arXiv subjects

D. Murali

Publications and source records attributed to D. Murali.

7 recordsLinked to original sources

arXiv2025

Thermoelectric properties of SbXY (X = Se, Te; Y = Br, I) Janus layers

We report a comprehensive investigation of the thermoelectric properties of SbXY (X = Se, Te; Y = Br, I) Janus layers (JL) using spin-polarized first-principles calculations. Ab initio molecular dynamics confirm that the 1T phase ($Pm31$) remains stable up to 1000 K, excluding any phase transitions. The calculated mean-square displacement further evidences the structural robustness. The thermal conductivity is strongly suppressed in Br-containing layers due to enhanced Froehlich interactions between optical and acoustic phonons. Electronic structure calculations reveal indirect band gaps of 1.1 to 1.3 eV, with valence and conduction bands dominated by the $p$-orbitals of halogen/chalcogen and of Sb, respectively. The carrier effective mass highlights anisotropic transport with lighter electrons being more mobile, while holes dominate the power factor, which attains values on the order of mW/mK$^2$. Direction-dependent transport indicates superior thermoelectric performance along the $xx$ direction, with negligible contribution along $yy$. The Figure of Merit reaches 0.6 at 1000 K in hole-doped SbSeBr, demonstrating strong potential for high-temperature applications. Our results reveal that the SbXY JLs, particularly SbSeBr, emerge as promising candidates for next-generation thermoelectric devices at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2025

Prediction of atomic H adsorption energies in metalloid doped MSSe (M = Mo/W) Janus layers: A combined DFT and machine learning study

Janus derivatives of 2H MX2 (M = Mo/W; X = S/Se), namely MSSe, have already been experimentally realized and explored for applications in photocatalysis, photovoltaics, and optoelectronics. Focusing on the photocatalytic properties of these layers, we investigate the adsorption of atomic hydrogen on the MSSe layers in the presence of metalloid dopants B, Si, and Ge. The layers in their pristine form exhibit positive adsorption energies, indicating an endothermic nature. Substitution of a dopant in the pristine MSSe layers alters the local symmetry, bonding character, and charge distribution, thereby increasing the number of active sites for hosting H adsorption and reducing the adsorption energy. We select distinct sites, both atomic and interstitial, for the substitution of dopants. The energetics of the H atom at various sites is studied to find the most favorable active site on the MSSe Janus layers. Our results based on density functional theory calculations show that the adsorption process becomes spontaneous and less attractive in the presence of atomic site dopant substitution, whereas the interstitial site results in an endothermic behavior. Moreover, having the data from DFT, we develop a supervised machine learning model for predicting the hydrogen adsorption energy. For this purpose, we utilize 23 elemental features of the atoms involved in the structure, thereby eliminating the need for DFT calculations in feature design. The dimensionality reduction technique, principal component analysis, is employed to reduce the dimensionality of the feature space, yielding independent features that are mutually orthogonal. The model is implemented as a multi-layer perceptron regressor with two hidden layers. The data augmentation technique is employed to artificially expand the dataset size, thereby enhancing the accuracy of the neural network model by 0.90% on the testing data.

cond-mat.mtrl-sci↗

arXiv2021

Bulk photovoltaic effect in BaTiO$_3$-based ferroelectric oxides: An experimental and theoretical study

The bulk photovoltaic effect exhibited by the non-centrosymmetric system gains research interest due to the observed large open-circuit voltage. The ferroelectric systems exhibiting anomalous photovoltaic effect are mostly crystallized with multiphase coexistence. Hence, the computational difficulty in building a multi-phase system restricts the detailed photovoltaic studies through phenomenological and shift current theory. In this work, ferroelectric Ba$_{1- x}$(Bi$_{0.5}$K$_{0.5}$)$_x$TiO$_3$ (BBKT) oxide is designed to crystallize in single-phase tetragonal symmetry with improved polarization characteristics, and it is found to exhibits large PV response. Both experimental and theoretical studies on BBKT samples reveal ~18% reduction in bandgap compared to the parent BaTiO$_3$. Short-circuit current measured as a function of light intensity and light polarization angle reveal linear and sinusoidal response, respectively. The observed features are in accordance with phenomenological theory. Remarkably, x = 0.125 sample displays ~8 times higher open-circuit voltage (7.39 V) than the parent compound. The enhanced PV effect is attributed to the large shift current along z-direction as evidenced from the additional charge-center shift of valence band occupied by O-2p orbital and conduction band occupied by Bi-6p orbital. Notably, the degenerate Bi-p$_z$ state at conduction band minimum in BBKT favours the large shift current response in the z-direction.

cond-mat.mtrl-sci↗

arXiv2017

Importance of phonon contribution to the free migration energy of the O vacancy in BaZrO3

