Effect of Compositional Fluctuations on the Phase Transitions in (Nd_1/2Sr_1/2)MnO₃

High-resolution synchrotron X-ray and neutron powder diffraction techniques have been used to investigate the structural and magnetic phase transitions in two polycrystalline samples with nominal composition (Nd0.5Sr0.5)MnO3. The first sample separates into three coexisting macroscopic phases at low temperature: a ferromagnetic (FM) phase, an orbitally ordered antiferromagnetic A-type (AFM-A) phase, and the majority phase, which shows charge, orbital, and antiferromagnetic CE-type (AFM-CE) ordering. The second sample shows a clean transition from the FM state into the AFM-CE state. The unusual behavior of the first sample is associated with a slight excess of Mn4+ and a wider compositional range with respect to the second sample and illustrates the very delicate competition between these three states near a Mn4+:Mn3+ ratio of unity, where both electron delocalization and charge-ordering are known to occur. The charge-ordered structure of Nd0.5Sr0.5MnO3 is reported.