Crystal structure and superconductivity of La_2-xBa_xCuO₄ (0.03≤x≤0.24)

High-resolution neutron powder diffraction has been performed to investigate the crystal structure of La_2-xBa_xCuO₄ as a function of x(0.03≤x≤0.24). The crystal parameters at 115 K refined by profile analysis show that the Cu-planar O(1) bond lengths decrease by 0.008 Å when x is increased by 0.1. This result indicates that the overlap of Cu and planar O orbitals, and thus the charge transfer, increases with x. The length of the Cu-apical O(2) bonds decreases at a rate of about one half of that for Cu-O(1), suggesting that the apical oxygens O(2) also play an important role in the charge transfer process. The changes of the Cu-O(1) bond lengths of the compound with x=0.125 at 15 K are further studied in connection with the structural phase transition to the low-temperature tetragonal (LTT) phase. These results are discussed in terms of their implication for the electronic structure and superconductivity of this system.