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In studying superstructures in transition metal oxides such as charge or orbital ordering, one usually considers site-centered superstructures. Such are for instance the standard checkerboard charge ordering in La0.5Ca0.5MnO3, or Verwey charge ordering in magnetite.
Hovewer there exist another possibility: bond-centered superstructures, such as e.g. the Peierls state in low-dimensional systems. In this talk I will consider the possibility of competition or coexistence of site-centered and bond-centered structures on a few examples. One is the charge ordering in less-than-half-doped manganites. We have recently shown [1] that in this case indeed a bond-centered ordering may exist, and, moreover, bond-centered ordering may coexist with site-centered one, in which case the resulting state would be ferroelectric. This as a rare case of ferroelectricity in magnetic material \x{2013} the so called multiferroic behaviour. The second topic is an orbitally-driven Peierls state [2]. I will show that in spinels and in some other frustrated systems a site-centered orbital ordering (ODW-Orbital Density Wave) may lead to the formation of bond-centered singlet Peierls-like states. This picture gives a simple explanation of extremely strange superstructures observed recently in MgTi2O4 [3] and CuIr2S4 [4], and may be relevant for several other materials, such as NaTiO2, La4Ru2O10 [5] and some others. I will also give a general discussion in which cases bond-centered structures may be favourable. [1] D.Efremov. J.van den Brink and D.Khomskii, Nature Materials, 3, 853 (2004), [2] D.Khomskii and T.Mizokawa, cond-mat/0407458 [3] M.Schmidt et al., Phys.Rev.Lett. 92, 056402 (2004) [4] P.G.Radaelli et al., Nature 416, 155 (2002) [5] P.Khalifah et al., Science 297, 2237 (2002) |