Ab Initio Investigation of Amorphous and Crystalline Arsenic Sesqui‐Chalcogenides: Optical Properties Explained by Metavalent Bonding

Phase‐change materials (PCMs) are employed in both electrical and optical devices exploiting the property contrast between their amorphous and crystalline states. Binary antimony sesqui‐chalcogenides such as Sb 2 Se 3 and Sb 2 S 3 are recently shown to be suitable PCMs for low‐loss optical applications. In this work, ab initio simulations of arsenic sesqui‐chalcogenides are carried out, including As 2 S 3 , As 2 Se 3 , and As 2 Te 3 to unravel the bonding and optical properties of their crystalline and amorphous phases. Due to the metavalent character of its chemical bonds, crystalline As 2 Te 3 shows a high optical response. However, in crystalline As 2 S 3 and As 2 Se 3 , the alignment of p orbitals is fully broken, which results in a very low‐extinction coefficient that is already comparable to their amorphous phase. Although As 2 S 3 and As 2 Se 3 display good low‐loss optical properties, the overall optical contrast upon phase transition is not sufficient for practical applications. Therefore, it is concluded that arsenic is a useful alloying element in reducing the optical loss of conventional PCMs, but its concentration should be kept at a relatively low level to balance the optical loss and contrast window.