Phase separation over an extended compositional range: Studies of the Ca1-xBixMnO3 (x <= 0.25) phase diagram

Phase transitions on the electron-doped side of the Ca1-xBixMnO3 system (x less than or equal to0.25) have been investigated using high-resolution syn chrotron x-ray and neutron powder-diffraction techniques,(-)electrical tran sport and magnetic susceptibility measurements. At room temperature all sam ples investigated were single phase, paramagnetic conductors (p<0.1 <Omega> cm), isostructural with GdFeO3 (space group Pnma). The Mn-O-Mn angles rema in nearly constant from x=0 to x=0.25, while the Mn-O distances steadily in crease with the Mn3+ content. Three distinct phases are observed at 25 K. T he first one, observed from 0.15 greater than or equal to x greater than or equal to0.03, is characterized by the absence of charge and orbital orderi ng, a canted G-type antiferromagnetic spin structure, and delocalized elect ron transport. The second phase, observed from 0.25 greater than or equal t ox greater than or equal to0.12 (single phase at x= 0.18), is characterized by pronounced orbital ordering, a C-type antiferromagnetic spin structure, and insulating behavior. The third low-temperature phase, observed for x g reater than or equal to 0.20, is characterized by orbital and magnetic orde ring similar to the Wigner crystal structure previously observed for Ca0.67 La0.33MnO3, but with a 4a x b x 2c unit cell. The most striking feature of the phase diagram is the wide compositional range over which low-temperatur e phase separation is observed. Only those samples with x<0.12 and x = 0.18 did not undergo phase separation upon cooling. We show that this behavior cannot be attributed to compositional variations, and therefore, propose th at anisotropic strain interactions between crystallites may be partially re sponsible for this behavior.