Abstract:
Charge ordering in rare earth manganates of the typeLn0.5A0.5MnO3(Ln=rare earth,A=alkaline earth) is highly sensitive to the average radius of the A-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 couplings JFM and JAFM with 〈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.
