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Title: Electronic Structure Theory
Electronic Structure Diagram
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Electronic properties and crystal structure of La0.5Ca0.5MnO3

Spin structure
Spin structure of La1-xCaxMnO3.
The orientations of magnetic moments obtained by neutron scattering are shown as arrows. Mn ions are shown as brown or green spheres and sites where holes may localise on O ions as O- are indicated by red or blue spheres.

Half-doped manganites such as La0.5Ca0.5MnO3 lie at the midpoint of the La1-xCaxMnO3 composition phase diagram. The doping level x = 0.5 means that half of the Mn ions are formally Mn3+ and half Mn4+. These ions order in a 'charge ordered state'. Two models have been proposed on the basis of diffraction experiments - conventional charge ordering and the Zener polaron state. In the former state, charge ordering occurs as shown in the figure on the left (Mn3+ green, Mn4+ brown) and all O ions are formally O2-. However, in the Zener polaron state an electron is transferred from an O ion in the centre of an Mn ion pair so that all Mn ions are formally Mn3+ and some O ions are formally O-. In the conventional Zener polaron picture the two Mn ions in the polaron fluctuate between Mn3+ Mn4+ and Mn4+ Mn3+ states and the O ion is formally O2-, but our Unrestricted Hartree-Fock (UHF) calculations do not support this picture.The figure on the left shows possible sites for O- ions at the midpoints of Zener polarons (red or blue circles). One O- ion is located in each quadrant (1-4) of the magnetic unit cell to form a polaron to the left (L) or right (R). O- orderings are denoted LLLL, LLLR, etc. Some of these result in bi-Mn ion polarons and some in tri-Mn polarons. We have investigated the relative energies of polaron orderings using UHF total energy calculations and found that the Zener (bi-Mn) polaron denoted LLLL is most favourable.


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Last updated: Mar 21 2012.

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