# Shifting of Fermi Level and Realization of Topological Insulating Phase in the Oxyfluoride BaBiO$_2$F

## TL;DR

Fluorination of BaBiO$_2$F shifts the Fermi level to realize a topological insulating phase, transforming non-topological BaBiO$_3$ into a topological insulator without affecting its spin-orbit coupling or parity.

## Contribution

Demonstrates that fluorination can effectively shift the Fermi level in BaBiO$_3$ derivatives, enabling topological insulating phases in these materials.

## Key findings

- 33% fluorination shifts E_F by ~2 eV
- BaBiO$_2$F is dynamically stable above 500 K
- Dirac states are robust against lattice distortions

## Abstract

The disadvantage of BaBiO$_3$ of not being a topological insulator despite having symmetry protected Dirac state is overcome by shifting the Fermi level (E$_F$) via fluorination. The DFT calculations reveal that the fluorination neither affects the spin-orbit coupling nor the parity of the states, but it acts as a perfect electron donor to shift the E$_F$. We find that 33 % fluorination is sufficient to shift the E$_F$ by $\sim$ 2 eV so that the invariant Dirac state lies on it to make BaBiO$_2$F a topological insulator. The fluorinated cubic compound can be experimentally synthesized as the phonon studies predict dynamical stability above $\sim$ 500 K. Furthermore, the Dirac states are found to be invariant against the low-temperature phase lattice distortion which makes the structure monoclinic. The results carry practical significance as they open up the possibility of converting the family of superconducting oxides, ABiO$_3$ (A = Na, K, Cs, Ba, Sr, Ca), to real topological insulator through appropriate fluorination.