[Cd4P2][I3] and [Cd2P][CdCl3]: two Millon's phase phosphides exhibiting wide band gaps and large birefringence

Abstract

Research on infrared birefringent crystals applicable to the 8–12 μm long-wave infrared region remains extremely scarce to date. Herein, two Millon's phase phosphide infrared birefringent crystals, [Cd₄P₂][I₃] and [Cd₂P][CdCl₃], are reported. Both [Cd₄P₂][I₃] and [Cd₂P][CdCl₃] exhibit a large birefringence (0.105, 0.139) at 546 nm, good IR transparency (up to 15 μm) and a wide band gap (2.20, 2.72 eV). The P–P covalent bonds in [Cd₄P₂][I₃] effectively enhance the structural anisotropy of this phosphide, which constitutes the intrinsic origin of its large birefringence. The chain-layer interconnected arrangement within [Cd₂P][CdCl₃] is responsible for the pronounced structural anisotropy of the crystal, a critical structural factor giving rise to its large birefringence. These results demonstrate the great potential of [Cd₄P₂][I₃] and [Cd₂P][CdCl₃] with the “Millon's phase” structure as birefringent materials for applications in the long-wave infrared region.