Paper & Patents

Decreasing the thermal conductivity of a thermoelectric material is always a prerequisite for its potential application. Using first-principle calculations, we examine the magnetism-induced change in lattice thermal transport in bismuth telluride. The source of magnetic moment, Cr in the doped system, weakly magnetizes the coordinated Te atoms to make the latter’s phonon softer than that in the pure compound. Although the transition metal dopants do not participate directly in the heat conduction process, the anharmonicity induced by them favors reducing the lattice thermal conductivity (κph). Large anharmonicity in thermodynamically stable (Bi0.67Cr0.33)2Te3 reduces the in-plane room temperature κph by approximately 79%. However, the latter is found to lie above Cahill’s limit (κmin) of the material which is an indication of the ability to achieve such a significant reduction of κph. The thermal conductivity, strictly, does not vary monotonically with doping concentration. For any particular doping level, the thermal conductivity is different for different configurations which is related to the internal energy of the system. We find that the internal energy variance of 0.03 eV would reduce the in-plane thermal conductivity of the room temperature lattice by at least 60% for 50% doping.

We systematically investigated the structural phase transition of Li[Ni1/3Co1/3Mn1/3]O2 (NCM) athode material depending on the state of charge (SOC) using cluster  expansion Monte Carlo simulation  (CE-MCS) combined with density functional theory (DFT). Considering the charging/discharging process involving lithium tercalation and deintercalation, the oxidation state of transition metal (TM) varies with SOC, resulting in a TM arrangement shift to a thermodynamically favorable NCM structure. Our results demonstrate that the phase transition from disordered to ordered happens at low SOC condition with high TM oxidation state, and that the phase transition is initiated by TM pop-up from the TM layer to the Li layer. Ni migration plays an especially fundamental role in the phase transition with diffusion energy barrier comparable to that of Li ion. Furthermore, based on a thorough understanding of the structural phase transition, we propose cation dopants (Zr, Ti and V) which inhibit the Ni pop-up as an initiating step of the phase transition by enhancing the chemical bonding of Ni ions in the NCM structure, thereby preventing the phase transition from causing undesirable structural degradation and severe capacity fading in NCM. Our theoretical investigations will provide insights into the structural phase transition mechanism and the design of new cathode materials for lithium ion batteries (LIBs).

  The number of published articles in the field of materials science is growing rapidly every year. This comparatively unstructured data source, which contains a large amount of information, has a restriction on its re-usability, as the information needed to carry out further calculations using the data in it must be extracted manually. It is very important to obtain valid and contextually correct information from the online (offline) data, as it can be useful not only to generate inputs for further calculations, but also to incorporate them into a querying framework. Retaining this context as a priority, we have developed an automated tool, MatScIE (Material Scince Information Extractor) that can extract relevant information from material science literature and make a structured database that is much easier to use for material simulations. Specifically, we extract the material details, methods, code, parameters, and structure from the various research articles. Finally, we created a web application where users can upload published articles and view/download the information obtained from this tool and can create their own databases for their personal uses.

High electron mobility transistors (HEMT) built using In\textsubscript{0.52}Al\textsubscript{0.48}As/In\textsubscript{0.53}Ga\textsubscript{0.47}As on InP substrates are a focus of considerable experimental studies due to their favourable performance for microwave, optical and digital applications. We present a detailed and comprehensive study of steady state and transient electronic transport in In\textsubscript{0.52}Al\textsubscript{0.48}As with the three valley model using the semi-classical ensemble Monte Carlo method and including all important scattering mechanisms. All electronic transport parameters such drift velocity, valley occupation, average electron energy, ionization coefficient and generation rate, electron effective mass, diffusion coefficient, energy and momentum relaxation time are extracted rigorously from the simulations. Using these, we present a complete characterization of the transient electronic transport showing the variation of drift velocity with distance and time. We have then estimated the optimal cut-off frequencies for various device lengths via the velocity overshoot effect. Our analysis shows that for device lengths shorter than 700 nm, transient effects are significant and should be taken into account for optimal device designs. As a critical example, at length scales of around 100 nm, we obtain a significant improvement in the cut-off frequency from 261 GHz to 663 GHz with the inclusion of transient effects. The field dependence of all extracted parameters here can prove to be helpful for further device analysis and design.

 We formulate Wannier orbital overlap population and Wannier orbital Hamilton population to describe the contribution of different orbitals to electron distribution and their interactions. These methods, which are analogous to the well known crystal orbital overlap population and crystal orbital Hamilton population, provide insight into the distribution of electrons at various atom centres and their bonding nature. We apply this formalism in the context of a plane-wave density functional theory calculation. This method provides a means to connect the non-local plane-wave basis to a localised basis by projecting the wave functions from a plane-wave density functional theory calculation on to localized Wannier orbital basis. The main advantage of this formulation is that the spilling factor is strictly zero for insulators and can systematically be made small for metals. We use our proposed method to study and obtain bonding and electron localization insights in five different materials.

