Ordering Phenomena in Transition Metal Oxides

Phase separation and charge order driven by orbital order   

The phenomenon of charge order (CO), a long range order of different metal oxidation states in a crystal lattice, was first proposed in 1939 for magnetite (Fe₃O₄) below the 120 K Verwey transition. CO has become important in recent years as CO stripes or other correlations may be important to the mechanism of superconductivity in cuprates, CMR in manganites, and other phenomena in oxides. However, it is only in the last few years that many CO structures have been experimentally determined.

We have recently studied CO in Fe₃O₄ [1,2] and in manganite perovskites; cation-ordered RBaMn₂O₆ (R = Tb, Y) [3,4] and (Pr_0.5Ca_0.5)MnO₃ [5], using high resolution powder X-ray and neutron diffraction. The results show that several CO arrangements are possible, with further variation of the orbital ordering (OO) associated with CO in manganites. The magnitude of CO, measured by Bond Valence Sum calculations using experimental bond distances, is always greatly reduced from theoretical values. The structurally observed CO is typically 20-60% of the ideal value in symmetry-broken CO structures.

Phase separation into CO insulator and ferromagnetic metallic regions is observed in many manganites. A simple example of long range phase separation driven only by OO has recently been discovered [6]. This produces both insulator-insulator and insulator-ferromagnetic metal co-existence as temperature varies.

[1] Wright, J P, Attfield, J P, Radaelli, P G Phys Rev Lett 2001, 87, 266401.
[2] Wright, J P, Attfield, J P, Radaelli, P G, Phys Rev B 2002, 66, 214422.
[3] Williams, A J, Attfield, J P, Phys. Rev. B, 2002, 66, 220405.
[4] Williams, A J, Attfield, J P, submitted.
[5] Goff R.J., Attfield, J P, submitted.
[6] J.P Attfield, J.P. Chapman, L. Lezama, T. Rojo and L.M. Rodriguez-Martinez, submitted.