High-temperature orbital and charge ordering in Bi1/2Sr1/2MnO3 -: art. no. 064415

We report a spectacularly high orbital/charge ordering temperature T-CO=475 K, in Bi1/2Sr1/2MnO3. The charges order approximately 350 K above the temperature predicted by the bandwidth tuning mechanism in the Ln(1/2)(Ca,Sr)(1/2)MnO3 family. This transition takes place 150 K above that in Bi1/2Ca1/2MnO3 which is much more distorted. Strikingly, the T-CO Variation along the Bi-1/2(Ca1-ySry)(1/2)MnO3 series (0 less than or equal toy less than or equal to1) is not monotonous but T-CO suddenly jumps 150 K for y greater than or similar to0.5 compositions. Neutron and synchrotron diffraction data allow us to propose a new mechanism to justify this behavior. For compositions with high Sr content, the manifestation of the 6s(2) character of the Bi3+ lone pair seems to markedly favor the charge ordering.