A[A6Ch][Si12P20] (A = Sr, Ba; Ch = S, Se, Te): achieving a wide band gap in pnictides by constructing [A6Ch] octahedral ionic units

Abstract

The design and synthesis of novel inorganic pnictides has long been challenging due to the difficulty in achieving a wide band gap, with most pnictides exhibiting a narrow band gap E_g <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 the 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 F3m. This work proposes a strategy based on regulating the electronic structure via [A₆Ch] octahedral ionic units, providing a previously unreported paradigm for the design and synthesis of wide-band-gap pnictides.