La(8-x)Sr(x)Cu8O(20-δ) (x = 1.3, 1.5, and 1.9) compounds were fluorinated using XeF2 as a fluorinating agent. The formation of a new tetragonal complex oxyfluoride La0.813Sr0.187Cu(O, F)(3-δ) was observed. The structure of the new phase was refined using X-ray powder data (a = 3.7921(3) Å, c = 4.0515(4) Å, S. G. P4/mmm, R1 = 0.020, R(P) = 0.045, R(wP) = 0.057) and was confirmed by electron diffraction and high-resolution electron microscopy (HREM). The tetragonal distortion of the perovskite unit cell seems to be due to a Jahn-Teller deformation of the copper coordination environment with the formation of four short (1.896 Å) and two long (2.026 Å) Cu-(O, F) distances. Domains of the La0.813)Sr0.187)Cu(O, F)(3-δ) phase were observed by HREM inside a matrix of a slightly fluorinated initial phase. (C) 2000 Academic Press.