Oxypnictide Anti-perovskite (Ca4Pn2– x N x )O and (Ca4– x Mg x

Abstract

We present the oxypnictide anti-perovskites A4Pn2O (A = Ca, Sr; Pn = P, As) as a class of visible-light-responsive photocatalytic materials. Substitutional doping is further explored through anionic N3–@Pn3– and cationic Mg2+@Ca2+ sites, yielding (Ca4Pn2–xNx)O and (Ca4–xMgx)Pn2O (with x = 0.125, 0.25, and 0.50). All systems are examined using first-principles calculations based on DFT and DFPT. The GGA-PBE functional demonstrates reliable reproduction of the experimental structural parameters for the pristine materials. In addition, the optoelectronic and water redox properties are accurately calculated using the screened Coulomb hybrid HSE06 functional. These materials exhibit direct band gaps within the optimal solar range (2.00–2.40 eV), strong optical absorption coefficients (>105 cm–1), high dielectric constants (26–85), weak exciton binding energies (<25 meV), and small electron and hole effective masses (< 0.50 m0) along the [100/010] and [001] directions. This leads to good exciton dissociation ability, good charge-carrier transport with efficient charge separation, and small recombination rate in the bulk. However, their VBM energy position is not appropriate enough for OER, and therefore, the substitutional doping strategy was adopted to overcome this limitation. Among all investigated doped materials, (Ca4Pn2–xNx)O and (Ca4–xMgx)Pn2O (for x = 0.25) achieve good thermodynamic stability and reveal the most suitable CBM and VBM energy positions for both H+ reduction and water oxidation while maintaining the excellent optoelectronic properties as in the pristine materials. This offers an opportunity for these promising candidates to be experimentally synthesized and tested for efficient solar-driven H2 production and/or CO2 reduction.