On the structural, electronic and magnetic properties of spinel

The structural, electronic and magnetic properties of spinel are investigated using the periodic ab initio Hartree - Fock program CRYSTAL. The geometry is fully optimized and the bulk modulus evaluated. The system is a large-gap insulator; the Mn - O bond is fully ionic, whereas in the Cr - O case there is a relatively large overlap between the Cr d and the O sp orbitals, and the resulting bond has some covalent character. The ferromagnetic solution (all spins in the same direction) and two ferrimagnetic solutions (one or two Mn atoms of the unit cell with spin down, the remaining transition metal atoms with spin up) have been considered; the latter are more stable than the former by 2.2 and 3.8 mHartree per unit cell, respectively. At zero pressure, the spinel is more stable than the mixture of simple oxides by about , in excellent agreement with experiment.