 Abstract Incessantly increasing capabilities of Density Functional Theory(DFT) to interpret and predict materials properties have established it as an important method in the domain of materials simulations. This naturally attracts several users beyond conventional computational theorists. In this work, we introduce Comprehensively INtregrated Environment for advanced MAterials Simulations (CINEMAS) [ www.ikst.res.in/cinemas]. CINEMAS is a completely graphical user interface(GUI) based simulation platform to facilitate DFT calculations and more, for both nonexpert and expert users. At the same time, different types of GUI enabled automation provides a platform to execute a large number of data-driven computations. The main workhorse of CINEMAS is 'workflows' which is a graphical representation of a single or a sequence of dependent and independent calculations. These workflows lead to complete preparation and execution of simulations from the beginning to end, providing handles for multiple analysis, and finally, prepare graphics meeting publication standards. External Python-based packages can be used with these workflows in CINEMAS. This workflow representation can trace back the calculations along a sequence, gives an overview of the whole project from a single point, and makes data sharing among collaborators very easy. We discuss the concept and features of CINEMAS, and how several tools required at different stages of a calculation are seamlessly integrated through its rich and easy GUI. We discuss how CINEMAS is self-sufficient to carry out calculations using a DFT code and more without requiring any other software application to prepare, execute, and analyze the simulations.Abstract Incessantly increasing capabilities of Density Functional Theory(DFT) to interpret and predict materials properties have established it as an important method in the domain of materials simulations. This naturally attracts several users beyond conventional computational theorists. In this work, we introduce Comprehensively INtregrated Environment for advanced MAterials Simulations (CINEMAS) [ www.ikst.res.in/cinemas]. CINEMAS is a completely graphical user interface(GUI) based simulation platform to facilitate DFT calculations and more, for both nonexpert and expert users. At the same time, different types of GUI enabled automation provides a platform to execute a large number of data-driven computations. The main workhorse of CINEMAS is 'workflows' which is a graphical representation of a single or a sequence of dependent and independent calculations. These workflows lead to complete preparation and execution of simulations from the beginning to end, providing handles for multiple analysis, and finally, prepare graphics meeting publication standards. External Python-based packages can be used with these workflows in CINEMAS. This workflow representation can trace back the calculations along a sequence, gives an overview of the whole project from a single point, and makes data sharing among collaborators very easy. We discuss the concept and features of CINEMAS, and how several tools required at different stages of a calculation are seamlessly integrated through its rich and easy GUI. We discuss how CINEMAS is self-sufficient to carry out calculations using a DFT code and more without requiring any other software application to prepare, execute, and analyze the simulations.

The solid solution of Be Mg1-O is examined as a candidate for high-k dielectric materials by considering the dielectric constant, bandgap, and phase stability at the same time. Using ab initio calculations including phonon calculations, the subtle interrelation between atomic structure and electrical properties is elucidated. Due to the different stable phases between Beo (wurtzite structure) and MgO (rock salt structure), Be and Mg atoms have a distinctive preference on the site occupation, leading to various unexpected configurations. Notably, the instability of Be atoms located at the octahedral sites in the rock salt structure Be M91-O X < 0.5) triggers the movement of Be atoms toward the tetrahedral-like sites. It results in the modified rock salt structure Be Mg1-O: shortened Be-O bonds at the tetrahedral-like sites in the rock salt structure composed of octahedral Mg-O bonds. The modified rock salt structure Be Mg1-O has a high bandgap over 7.3 eV irrespective of the composition and atomic configuration. In contrast, the energetic stability and dielectric constant highly depend on the atomic configuration, where a configuration with longer apical Be-O bond length tends to show lower energetic stability and higher dielectric constant. From this key finding, superlattice structures in Beo25Mg0.750 are proposed as a suitable high-k material providing the opportunities to systematically control the dielectric constant by the design of the atomic arrangement. Further examination reveals that the proposed superlattice structures are stable, and their high-k values slightly increase as temperature increases.

 Chemisorption on ferromagnetic and non-magnetic surfaces is discussed within the Newns–Anderson–Grimley model along with the Stoner model of ferromagnetism. In the case of ferromagnetic surfaces, the adsorption energy is formulated in terms of the change in surface magnetic moments. Using such a formulation, we address the issue of how an adsorbate's binding strength depends on the magnetic moments of the surface and how the adsorption process reduces/enhances the magnetic moments of the surface. Our results indicate a possible scaling relationship of adsorption energy in terms of surface magnetic moments. In the case of non-magnetic surfaces, we formulate a modified Stoner criterion and discuss the condition for the appearance of magnetism due to chemisorption on an otherwise non-magnetic surface.

Structural studies of FeP powder is done using X-ray diffraction data. In addition to the pure hexagonal phase, we noticed the existence of an additional phase in pristine Fe,P powder. The additional phase is found to be Fe, P-type tetragonal phase as confirmed by two phases Rietveld analysis of XRD data of ball milled powder. The heat treatment method used to get homogeneous FeP phase which is hexagonal. The current findings are important as they can affect the physical properties such as magnetoelastic coupling and magnetocaloric effect.

We present a module to calculate the mobility and conductivity of semiconducting materials using Rode's algorithm. This module uses a variety of electronic structure inputs derived from the Density Functional Theory (DFT). We have demonstrated good agreement with experimental results for the case of Cadmium Sulfide (Cds). We also provide a comparison with the widely used method, the so-called relaxation time approximation (RTA) and demonstrated a favorable improvement of the results compared to RTA. The present version of the module is interfaced with the Vienna ab initio simulation package (VASP).