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First-principles study of defect formation energies in LaO$X$S$_2$ ($X=$ Sb, Bi)

We theoretically investigate defect formation energies in LaO$X$S$_2$ ($X=$Sb, Bi) using first-principles calculation. We find that the oxygen vacancy is relatively stable, where its formation energy is higher in $X=$ Sb than in $X=$ Bi. An interesting feature of $X=$ Sb is that the vacancy of the in-plane sulfur atom becomes more stable than in $X=$ Bi, caused by the formation of an Sb$_2$ dimer and the electron occupation of the impurity energy levels. The formation energies of cation defects and anion-cation antisite defects are positive for the chemical equilibrium condition used in this study. Fluorine likely replaces oxygen, and its defect formation energy is negative for both $X=$ Sb and Bi, while that for $X=$ Sb is much higher than $X=$ Bi. Our study clarifies the stability of several point defects and suggests that the in-plane structural instability is enhanced in $X=$ Sb, which seems to affect a structural change caused by some in-plane point defects.