Distinction between Two Types of Charge-Ordered States in the Rare Earth Manganates,Ln0.5A0.5MnO3, Based on Chemical Melting

Effects of cation substitution in the manganese site of charge-ordered (CO) rare earth manganates have been studied to find out how the properties vary with the average radius of theAsite cations, 〈r A 〉, and the substituent cation. The insulating CO state ofLn 0.5Ca0.5MnO3(Ln=Nd, Gd, or Y) is unaffected by substitution with Al3+and Fe3+, but the CO state in the Nd compound (〈r A 〉=1.17 Å) is converted into a ferromagnetic metallic state by Cr3+substitution. Chemical conversion of the charge-ordered state in the insulating manganates by Cr3+substitution seems to occur only when 〈r A 〉≥1.17 Å. Progressive substitution of Mn3+in Nd0.5Sr0.5MnO3(〈r A 〉=1.24 Å) with Al3+and Fe3+renders the material an insulator. Substitution with Cr3+, however, makes it a ferromagnetic metal from 250 K (∼T C) down to low temperatures, without the intervening CO state as in the parent manganate.