Effect of compositional fluctuations on the phase transitions in (Nd_½Sr_½)MnO₃

Abstract

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 (Nd_0.5Sr_0.5)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 Nd_0.5Sr_0.5MnO₃ is reported.