice versa.

Structural, stability and thermoelectric properties for the monoclinic phase of NaSbS2 and NaSbSe2: A theoretical investigation

Abstract: This study is the first attempt towards establishing computational insight into the structural, electronic, mechanical, dynamical and thermoelectric properties of the monoclinic phases of NaSbS2 and NaSbSe2. The mechanical properties are predicted using the Hill approximation. Dynamical stability was investigated by computing the phonon frequency to check for the absence of imaginary modes. Lattice thermal conductivity was calculated by using a single-mode relaxation-time approximation in the linearized phonon Boltzmann equation from first-principles an-harmonic lattice dynamics calculations. We found that the lattice thermal conductivity of NaSbS2 and NaSbSe2 are anisotropic, with values ranging between 0.753 and 1.173 Wm−1 K−1 at room temperature (300 K). The calculated values of the lattice thermal conductivity are small, especially along the x-axis. The charge transport properties are predicted using Boltzmann transport equations. The highest values attained for the figure of merit are high as 4.22 and 2.88 when the electron concentration is 1018 cm−3 at 600 K for NaSbS2 and NaSbSe2, respectively. This highlights the potential of using NaSbS2 and NaSbSe2 in designing thermoelectric materials since low lattice thermal conductivity and high figure of merit are a requisite for maximizing the efficiency of thermoelectric materials.

First-principles study of Ti50Ru50-xYx (Y = Ni, Pd and Pt) with a prospect to induce SME on stable CsCl-type Ti50Ru50 phase

Abstract: Most CsCl-type intermetallics composed of group IV and VIII–XI transition metals have shape memory effect (SME), a phenomenon that occurs on a certain class of materials with an ability to undergo martensitic transformation (MT) during cooling. This advanced functional materials’ property is enabled by MT from high-temperature B2 phase of high symmetry to lower symmetry phases such as L10, B19 or B19’ upon cooling. Peculiarly, Ti50Ru50 with similar ordered B2 at high temperature remains ordered and stable with no phase transition down to room temperature. In this study, first-principles calculations based on density functional theory (DFT) are used to investigate the structural, thermodynamic and electronic properties of the stable Ti50Ru50 compound by systematically substituting part of the Ru atoms with ductile group 10 metal (Ni, Pd and Pt). This is an attempt to destabilizing B2 phase at 0 K through Ti50Ru50-xYx ternary alloying to promote MT that could yield SME. Graphical Abstract

Elastic stability and electronic structure of pyrite type PtN2: A hard semiconductor

Abstract: The elastic properties and electronic structure of PtN2 with the pyrite structure (PtN2(C2)) were studied with first-principles calculations. The crystal structure is demonstrated to be elastically stable with a lower energy than the metastable fluorite structure proposed before. The calculated shear modulus of 214 GPa suggests that PtN2(C2) is harder than some well known hard materials such as TiN and SiC. The high elastic moduli are attributed to a stacking of corner-shared PtN6 octahedra bonded by strong N-N covalent bonding. In contrast to the metallic fluorite-type phase, PtN2(C2) is semiconducting with an indirect band gap.

KAgX (X = S, Se): High-Performance Layered Thermoelectric Materials for Medium-Temperature Applications.

Abstract: Monolayer KAgX are a class of novel two-dimensional (2D) layered materials with efficient optical absorption and superior carrier mobility, signifying their potential application prospect in photovoltaic (PV) and thermoelectric (TE) fields. Motivated by the recent theoretical studies on the KAgX monolayer, we carried out systematic investigations on the TE performance of KAgS and KAgSe monolayers, employing density functional theory (DFT) and semiclassical Boltzmann transport equation (BTE). For both KAgSe and KAgS monolayers, large Gruneisen parameters, low group velocities, and short phonon scattering time greatly hinder their heat transport and result in an ultralow thermal conductivity, 0.26 and 0.33 W m-1 K-1 at 300 K, respectively. A twofold degeneracy appearing at the Γ point and the abrupt slope of the density of states (DOS) near the Fermi level give rise to high Seebeck coefficients of KAgX monolayers. Due to the ultralow thermal conductivity and excellent electronic transport performance, the ZT values as high as 4.65 (3.11) and 4.05 (2.63) at 500 (300) K in the n-type doping for KAgSe and KAgS monolayers are obtained. The exceptional performance of KAgX monolayers sheds light on their immense potential applications in the medium-temperature (around 300-500 K) thermoelectric devices and greatly stimulates further experimental synthesis and validation.

Polymorphic energy ordering of MgO, ZnO, GaN, and MnO within the random phase approximation

Abstract: Accurate relative energetic stabilities between the tetrahedrally coordinated (zinc-blende or wurtzite) and octahedrally coordinated (rock-salt) phases of MgO, ZnO, GaN, and MnO are obtained by first-principles calculations within the framework of adiabatic connection fluctuation-dissipation theorem (ACFDT) and with the random phase approximation (RPA) to the correlation energy. The RPA-ACFDT correctly recovers the rock-salt structure of MnO as the ground-state phase, as observed experimentally, whereas previous density and hybrid functional methods obtained the wrong energy ordering. Even though standard density functionals give the correct ordering of the non-transition-metal compounds, significant quantitative changes occur also for MgO and ZnO. We conclude that the RPA can serve as an important benchmark for structural preferences in polymorphic materials. The present study suggests that density functional predictions for open $d$-shell materials such as transition metal compounds might be more prone to erroneous structure prediction than commonly expected.

Excited state calculations in solids by auxiliary-field quantum Monte Carlo

Abstract: We present an approach for ab initio many-body calculations of excited states in solids. Using auxiliary-field quantum Monte Carlo, we introduce an orthogonalization constraint with virtual orbitals to prevent collapse of the stochastic Slater determinants in the imaginary-time propagation. Trial wave functions from density-functional calculations are used for the constraints. Detailed band structures can be calculated. Results for standard semiconductors are in good agreement with experiments; comparisons are also made with GW calculations and the connections and differences are discussed. For the challenging ZnO wurtzite structure, we obtain a fundamental band gap of 3.26(16) eV, consistent with experiments.

From the ZnO Hollow Cage Clusters to ZnO Nanoporous Phases: A First-Principles Bottom-Up Prediction

Abstract: A family of ZnkOk (k = 12, 16) cluster-assembled solid phases with novel structures and properties has been characterized utilizing a bottom-up approach with density functional calculations. Geometries, stabilities, equation of states, phase transitions, and electronic properties of these ZnO polymorphs have been systematically investigated. First-principles molecular dynamics (FPMD) study of the two selected building blocks, Zn12O12 and Zn16O16, with hollow cage structure and large HOMO–LUMO gap shows that both of them are thermodynamically stable enough to survive up to at least 500 K. Via the coalescence of building blocks, we find that the Zn12O12 cages are able to form eight stable phases by four types of Zn12O12–Zn12O12 interactions, and the Zn16O16 cages can bind into three phases by the Zn16O16–Zn16O16 links of H′, C′, and S′. Among these phases, six ones are reported for the first time. This has greatly extended the family of ZnO nanoporous phases. Notably, some of these phases are even more stab...