X-ray absorption and neutron diffraction studies of (Sr _1-xCe_x)MnO₃: Transition from coherent to incoherent static Jahn-Teller distortions

We have carried out Ce L- and Mn K-edge x-ray absorption near edge structure (XANES) measurements to experimentally determine the oxidation states of both Ce and Mn in (Sr_1-xCe_x )MnO3 (x = 0.1-0.4). It was found that although Ce is predominantly 4+ at low doping levels (x = 0.1 and 0.15), the Ce valency decreases with increasing Ce doping (reaching a value of around 3.5+ at x = 0.4). The average Mn oxidation state decreases with the increase of Ce content, with the percentage of Jahn-Teller active Mn ³⁺ ions increasing from 26% (x = 0.1) to 57% (x = 0.4). Precise structural parameters were also obtained from high resolution neutron diffraction studies for samples with x = 0.1-0.3. The crystal structure remains tetragonal in I 4/mcm for x ≤ 0.3. The octahedral tilt angle increases with increasing Ce content, but the distortion of the MnO₆ octahedra is reduced significantly at x ≥ 0.2 due to a transition from long-range ordered Jahn-Teller distortions to incoherent static distortions.