Phase separation over an extended compositional range: Studies of theCa1−xBixMnO3(x<~0.25)phase diagram

Abstract

Phase transitions on the electron-doped side of the ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}{\mathrm{MnO}}_{3}$ system $(x<~0.25)$ have been investigated using high-resolution synchrotron x-ray and neutron powder-diffraction techniques, electrical transport and magnetic susceptibility measurements. At room temperature all samples investigated were single phase, paramagnetic conductors $(\ensuremath{\rho}<0.1\ensuremath{\Omega}\mathrm{cm}),$ isostructural with ${\mathrm{GdFeO}}_{3}$ (space group Pnma). The Mn-O-Mn angles remain nearly constant from $x=0$ to $x=0.25,$ while the Mn-O distances steadily increase with the ${\mathrm{Mn}}^{3+}$ content. Three distinct phases are observed at 25 K. The first one, observed from $0.15>~x>~0.03,$ is characterized by the absence of charge and orbital ordering, a canted G-type antiferromagnetic spin structure, and delocalized electron transport. The second phase, observed from $0.25>~x>~0.12$ (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 $x>~0.20,$ is characterized by orbital and magnetic ordering similar to the Wigner crystal structure previously observed for ${\mathrm{Ca}}_{0.67}{\mathrm{La}}_{0.33}{\mathrm{MnO}}_{3},$ but with a $4a\ifmmode\times\else\texttimes\fi{}b\ifmmode\times\else\texttimes\fi{}2c$ unit cell. The most striking feature of the phase diagram is the wide compositional range over which low-temperature 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 that anisotropic strain interactions between crystallites may be partially responsible for this behavior.