Investigation of electronic, optical, and thermoelectric properties of half-metallic spinel X2NO4 (X=B, Al): First-principles calculations

The cubic spinel structure of B2NO4 and Al2NO4 was investigated using ab-initio calculations for a better understanding of their electronic, bonding, and thermo-optical properties in spin-up and spin-down cases. The calculations were performed by integrating the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (PBE-GGA) with the full potential linearized augmented plane wave (FP-LAPW). The elastic constants as well as the negative formation energies indicate that the studied compounds are stable. It was found that A2NO4 has a unit cell volume of about 30% larger than that of B2NO4. The band gap values for B2NO4 and Al2NO4 are 1.65 eV and 1.94 eV, respectively, for the spin-down case. Oxygen and nitrogen are the major elements in the determined total density of states (T-DOS). The bandgap results are consistent with the DOS results. This implies that the structures behave as half-metallic, going through a metallic phase during spin-up and a semiconductor phase during spin-down. The optical results for both compounds indicate distinct behaviors. Al2NO4 has a higher conductivity than B2NO4, but when all other studied optical properties were considered, B2NO4 is superior. A covalent bond that is close to an ionic bond was observed from the electron density. Additionally, the charge carriers for B2NO4 and Al2NO4 are holes for the spin-up case and electrons for the spin-down case.