Anomalous effective charge of hypervalent chalcogenide As$_2$Te$_3$: First-principles study

In order to understand the electron-lattice interaction of hypervalent chalcogenide As$_2$Te$_3$, we performed first-principles calculations based on density functional theory. The material exhibits an intermetalic character with a narrow gap $\sim 0.2$ eV, and thus is expected to have a strong electron-lattice coupling. To estimate such couplings we calculated Born effective charge tensors, and found that the resulting charge is $6$ for As and $4$ for Te, which are about three times larger than other chalcogenides, As$_2$Se$_3$ and As$_2$S$_3$. The microscopic mechanism for the anomalous charges is discussed with a tight-binding model.