Electron rich manganites Pr0.4A0.6MnO3: influence of A cations upon structure, magnetism and conductivity

The role of the A-site average cationic size border=0 src="/.dejavu/.tact/lib1/art410/pcdata/html_gif/charset.gif/bea.gif" upon the structure, the magnetic and resistive properties of Pr0.4Ca0.6MnO3 has been studied by substituting strontium and barium for calcium. At room temperature, the electron microscopy study and refinements of the X-ray diffraction data demonstrate a progressive transformation from an orthorhombic Pnma structure to a cubic Pm3m one, through a tetragonal I4/mcm cell as border=0 src="/.dejavu/.tact/lib-1/art410/pcdata/html_gif/charset.gif/bea.gif" increases. At low temperature (92K), the electron diffraction study reveals a more complex situation depending on border=0 src="/.dejavu/.tact/lib1/art410/pcdata/html_gif/charset.gif/bea.gif". Three types of charge-ordered (CO) phases with modulated structures are observed for the smaller border=0 src="/.dejavu/.tact/lib-1/art410/pcdata/html_gif/charset.gif/bea.gif" values. For the largest border=0 src="/.dejavu/.tact/lib1/art410/pcdata/html_gif/charset.gif/bea.gif" values, two phases (without CO) are observed: a monoclinic distorted structure (involving an orthorhombic Fmmm supercell) and a cubic Pm3m structure. Accordingly, the resistivity (rho)(T) and magnetization M (T) curves exhibit the characteristic signature of CO below TCO, i.e. (rho)increases and M decreases. In contrast, the CO is suppressed by increasing border=0 src="/.dejavu/.tact/lib-1/art410/pcdata/html_gif/charset.gif/bea.gif" but (rho) shows a slight decrease, which is reminiscent of the A-type antiferromagnetism. This suggests a change of orbital filling, from dz2 to dx2-y2 for the eg electron, as border= 0 src="/.dejavu/.tact/lib-1/art410/pcdata/html_gif/charset.gif/bea.gif" is increased.