A systematic FTIR study of thorium-doped electron superconductors (T' structure) series is presented in this paper. Even though all Ln(1.85)Th(0.15)CuO(4) compounds belong to T' structure, due to different sizes of rare earth ionic radii, the structure stabilization is very difficult to achieve since copper-oxygen distance varies for different members of the series. The origin of disorder is due to the non-matching of Ln-O/Cu-O bonds in T' structure. The built-in internal pressure is responsible for the change of CuO polyhedron (CuO6 octahedra to CuO2 planar) as relaxing mechanism in the formation of T' compounds from T-structured compounds. This is well established from the appearance or disappearance of Cu-O vibration peaks in the FTIR spectra.