# Computational Materials Engineering

# Introduction

Hello there,

Welcome to our group website.

My name is Appala Naidu Gandi. I am working as an Assistant Professor in the Department of Metallurgical and Materials Engineering. Our group works in the area of computational Materials Engineering. Mostly, we use first-principles calculations to obtain system specific information. Using the structural information of a system as input the SchrĂ¶dinger equation is solved for a given configuration of atoms in the first-principles calculations. As output, system specific information such as total energy, electronic band structure, density of states, force constants, etc. is obtained. Using this information, we try to understand the mechanical, transport, and dynamical properties of materials. Presently, we are working in the following areas: (a) thermoelectric transport, (b) lattice dynamics, (c) mechanical behavior, and (d) structural characterization.

# Thermoelectric transport

Thermoelectric materials are used for electricity-generation from waste heat and for low-temperature solid-state refrigeration. Figure of merit is used to measure efficiency of a given thermoelectric material. At a given temperature, three fundamental quantities determine figure of merit of a material: Seebeck coefficient, electrical conductivity, and thermal conductivity. Electronic band structure determines the Seebeck coefficient, electrical conductivity, and part of the thermal conductivity, while lattice vibrations determine the rest of the thermal conductivity. We study these fundamental properties of various thermoelectric materials with an objective to estimate the figure of merit and to suggest ways to further improve the figure of merit.

With the electronic bandstructure as input, we solve the Boltzmann transport equation for electrons and estimate the electronic contribution to the figure of merit using BoltzTrap. We study the dynamical stability of structures by calculating the phonon dispersion relations. After confirming the dynamical stability, we use the force constants as input, and solve the Boltzmann transport equation for phonons to estimate the lattice contribution to the thermal conductivity using ShengBTE. By combining the electronic and lattice contributions to the figure of merit, we estimate the thermoelectric efficiency of materials.

# Lattice dynamics

Atoms vibrate about their mean positions in the structure due to thermal energy. These vibrations are responsible for many dynamical properties. We calculate the harmonic phonon dispersion from the force constants and estimate the vibrational contributions to the thermodynamical properties.

In phase transitions induced by a soft phonon, there is a definite structural relationship between the parent phase and the product phase. We study such relations using group theoretical methods.

# Mechanical behavior

Elastic stiffness tensor, stacking fault energy, and surface energy are three fundamental quantities of a material in determining its mechanical behaviour. By calculating the energy of a strained crystal as a function of strain tensor and strain magnitude, we calculate the stiffness tensor. We evaluate the surface energy by calculating the variation of system energy as a function of separation distances between two slabs. We estimate the stacking fault energies by comparing the energies of a perfect crystal and one with a stacking fault. From the results, we try to understand mechanical behavior of materials.

# Structural characterization

We also use different ab initio characterisation technique to support experimental studies. By analyzing the normal modes at the zone center, we provide the frequencies and wave motion for the Infra Red active and Raman active phonons. From the calculated charge density for a given configuration of atoms, we simulate the Scanning Tunnelling Microscopic images.