CHARGE-ORDERING IN MANGANATES

Ordering of Mn3+ and Mn4+ ions occurs in the rare earth manganates of the general composition Ln(1-x)A(x)MnO(3) (Ln rare earth, A = Ca, Sr). Such charge-ordering is associated with antiferromagnetic and insulat ing properties. This phenomenon is to be contrasted with the ferromagn etic metallic behavior that occurs when double-exchange between the Mn 3+ and Mn4+ ions predominates. Two distinct types of charge-ordering c an be delineated. In one, a ferromagnetic metallic (FMM) state transfo rms to the charge-ordered (CO) state on cooling. In the other scenario , the CO state is found in the paramagnetic ground stale and there is no ferromagnetism down to the lowest temperatures. Magnetic fields tra nsform the CO state to the FMM state, when the average radius of the A -site cations is sufficiently large ([r(A)] > 1.17 Angstrom). Chemical melting of the CO state by Cr3+ substitution in the Mn site is also f ound only when [r(A)] greater than or similar to 1.17 Angstrom. The ef fect of the size of the A-cations on the Mn-O-Mn angle is not enough t o explain the observed variations of the charge-ordering temperature a s well as the ferromagnetic Curie temperature T-c. An explanation base d on a competition between the Mn and A-cation orbitals for sigma-bond ing with the oxygen rho(sigma) orbitals is considered to account for t he large changes in T-c and hence the true bandwidth, with [r(A]). Eff ects of radiation, electric field, and other factors on the CO state a re discussed along with charge-ordering in other manganate systems. Co mplex phase transitions, accompanied by changes in electronic and magn etic properties, occur in manganates with critical values of(rA) Or ba ndwidth. Charge-ordering is found in layered manganates, BixCa1-xMnO3 and CaMnO3-delta.