The Nature of Charge Ordering in Rare Earth Manganates and Its Strong Dependence on the Size of theA-Site Cations

Charge ordering in rare earth manganates of the typeLn 0.5 A 0.5MnO3(Ln=rare earth,A=alkaline earth) is highly sensitive to the average radius of theA-site cations, 〈rA 〉. In the small 〈rA 〉 regime (e.g., Y0.5Ca0.5MnO3), charge ordering occurs in the paramagnetic state, the transformation to an antiferro-magnetic state occurring at still lower temperatures. At moderate 〈rA 〉 values (e.g., Nd0.5Sr0.5MnO3), a ferromagnetic metallic state transforms to a charge-ordered antiferromagnetic state with cooling. These two distinct types of charge ordering and associated properties are explained in terms of the variation of the exchange couplingsJ FMandJ AFMwith 〈rA 〉 and the invariance of the single-ion Jahn–Teller energy with 〈rA 〉. A qualitative temperature–〈rA 〉 phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different 〈rA 〉 regimes.