A[A6Ch][Si12P20] (A = Sr, Ba; Ch = S, Se, Te): Achieving Wide Band Gap of Pnictides by Constructing [A6Ch] octahedral ionic units

Abstract

The design and synthesis of novel inorganic pnictides have long been challenged by the difficulty of achieving wide band gap, with most pnictides exhibiting narrow band gap of Eg < 2.0 eV. This work reports the first synthesis of A-M-Pn-Ch phase, A[A6Ch][Si12P20] (A = Sr, Ba; Ch = S, Se, Te) by introducing highly electronegative chalcogen elements (S, Se, Te) into Ba/Sr-Si-P system. They exhibit wide band gaps of 1.91-2.27 eV, significantly outperforming known compounds in the Ba/Sr-Si-P and A-M-Pn-X (X = halogen) systems. Theoretical calculations reveal that the wide band gaps originate from the electronic regulation effect of the [A6Ch] octahedral ionic units, whose moderate ionic-covalent hybrid bonding characteristics promote charge localization and effectively suppress the metallic behavior of the system. Moreover, by constructing the mixed octahedral ionic unit [Ba2Sr4Ch] as an interpenetrated guest, the inversion symmetry of the interpenetrated host [Si12P20] covalent framework is successfully broken, enabling Ba[Ba2Sr4Ch][Si12P20] to crystallize in the non-centrosymmetric space group F43m. This work proposes a strategy based on regulating the electronic structure via [A6Ch] octahedral ionic units, provides an unreported paradigm for the design and synthesis of wide band gap pnictides.