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

Abstract

Infrared (IR) birefringent crystals applicable to the 8-12 μm long-wave IR region remain scarce to date. Herein, two Millon’s phase phosphides IR 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, and good IR transparency (up to 15 μm) and wide band gap (2.20, 2.72 eV). The P–P bonds in [Cd4P2][I3] effectively enhance the structural anisotropy, which constitutes the intrinsic origin of its large birefringence. The chain-layer interconnected arrangement within [Cd2P][CdCl3] is responsible for the pronounced structural anisotropy, a critical structural factor giving rise to its large birefringence. These results demonstrate the potential of [Cd4P2][I3] and [Cd2P][CdCl3] with “Millon's phase” structure as birefringent materials for applications in the long-wave infrared region.