Tight-binding theory of lanthanum strontium manganate
An earlier analysis of manganese oxides in various charge states indicated that free-atom term values and universal coupling gave a reasonable account of the cohesion. This approach is here extended to LaxSr(1-x)MnO3 in a perovskite structure, and a wide range of properties, with comparable success, including the cohesion, as a function of x. Magnetic and electronic properties are treated in terms of the same parameters and the cluster orbitals used for cohesion. This includes an estimate of the Neel and Curie-Weiss temperatures for SrMnO3, an antiferromagnetic insulator, and the magnitude of a Jahn-Teller distortion in LaMnO3 which makes it also insulating with (100) ferromagnetic planes (due to a novel double-exchange for the distorted state), antiferromagnetically stacked, as observed. We estimate the Neel temperature and its volume dependence, and the ferromagnetic Curie-Weiss temperature which applies between the Neel and Jahn-Teller temperatures. We expect hopping conductivity when there is doping (0<x<1) and estimate it in the context of small-polaron theory. It is in accord with experiment between the Neel and Jahn-Teller temperatures, but below the Neel temperature the conduction appears to be band-like, for which we estimate a hole mass as enhanced in large-polaron theory. We see that above the Jahn-Teller temperature LaMnO3 should be metallic as observed, and paramagnetic with a ferromagnetic Curie-Weiss constant which we estimate. Many of these predictions are not so accurate, but are sufficiently close to provide a clear understanding of all of these properties in terms of a simple theory and parameters known at the outset. We provide also these parameters for Fe, Co, and Ca so that formulae for the properties can readily be evaluated for similar systems.