Charge ordered oxygen ions and bi- and tri-Mn polarons in La_0.5Ca_0.5MnO₃

Abstract

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.

Acknowledgments

It is a pleasure to acknowledge the contribution of Vic Saunders to this work through the development of the CRYSTAL and GAMESS codes. This work was supported by Enterprise Ireland under grant number SC/00.267 and by the Irish Higher Education Authority under the PRTLI-IITAC2 programme. The author wishes to acknowledge the assistance of C. Smith in preparing figures for this article.

Notes

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Notes on contributors

C. H. Patterson *

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