Wu, Xifan; Ruzsinszky, Adrienn; Yan, Qimin; Carnevale, Vincenzo (Temple University. Libraries, 2018)
      This thesis discusses structural and ferroelectric properties of two well-known classes of materials, perovskite oxides and Hydrogen bonded ferroelectrics, using first-principles calculations. Certain aspects of first principles calculations are central to the problems presented in this thesis. Such as the ability to calculate polarization based on the modern theory of polarization and calculation of ferroelectric property under finite electric displacement field. Therefore, these fundamental theoretical approaches are discussed following an opening section on the basic methodology of density-functional theory. In addition to the discussion on theoretical methods, a brief review of different phenomena and techniques crucial to alter/enhance ferroelectric properties at the interfaces of perovskite materials has been presented along with examples. The first problem presented in this thesis proposes and validates an alternative quantitative measure of ferroelectric(FE) and antiferrodistortive(AFD) instabilities by means of calculating inverse capacitance and layer inverse capacitance of layered perovskites. The presented methodological approach is applied to BaTiO$_{3}$/CaTiO$_{3}$ and PbTiO$_{3}$/SrTiO$_{3}$ superlattices and it precisely estimates FE and AFD instabilities. Here we also present an approach to accurately predict the ferroelectric instabilities in large period superlattices from the statistical coefficients obtained from short period superlattices. In the second problem, we study ferroelectricity in an organic crystal(croconic acid) for which ferroelectric polarization is close to that of bulk BaTiO$_{3}$. We employ new meta-GGA functional named SCAN and revisit all structural and ferroelectric properties. Calculated X-ray absorption spectra(XAS) qualitatively and quantitatively agrees well with experimental O K-edge spectra. By discussing the origin of each XAS peak and their characteristic we demonstrate with a systematic approach the connection between ferroelectricity and XAS in croconic acid. Best to our knowledge such relation has not been realized in past. This study could prove XAS as a new way to measure ferroelectric instability in hydrogen-bonded organic ferroelectrics.
    • Properties of Liquid Water and Solvated Ions Based on First Principles Calculations

      Wu, Xifan; Riseborough, Peter; Ruzsinszky, Adrienn (Temple University. Libraries, 2018)
      Water is of essential importance for life on earth, yet the physics concerning its various anomalous properties has not been fully illuminated. This thesis is dedicated to the understanding of liquid water from aspects of microscopic structures, dynamics, electronic structures, X-ray absorption spectra, and proton transfer mechanism. This thesis use the computational simulation techniques including density functional theory (DFT), ab initio molecular dynamics (AIMD), and theoretical models for X-ray absorption spectra (XAS) to investigate the dynamics and electronic structures of liquid water system. The topics investigated in this thesis include a comprehensive evaluation on the simulation of liquid water using the newly developed SCAN meta-GGA functional, a systematic modeling of the liquid-water XAS using advanced ab initio approaches, and an explanation for a long-puzzling question that why hydronium diffuses faster than hydroxide in liquid water. Overall, significant contributions have been made to the understanding of liquid water and ionic solutions in the microscopic level through the aid of ab initio computational modeling.