Structure and superconducting properties of the Tl 1−δ CaBa 2 Cu 2 O 7 and Tl 1−δ Y 1−x Ca x Ba 2 Cu 2 O 7 series

Superconducting Tl 1−δ CaBa 2 Cu 2 O 7 can be prepared without impurity from the corresponding two Tl-O layer cuprate only when some Tl deficiency is incorporated. The Tl 1−δ CaBa 2 Cu 2 O 7 and Tl 1−δ Y 1−x Ca x Ba 2 Cu 2 O 7 series show maximum T c at an optimal hole concentration. The T c maximum occurs at a Cu—O distance of ca. 1.924 A.