First-principles study on control of optical properties with defect and doping in $Bi_{1-x}Ca_xFeO_{3-\delta}$

Abstract

In this dissertation, we study the defect and doping characteristics of bismuth ferrite ($BiFeO_3$) and calcium ferrite ($CaFeO_{3-x}$) using first-principles calculations. In addition, we analyze adjustable optical properties and propose microscopic electrochromic mechanisms in these materials. The stabilities and electronic structures of oxygen vacancy and calcium substitutional defects in $BiFeO_3$ are analyzed. In Ca doped $BiFeO_3$, the hole polaron and bipolaron can be formed and they become absorption centers in wider energy range than the absorption range described by oxygen defects. Such a hole polaron and bipolaron can be a successful model for explaining the mechanism of the prominent electrochromic phenomenon of recently reported Ca-doped $BiFeO_3$. The topotactic transition between the brownmillerite $CaFeO_{2.5}$ and the perovskite $CaFeO_3$ is analyzed with atomic and electronic structure. The changes in structure and electron correlation caused by the migration of oxygen vacancies induce drastic changes in optical properties in the visible light region. This transition can be the atomic origin of the electrochromic phenomena.