BaZrO3 exhibits excellent proton conductivity and good high-temperature stability. It is therefore a promising electrolyte material for solid oxide fuel cells. The stability of BaZrO3 at high temperatures is generally explained by the low diffusivity of the O vacancy. Present first-principle Density-Functional-Theory calculations show that the slow migration of the doubly positively charged O vacancy at high temperature cannot be solely caused by the ground-state migration energy but by the contribution of phonon excitations to the free migration energy. With increasing temperature, the effective barrier for oxygen vacancy migration increases. At about 1000K, which is the operating temperature of fuel cells, the calculated O vacancy diffusivity is more than one order of magnitude lower than that determined using solely the ground-state migration barrier. The calculated diffusivity data agree well with experimental results from literature. The present work reveals that the high-temperature stability of BaZrO3 is mainly due to the phonon contribution to the free migration energy of the O vacancy.

cond-mat.mtrl-sci↗

arXiv2016

Influence of phonon and electron excitations on the free energy of defect clusters in solids: A first-principles study

Although many processes of nanostructure evolution in solids occur at elevated temperatures, basic data obtained from ground state energetics are used in the modeling of these phenomena. In order to illustrate the effect of phonon and electron excitations on the free binding energy of defect clusters, first-principles calculations are performed for vacancy-solute pairs as well as vacancy and Cu dimers, trimers, and quadromers in bcc Fe. Based on the equilibrium atomic positions determined by the relaxation of the supercell with the defect in the ground state under constant volume (CV) as well as zero pressure (ZP) conditions, the contribution of phonon excitations to the free binding energy is calculated within the framework of the harmonic approximation. The contribution of electron excitations is obtained using the corresponding ground state data for the electronic density of states. Quasi-harmonic corrections to the ZP-based results do not yield significant changes in the temperature range relevant for applications. At 1000 K the maximum decrease/increase of the ZP-based data for the absolute value of the free binding energy with respect to the corresponding ground state value is found for the vacancy-W (43%) / vacancy-Mn (35%) pair. These results clearly demonstrate that contributions of phonon and electron excitation to the free binding energy of the defect clusters are generally not negligible. The general behavior of the free binding energy of vacancy and Cu dimers, trimers and quadromers is similar to that of the vacancy-solute pairs. The results obtained in this work are of general importance for studies on the thermodynamics and kinetics of defect clusters in solids.

cond-mat.mtrl-sci↗

arXiv2015

First-principles calculation of defect free energies: General aspects illustrated in the case of bcc-Fe

The first-principles calculation of contributions of phonon and electron excitations to free formation, binding, and migration energies of defects is illustrated in the case of bcc-Fe. First of all, the ground state properties of the vacancy, the foreign atoms Cu, Y, Ti, Cr, Mn, Ni, V, Mo, Si, Al, Co, O, and the O-vacancy pair are determined under constant volume (CV) as well as zero pressure (ZP) conditions. Second, the phonon contribution to defect free energies is calculated within the harmonic approximation using the equilibrium atomic positions determined in the ground state under CV and ZP conditions. Additionally, quasi-harmonic corrections are applied to the ZP-based data. A simple transformation similar to the quasi-harmonic approach is found between the CV- and ZP-based frequencies. Therefore, it is not necessary to calculate these quantities and the corresponding defect free energies separately. In contrast to ground state energetics the CV- and ZP-based defect free energies do not become equal with increasing supercell size. Third, it was found that the contribution of electron excitations to the defect free energy can lead to an additional deviation of the total free energy from the ground state value or can compensate the deviation caused by the phonon contribution. Finally, self-diffusion via the vacancy mechanism is investigated. The ratio of the respective CV- and ZP-based results for the vacancy diffusivity is nearly equal to the reciprocal of that for the equilibrium concentration. This behaviour leads to almost identical CV- and ZP-based values for the self-diffusion coefficient. Obviously, this agreement is accidental. The consideration of the temperature dependence of the magnetization yields self-diffusion data in very good agreement with experiments.

cond-mat.mtrl-sci↗

arXiv2014

Energetics, structure, and composition of nanoclusters in Oxide Dispersion Strengthened Fe-Cr alloys

Extensive first-principle calculations on embedded clusters containing few O, Y, Ti, and Cr atoms as well as vacancies are performed to obtain interaction parameters to be applied in Metropolis Monte Carlo simulations, within the framework of a rigid lattice model. A novel description using both pair and triple parameters is shown to be more precise than the commonly used pair parameterization. Simulated annealing provides comprehensive data on the energetics, structure and stoichiometry of nm-size clusters at T=0. The results are fully consistent with the experimental finding of negligible coarsening and a high dispersion of the clusters, with the observation that the presence of Ti reduces the cluster size, and with the reported radiation tolerance of the clusters. In alloys without vacancies clusters show a planar structure, whereas the presence of vacancies leads to three-dimensional configurations. Additionally, Metropolis Monte Carlo simulations are carried out at high temperature in order to investigate the dependence of nanocluster composition on temperature. A good agreement between the existing experimental data on the ratios (Y+Ti):O, Y:Ti, (Y+Cr):O, and Y:Cr, and the simulation results is found. In some cases it is even possible to draw the conclusion that the respective alloys contained a certain amount of vacancies, and that the clusters analyzed were frozen-in high-temperature configurations. The comparison of experimental data with those obtained by simulations demonstrates that the assumption of nanoclusters consisting of nonstoichiometric oxides which are essentially coherent with the bcc lattice of the Fe-Cr matrix leads to reasonable results.

cond-mat.mtrl-sci↗