AbstractThe structural, magnetic and transport phase diagrams of the manganese perovskites (general formula: A1-xA'xMnO3) are characterised by a variety of exotic phenomena, including high-temperature polaronic behaviour, charge, orbital and magnetic ordering and colossal magnetoresistance (CMR). These properties can be “tuned” by changing the doping level x, the electronic bandwidth (through the average A-site ionic radius, <rA>), and the A-site disorder, and are believed to be a manifestation of the underlying competition between electron-lattice coupling and double exchange. Usually, at low temperatures, one of these two interactions is dominant, resulting in a homogeneous ground state, which is either a metallic ferromagnet or a charge-ordered insulator. We have recently found, however, that, for special points in the phase diagram (x ~ 0.3,,<rA> ~ 1.18 Å), the competition can be preserved down to low temperatures, resulting in an inhomogeneous ground state at the microscopic level. This unusual state is characterised by the coexistence of charge-ordered and metallic domains, which are intertwined over a variety of length-scales, and appear to show spin-glass-like dynamics. Upon application of an external field (magnetic field, pressure or even x-rays), the domains grow to macroscopic sizes, resulting in phase segregation. We speculate that the evolution of the local magnetic and crystal structures during this phase segregation process may parallel those occurring, at much higher temperatures, for compounds displaying CMR behaviour at the paramagnetic-toferromagnetic transition. Very recently, it has been suggested that the charge-ordered state, which is stable for higher values of the Mn oxidation state (x ≥ 0.5), may also be associated with modulated mesoscopic phase segregation, in the form of “stripes”. This hypothesis will be discussed in the light of recent x-ray synchrotron and neutron diffraction data on the crystallographic and magnetic modulation in La0.33Ca0.67MnO3
A-site phase segregation in mixed cation perovskite