Determination of the Raman Spectra of Molten ZnCl2 and Thermophysical Properties of Polymerized C60 Solids Using Atomistic Computational Techniques

Abstract

Recent advances in high performance computing technology have enabled the implementation of computational materials science algorithms to examine the structure-property relations of a wide variety of materials. Fundamental insights gained from these studies can thus provide guidelines for the appropriate selection of materials (or combination of materials) for targeted engineering and technological applications. In this regard, this dissertation primarily focuses on using atomistic computational techniques for characterizing the vibrational properties of two distinct material-systems, namely molten ZnCl_2 and polymerized C_60 solids, and thus gain new knowledge on their structure and thermophysical properties respectively. In the first study, ab initio methods were used to interpret the experimentally determined Raman spectrum of molten ZnCl_2, thereby providing never before available insights on the short range structural ordering in ZnCl_2, melts. In particular, using ab initio molecular dynamics (AIMD) the different structural motifs present in molten ZnCl_2 were determined at 600K. The accuracy of the result was confirmed by the good agreement between the structural factors and radial distribution functions as determined from AIMD and neutron diffraction (ND) experiments. To study the Raman signatures of the ZnCl_2 melt structures, appropriate solid-state ZnCl_2 prototypes were chosen to represent the different observed structural motifs. The respective Raman signatures of the different prototypes were calculated from density functional perturbation theory (DFPT) within the density functional theory (DFT) framework. The identification of the respective Raman signatures provided the ability to accurately deconvolute the experimentally determined Raman spectrum at 600K as well as identify the relative population of the different short range ordering structures. The findings of this study has implications for optimizing the composition and operating temperatures of ZnCl_2 based salts (e.g. KCl-NaCl-ZnCl_2-AlCl_3) for utilization as thermal storage fluids in concentrating solar power plants. In the second study, classical molecular dynamics (MD) was used to compare and contrast phonon-driven thermophysical properties of polymerized and unpolymerized solid state polymorphs of C_60. To achieve this, a newly parameterized interatomic potential was developed for accurately modeling the short range as well as long range interactions in the C_60 solids. Using this potential, it was unambiguously shown that polymerized C_60 polymorphs exhibit a two order of magnitude enhancement in the thermal conductivity and an order of magnitude change in the elastic stiffness. The significant increase in the thermal conductivity was correlated to the presence of new THz thermal phonon modes, characterized by larger mean free paths. In addition, it was also seen that the Debye temperature of the C_60 structures was strongly dependent on the extent of polymerization. The new understanding obtained in this work provides valuable guidelines for the design and development of novel C_60 based phononic metamaterials for applications as vibrational and thermal management systems.