Electron-doped rare-earth manganates of the type Ca 1−x Ln x MnO 3 (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, T CO , 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, T CO increases with x or the electron concentration. These trends are also seen for Ca 1−x Bi x MnO 3 compounds (T CO = 300 K for x = 0.3) which show transitions to a more distorted orthorhombic structure below T CO . In Ca 1−x Ln x MnO 3 , Cr doping does not melt the charge-ordered state, unlike the case for the hole-doped systems. CaMnO 2.82 , for which electron doping is affected by anion vacancies, appears to show charge ordering at around 200 K.Investigations of colossal magnetoresistance and related phenomena for the rare-earth manganates, reported extensively in the last few years, by and large pertain to solids of the composition Ln x A 1−x MnO 3 (Ln = rare earth, A = Ca, Sr, x 0.5), which involves hole doping [1,2]. The hole-doped manganates exhibit charge ordering, particularly when x = 4/8 or 5/8, and the charge-ordering transition is highly sensitive to the average radius of the Asite cations, r A [2][3][4]. The charge-ordering transition temperature, T CO , decreases with increase in r A , unlike the ferromagnetic Curie temperature, T C , which increases with r A . Furthermore, the strain arising from the size mismatch between A-site cations affects T C . It is found that T C decreases with the variance, σ 2 , which is a measure of the mismatch between the A-site cations [5]. There has been some interest recently in studying the properties of electrondoped manganates where Ca is partly substituted for with a trivalent or a tetravalent cation. These studies [6] have shown that the electron concentration determines the magneto-transport properties of these materials. A study of Ca 1−x Bi x MnO 3 (x = 0.18) has shown the occurrence of charge ordering accompanied by a structural transition [7]. We considered it important to investigate charge ordering in the electron-doped systems to examine the similarities, if any, with the hole-doped manganates. For this purpose, we have investigated manganates of the type Ca 1−x Ln x MnO 3 (Ln = La, Nd, Gd or Y), in particular the composition corresponding to x = 3/8 for which charge ordering should be favoured [8]. We have also studied the effect of electron doping on charge ordering in Ca 1−x Bi x MnO 3 (x = 0.2, 0.3), paying attention to the structure changes at the charge-ordering transition. Lastly, we have examined the effect of electron doping in the parent CaMnO 3 , brought about by oxygen deficiency.
