Understanding Phonon Transport in Extended Solids Using First Principles Calculations

Abstract

Materials with an extreme lattice thermal conductivity (L) are indispensable for thermal energy management applications. Therefore, microscopic understanding of phonon transport is critically important for designing functional materials. In the present thesis, a systematic investigation has been made for in-depth understanding of phonon transport in binary and ternary compounds using first principles calculations in combination with Boltzmann transport theory. In contrast to the expected trend based on their atomic mass, anomalous trends for L are observed in binary systems, namely Alkaline-earth chalcogenides and Alkali halides. It has been shown how atomic mass contrast can tune the contribution of optical phonons to L and its implications on scattering rates either enhancing or suppressing L. Ternary alkaline-earth halofluorides and Bismuth halooxides provide an avenue for designing functional materials with low L due to their intrinsic bonding heterogeneity. Investigation of iso-structural layered materials with varying average atomic mass is intriguing because they allow to make structure-property correlations by exploring the interplay between bonding heterogeneity and atomic mass and their implications on lattice dynamics thereby tailoring the phonon transport properties. Overall, the present thesis focused on understanding interplay amongst crystal structure, atomic mass, chemical bonding, mechanical properties, lone pair activity, and their role in phonon transport properties, which would aid in designing extremely low L materials. This is indispensable for the development of sustainable energy conversion devices for future thermal energy management applications. The Thesis consists of six chapters, the finer details are provided below. Chapter 1: It provides an introduction to the domain of phonon transport in extended solids and why low L plays a crucial role in thermal management applications. It also provides a comprehensive view of the various mechanisms affecting the phonon transport, both extrinsic and intrinsic with a comprehensive literature survey of the mechanisms to lower L. Chapter 2: This chapter provides the theoretical background for the current work and the computational methodology utilized for this work. iii The Density Functional Theory (DFT) formalism and an overview of first principles calculations are discussed. Both harmonic and anharmonic approximations concerning the phonons and phonon transport has been discussed followed by the Boltzmann transport theory for obtaining L, phonon-phonon scattering mechanism has been discussed, specifically three phonon scattering, as the same has been considered in the current work. The methodology employed in the present study, known as Temperature Dependent Effective Potential (TDEP), is elaborated upon, this is followed by an overview of list of packages utilized for the current work. Chapter 3: The first part of the chapter focuses on a detailed and comparative study on phonon transport of Alkaline Earth Chalcogenides (AEC’s) MCh (M = Mg, Ca, Sr, Ba and Ch = O, S, Se,Te) compounds in order to provide insights to achieve low L materials through phonon engineering. More light is shed on understanding lattice dynamics, phonon transport, and mechanical properties of 16 MCh (M = Mg, Ca, Sr, Ba and Ch = O, S, Se, Te) compounds. The second part of this chapter deals with another set of isostructural binary systems, Alkali Halides (AH’s), consisting of 20 MX ( M = Li, Na, K, Rb, Cs and X = F, Cl, Br, I) compounds and presented in comparison with the results obtained with AEC’s. This chapter provides an in-depth understanding of atomic mass and its effect on phonon transport properties of AH’s and AEC’s. Furthermore, this reveals that by manipulating the atomic masses, one can engineer materials with both high and low values of L, providing exciting possibilities for tailored thermal conductivity in various applications. Chapter 4: This chapter explores layered materials which are bonded through strong covalent/ionic bonds within the plane (in-plane) and coupled by weak van der Waals (vdW) interactions in the perpendicular (out-of-plane) direction i.e., bonding heterogeneity, thus resulting in a strong structural anisotropy. Therefore, through bonding heterogeneity, these layered materials provide an avenue for tailoring phonon transport properties. Investigation of iso-structural layered materials with varying average atomic mass is intriguing because they allow structure-property correlations by exploring the inter-play between bonding heterogeneity and atomic mass and their implications on lattice dynamics, thereby fine-tuning the phonon transport properties. Consequently, for layered materials, a microscopic understanding of crystal structure, iv bonding, anharmonic lattice dynamics, and phonon transport properties is of the utmost importance. Alkaline-earth halofluorides, MXF (M = Ca, Sr, Ba and X = Cl, Br, I) belong to the class of matlockite (PbClF)-type layered materials and they provide an avenue to ex plore the interplay between crystal structure, atomic mass, and bonding heterogeneity and thereby to fine tune their phonon transport properties. The outcomes of the chapter are that structural anisotropy and/or bonding plays a crucial role along with atomic mass in determining the L in these iso-structural MXF compounds. This study on MXF compounds provides an in-depth understanding on interplay among crystal structure, atomic mass and bonding heterogeneity, which would aid in designing extreme L materials by manipulating in-plane and out-of-plane bonding for future thermal energy management applications. Chapter 5: This chapter explores another family of layered materials known as Bismuth halooxides, BiXO (X= Cl, Br, I). BiXO is composed of a series of ionically bonded X-Bi-O-O-Bi-X layers stacked perpendicular to the c-axis. These layers are held together by weak van der Waals (vdW) interactions; consequently, these compounds exhibit bonding heterogeneity, featuring in-plane ionic bonding and out-of plane weak vdW bonding. The rattling mechanism owing to bonding heterogeneity that results in an ultralow L has been described. Chapter 6: This chapter summarizes the results that are obtained and consolidates the same by proposing few design principles for obtaining low L materials followed by the future scope of work.