Effect of substituting Ru4+ and other tetravalent ions in the B-site of rare earth manganates on the magnetotransport properties and charge-ordering

Substitution of Ti and Zr in the B-site of La0.7Ca0.3MnO3 lowers the T-C and the temperature of insulator-metal transition. Substitution of Ru in the B-site of La0.7Ca0.3MnO3, however, does not alter the T-C or T-IM significantly. More importantly, the effect of substitution of Ti and Zr in the B-site of the charge-ordered Nd0.5Ca0.5MnO3 progressively destroys the charge-ordering in this insulating material. In contrast, substitution by Ru in the B-site renders the material ferromagnetic with the T-C increasing with the Ru content and the material also shows insulator-metal transition. This marked effect of Ru substitution on the charge-ordering is also seen in Nd0.5Sr0.5MnO3 where the T-C increases with Ru content while charge-ordering gets destroyed. The extraordinary effect of Ru in favouring the ferromagnetic metallic state and destroying the charge-ordering is attributed mainly to its unique electronic configuration (t(2g)(4)e(g)(0)). (C) 1999 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.