Band Gap Tuneability in Antiperovskite-Based Nitrides AE3PnN and Imides AE5Pn2(NH)2 (AE = Ca, Sr; Pn = As, Sb, Bi).

Inorganic antiperovskites with the formula X3AN (X = Ba, Sr, Ca, Mg; A = As, Sb) have recently been reported to exhibit excellent optoelectronic properties including small carrier effective masses, suitable direct bandgaps, high optical absorption coefficients as well as allowed optical transitions at the band edges. Using the ammonothermal method, we have synthesized the imide antiperovskites AE5Pn2(NH)2 (AE = Ca, Sr; Pn = As, Sb, Bi). The crystal structures of AE5Pn2(NH)2 were solved and refined in the orthorhombic space group Pbam by single-crystal x-ray diffraction (scXRD), and further confirmed using powder X-ray diffraction (pXRD) and Raman spectroscopy. Depending on the ion size ratio between AE2+ and Pn3-, different degrees of octahedral tilting can be observed. Soft X-ray spectroscopy was used to study the band gap and electronic structure, and revealed the presence of oxygen impurities. The AE5Pn2(NH)2 compounds can further react to form the ternary antiperovskites AE3PnN. Density functional theory calculations reveal favorable transport and optical properties. Narrow direct band gaps in the range of 0.87-1.76 eV could be verified experimentally, making AE5Pn2(NH)2 not only suitable as precursor materials for the corresponding AE3PnN antiperovskites, but also as promising candidates for solar cell absorber materials.