Oxygen ordering and electrochromism in Ca-doped BiFeO$_3 $*

We show that calcium-doped bismuth ferrite thin films exhibit an electrochromic effect arising from an intrinsic mechanism due to redistribution of carriers, without the need for additional electrolytes that are needed in common electrochromic devices. The absorption change and coloration efficiency at the band edge are $4.8\times 10^6m^{-1}$and $190cm^2C^{-1}$, respectively, which are among the highest reported values for inorganic electrochromes. These experimental findings are supported by optical absorption calculations from first-principles theory, confirming the strong absorption change at the band edge.