Charge ordering in electron-doped manganates

Electron-doped rare-earth manganates of the type Ca1-xLnxMnO3 (Ln = La, Nd, Gd or Y) with x = 0.2 and 0.3 show charge ordering in the 150 - 270 K range, but the charge-ordering transition temperature, Tco, generally decreases with the decrease in the size of the A-site cations, a trend exactly opposite to that for hole-doped manganates. On the other hand, Tco increases with x or the electron concentration. These trends are also seen for Ca1-xLnxMnO3 compounds (Tco = 300K for x = 0.3) which show transitions to a more distorted orthorhombic structure below Tco. In Ca1-xLnxMnO3, Cr doping does not melt the charge-ordered state, unlike the case for the hole-doped systems. CaMnO2.82, for which electron doping is affected by anion vacancies, appears to show charge ordering at around 200 K.