Long period mixed polytypes in the Ti2Ba2CanCu1+nO6+2n superconductors

High Tc superconductors in the homologous series Tl2-xBa2CanCu1+nO6+2n can exist up to at least n = 4. It is found by electron microscopy and electron diffraction that mixed layer polytypes with very large unit cells, up to several hundred Angstrom occur. These large repeat units originate from a periodic mixture of compounds with n = 4 and n = 3 combined with the fact that both single and double Tl-O layers may occur. The critical temperature is slightly affected by these modulations. Susceptibility measurements indicate a Tc of 117K is found; R = O was obtained at 110 K