Bismuth ferrite (BiFeO₃) has received much attention as a multiferroic material, which simultaneously exhibits both ferroelectric and antiferromagnetic properties. Cation doping has been widely used to improve fundamental properties such as the electronic, magnetic and ferroelectric properties of multiferroic oxides. Particularly, it has been reported that Ca-doped BiFeO3 exhibit a prominent electrochromic effect.
In this study, we performed first-principles hybrid functional calculations to understand the microscopic mechanism of electrochromism phenomena in Ca-doped BiFeO₃. Based on the defect study of calcium substitutional at the bismuth site (Ca_Bi) and oxygen vacancy defect (V_O), we compared electronic structures of V_O and Ca_Bi defects under thermodynamic stability. We found that the isolated neutral charged ((+)-charged) Ca_Bi defect induces a deep state in the band gap, indicating that the hole-induced trap state can be regarded as a small polaron (bipolaron). Our theoretical results from Ca dopant and oxygen vacancy in Ca-doped BiFeO₃ can provide an explanation for recent experimental observations and a correlation between hole polaron and electrochromism.