Charge ordered oxygen ions and bi- and tri-Mn polarons in La0.5Ca0.5MnO3

Charge ordering (CO) in manganites is widely believed to consist of ordering of Mn ions as Mn3+ and Mn4+. Unrestricted Hartree–Fock calculations show that CO in manganites is actually ordering of O ions as O− and O2− on sites adjacent to Jahn–Teller distorted Mn ions, along the direction of long Mn–O bonds. The energetics of CO of O ions is investigated by calculating the UHF total energies of all unique charge orderings and the lowest energy CO state is found to be one where all Mn ions are bound into bi-Mn polarons. The energetics of several spin states of these polarons is investigated using configuration interaction calculations on a cluster. Bi-Mn polarons are elongated along their axes which results in transverse Mn ion displacements within the perovskite unit cell and this may explain the lattice modulations observed in manganites by transmission electron microscopy.