Atomistic computer modelling was used to investigate the structures of the YBCO family of superconductors, and the diffusion of Li+ ions through Ba channels. The calculated structures agreed well with the experimental data for both the 123 and 124 structures. However, a slight modification of the 124 structure was necessary in order to accord with experimental evidence. The calculated activation energy (0.8eV) for Li+ diffusion in the 124 structure agreed well with experimental results (about 0.5eV). It was found that the calculated a- and b-axes for the 125 structure were significantly different to those which were obtained for the 123 and 124 structures; thus suggesting that these structures might be incommensurate with the 125 structure and implying that this could explain the difficulty which was experienced in creating the 125 structure. It was noted that there was a significant movement of the Li ion in the c-axis of the 124 structure during calculations of the diffusion profile. Preliminary calculations suggested that this value would not be significantly lower in the 125 structure. It was concluded that the O(4) site in the 124 system should be regarded as consisting of 2 sites of 0.5 occupancy which were about 0.08nm apart along the a-axis.

G.Morrison, M.A.Alario-Franco, E.Moran, A.V.Chadwick: Materials Science Forum, 1997, 239-241, 403-6