Introduction Computational Details Aim of the work

“First principles calculations of charge ordering in manganites” D. Baldomir1,2, V. Pardo1,2, J. Castro1,2, M. Iglesias1,2, J.E. Arias1,2 and J.Rivas1 1

2

Departamento de Física Aplicada, Universidade de Santiago de Compostela, E-15782, Spain Instituto de Investigacións Tecnolóxicas, Universidade de Santiago de Compostela, E-15782, Spain

Introduction

Computational Details

 Charge ordering at half-filling in La1-xCaxMnO3  Full charge disproportionation into Mn3+ and Mn4+ ions?  Structural distortion associated to the charge disproportionation  Origin of the charge ordering: Jahn Teller vs. on-site Coulomb interaction

• Electronic Structure DFT calculations  GGA-based structure optimization • APW+lo method (WIEN2k)1 • LDA+U (strong correlations)2 • all-electron, full-potential • RKmax= 7, 1000 k-points

Aim of the work  Study the relationship between charge disproportionation and magnetic ordering in La0.5Ca0.5MnO3  Structural relaxation in the different possible magnetic configurations  Comparison with the experimental geometry

GGA structure optimization

Structural Data  Geometry optimized within GGA for several AF structures  Charge disproportionation is associated with the deformation of one of the octahedra (d4 ion)  GGA predicts a shortened octahedron for Mn3+  Experimental geometry: lowest E state within LDA+U is an elongated octahedron  Full charge disproportionation Mn3+/Mn4+  AF ordering produces charge disproportionation  on-site Coulomb interactions yield the correct Jahn-Teller distortion that stabilizes the system  A-type and FM ordering yield a Mn3.5+ metallic picture for both Mn cations

Exp.

Distances (Å)

C-, Etype

G-type

A-type

FM

Mn1-O

1.92 x 4 1.93 x 2

1.93 x 2 1.94 x 4

1.93 x 2 1.96 x 4

1.92 x 2 1.95 x 4

Mn2-O

1.91 x 2 2.00 x 4

1.93 x 2 2.01 x 4

1.91 x 2 1.96 x 4

1.92 x 2 1.95 x 4

1.91 2.0

 A shortened octahedron is predicted within GGA for a Mn3+ ion in an AF ordering  Mn4+ would be in an undistorted environment

MnO6 shortened octahedron

2.05 1.92

Radaelli et al.3 1.91 x 6 1.92 x 4 2.05 x 2

 An elongated octahedron is experimentally found for a Mn3+ ion in a CE-type AF ordering  Such a solution gives the lowest E within LDA+U

MnO6 elongated octahedron

Electronic Structure of the C.O. state  Full disproportionation is observed in the DOS plots into a Mn3+ and a Mn4+ ion.  In an elongated octahedron, the d4 configuration contains one electron in the dz2 orbital and the dx2-y2 orbital is unpopulated  Mn2 is a Mn4+ ion with a d3 configuration and all the eg bands fully unoccupied  The charge ordered AF ordering produces the opening of a gap leading to the insulating behaviour experimentally found.

Conclusions

Density of states (DOS) plots for the inequivalent Mn atoms in the experimental geometry and an AF ordering established. Mn1 corresponds to the Mn3+ ion and Mn2 would be the Mn4+ ion, as can be seen in the figure. The coordinate system was chosen such that the z-direction goes along the elongated axis.

Acknowledgements The authors acknowledge fruitful discussions with F. Rivadulla. We also wish to thank the Centro de Supercomputación de Galicia (CESGA) for the computing facilities, Xunta de Galicia (Project No. PGIDIT02TMT20601PR), Ministry of Science and Education of Spain (Project No. MAT2004-05130) and University Santiago de Compostela for financial support.

 A full charge disproportionation into Mn3+ and Mn4+ occurs in La0.5Ca0.5MnO3  The charge disproportionation is not inherent of the CE-type phase: AF ordering produces some degree of charge disproponation but on-site Coulomb repulsion is also necessary to give the correct distortion  GGA produces the wrong distortion of the octahedron

References 1 K.

Scwharz and P. Blaha, Comput. Mater. Sci. 28, 259 (2003). Lichtenstein et al., Phys. Rev. B 52, R5467 (2003). 3 P.G. Radaelli et al. Phys. Rev. B 55, 3015 (1997). 2 A.I.