LONG PERIOD MIXED LAYER POLYTYPES IN THE TL2BA2CANCUN+1O2N+6 SUPERCONDUCTOR

By means of high resolution electron microscopy and electron diffraction it is shown that the high-Tc superconductors in the homologous series Tl2Ba2Cun+1O2n+6 exist up to at least n=4. It is found futhermore that mixed-layer polytypes with very large unit cells of several tens of nm, built on complicated layer sequences, occur. The possibility of polytype formation is shown to be related to the simultaneous occurrence of perovskite lamellae with different thicknesses and moreover to the fact that single layer as well as double layer Tl-O lamellae can occur in the same crystal. The most probable formation mechanism is considered to be spiral growth.