Hole-concentration-induced transformation of the magnetic and orbital structures inNd1−xSrxMnO3

We have peformed neutron diffraction measurements on melt-grown polycrystalline samples of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ $(0.49\mathrm{}<~x<~0.75).$ A systematic transformation of the crystalline and magnetic structures of this system was observed. This can be consistently explained by the change of the character of the Mn $e_g$ orbitals. When increasing X, the ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ system exhibits an evolution from a metallic ferromagnetic state, to a metallic A-type antiferromagnetic (AFM), and then to an insulating C-type AFM state. The CE-type charge-ordered AFM state was observed only in the vicinity of $x=1/2\mathrm{}$ and it coexists with the A-type AFM state for $x\gtrsim 1/2,$ indicating that the energy difference between these two states is very small. We also found that the ${\mathrm{MnO}}_6$ octahedra are apically compressed in the CE-type and A-type AFM states due to the ${d(3x}^2-r^2{)/d(3y}^2-r^2)$ or ${d(x}^2-y^2)$ orbital ordering, whereas they are apically elongated by the rod-type ${d(3z}^2-r^2)$ orbital ordering in the C-type AFM state. Finally, a selective broadening of Bragg peaks was observed in the C-type AFM phase and its x dependence strongly suggests the onset of charge ordering for either $x=4/5\mathrm{}$ or $x=3/4.\mathrm{}$