Structural effects on the magnetic and transport properties of perovskite $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ ($x=0.25,$ 0.30)

The evolution of the structural properties of $A_{1-x}{A}_x^{\prime }{\mathrm{MnO}}_3$ was determined as a function of temperature, average $A$-site radius $〈r_A〉,$ and applied pressure for the “optimal” doping range $x=0.25,$ 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, 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 temperatures for all values of $〈r_A〉,$ and the spin-canted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of $〈r_A〉,$ are characterized by large metric distortions of the ${\mathrm{MnO}}_6$ octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low $〈r_A〉.$ These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at $T_C$ as a function of “chemical” and applied pressure.