STRUCTURAL EFFECTS ON THE MAGNETIC AND TRANSPORT-PROPERTIES OF PEROVSKITE A(1-X)A(X)'MNO3 (X=0.25, 0.30)

The evolution of the structural properties of A(1-x)A'xMnO3 was determ ined as a function of temperature, average A-site radius (r(A)), and a pplied pressure for the ''optimal'' doping range x = 0.25, 0.30, by us ing high-resolution neutron powder diffraction. The metal-insulator tr ansition,which can be induced both as a function of temperature and of (r(A)) was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high te mperatures for all values of (r(A)), and the spincanted ferromagnetic charge-ordered phase, which is found at low temperatures for low value s of (r(A)), are characterized by large metric distortions of the MnO6 octahedra. These structural distortions are mainly incoherent with re spect to the space-group symmetry, with a significant coherent compone nt only at low (r(A)). These distortions decrease abruptly at the tran sition into the ferromagnetic metal phase. These observations are cons istent with the hypothesis that, in the insulating phases, lattice dis tortions of the Jahn-Teller type, in addition to spin scattering, prov ide a charge-localization mechanism. The evolution of the average stru ctural parameters indicates that the variation of the electronic bandw idth is the driving force for the evolution of the insulator-to-metal transition al T-C as a function of ''chemical'' and applied pressure.