The structural and electronic properties of spinel MnCo2O4 bulk and low-index surfaces: From first principles studies

The structural and electronic properties of the bulk and low-index (0 0 1) and (1 1 0) surfaces of the MnCo2O4 spinel have been studied from the spin-polarized GGA + U calculations. Results show that Mn prefers to occupy the octahedral sites and exhibits as high-spin Mn3+ while half Co takes the tetrahedral sites as high-spin Co2+ and the other half takes the octahedral sites as nonmagnetic Co3+. The electronic structure of the MnCo2O4 bulk is insulating with very small gap of about 0.03 eV. The surface atomic relaxations and surface energies are calculated with different terminations and stable MnCo2O4 (0 0 1) and (1 1 0) surface models are obtained. The electronic structures of the Mn and the tetrahedral site Co atoms at the MnCo2O4 (0 0 1) and (1 1 0) surfaces resemble their bulk characters. However, due to the lowered Co–O coordination, the electronic structure of the octahedral Co atoms changes substantially from the bulk state t32g(↑)t32g(↓) into the surface state t32g(↑)e1g(↓) t22g(↓) at the MnCo2O4 (0 0 1) surface (5-coordinated Co) and t32g(↓)e2g(↓)t12g(↓)e1g(↓) at the MnCo2O4 (1 1 0) surface (4-coordinated Co).