General Considerations
2001; Springer Science+Business Media; Linguagem: Inglês
10.1007/3-540-45503-5_1
ISSN1616-8550
Autores Tópico(s)Multiferroics and related materials
ResumoFor reference a few properties of AMO 3 perovskites are reviewed. Lattice distortions arising from the mismatch between equilibrium A-O and M-O bond lengths are discussed; a compressive stress on the MO 3 array gives cooperative rotations of the MO 6/2 octahedra that bend the (180° - φ) M-O-M bonds by an angle φ. Ligand-field wavefunctions for d electrons at transition-metal M atoms are used to distinguish, for single-valent MO3 arrays, where the M-O-M interactions should be treated by superexchange perturbation theory from where they should be described by tight-binding band theory. Double-exchange and indirect-exchange interactions are also reviewed. The possible symmetries of the spin configurations of magnetic perovskites are specified. The virial theorem is employed to demonstrate the existence of lattice instabilities at a cross-over from localized to itinerant electronic behavior due to a double-well potential for the equilibrium M-O bond length with (M-O) loc > (M-O) itin . Long-range and short-range cooperative Jahn-Teller distortions are contrasted with charge-density-wave instabilities; polarons and correlation bags are contrasted with strong-correlation fluctuations.
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