High-temperature orbital and charge ordering inBi1/2Sr1/2MnO3

Abstract

We report a spectacularly high orbital/charge ordering temperature ${T}_{\mathrm{CO}}=475\mathrm{K},$ in ${\mathrm{Bi}}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}.$ The charges order approximately $350\mathrm{K}$ above the temperature predicted by the bandwidth tuning mechanism in the ${\mathrm{Ln}}_{1/2}(\mathrm{C}\mathrm{a},\mathrm{S}\mathrm{r}{)}_{1/2}{\mathrm{MnO}}_{3}$ family. This transition takes place $150\mathrm{K}$ above that in ${\mathrm{Bi}}_{1/2}{\mathrm{Ca}}_{1/2}{\mathrm{MnO}}_{3}$ which is much more distorted. Strikingly, the ${T}_{\mathrm{CO}}$ variation along the ${\mathrm{Bi}}_{1/2}({\mathrm{Ca}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{)}_{1/2}{\mathrm{MnO}}_{3}$ series $(0<~y<~1)$ is not monotonous but ${T}_{\mathrm{CO}}$ suddenly jumps $150\mathrm{K}$ for $y\ensuremath{\gtrsim}0.5$ 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 ${\mathrm{Bi}}^{3+}$ lone pair seems to markedly favor the charge ordering.