Effect of compositional fluctuations on the phase transitions in (Nd1/2Sr1/2)MnO3

High-resolution synchrotron X-ray and neutron powder diffraction techniques have been used to investigate the structural and magnetic phase transition s 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 antiferromag netic A-type (AFM-A) phase, and the majority phase, which shows charge, orb ital, and antiferromagnetic CE-type (AFM-CE) ordering. The second sample sh ows 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+ an d a wider compositional range with respect to the second sample and illustr ates the very delicate competition between these three states near a Mn4+:M n3+ ratio of unity, where both electron delocalization and charge-ordering are known to occur. The charge-ordered structure of Nd0.5Sr0.5MnO3 is repor ted.