Phase separation, orbital ordering and magnetism in (La0.375Ca0.625)MnO3

At 300 K (La0.375Ca0.625)MnO3 crystallizes in the orthorhombic Pnma space group; on cooling a Pnma -> Pnma structural transition occurs due to charge-orbital ordering within the Mn sub-lattice, producing a superstructure consistent with a Wigner-crystal model with a tripling of the cell parameter a. The primary active mode yielding the observed ordered structure corresponds to the irreducible representation labelled , with wave vector ( 1/3 ,0,0). Nevertheless, the disordered polymorph stable at room temperature is retained at low temperature as a secondary phase, coexisting with the charge-orbital ordered structure. These two phases display different spin orderings; the antiferromagnetic structure associated to the charge-orbital ordered phase is characterized by a magnetic propagation wave vector k=(0,0,½), with a canted spin ordering in the ac plane, whereas a Cy-type arrangement develops within the disordered polymorph