Long period mixed layer polytypes in the Tl2Ba2CanCu1+nO6+2n superconductors

Abstract

High Tc superconductors in the homologous series Tl2−xBa2CanCu1+nO6+2n can exist up to at least n=4. It is found furthermore by electron microscopy and electron diffraction that mixed layer polytypes with very large unit cells, up to several hundred Angström 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 TlO layers may occur. The critical temperature is slightly affected by these modulations. Susceptibility measurements indicate a Tc of 117K is found; R=0 was obtained at 110K.