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

Superconducting Tl<SUB>1&#8722;&#948;</SUB>CaBa<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>7</SUB> can be prepared without impurity from the corresponding two Tl-O layer cuprate only when some Tl deficiency is incorporated. The Tl<SUB>1&#8722;&#948;</SUB>CaBa<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>7</SUB> and Tl<SUB>1&#8722;&#948;</SUB>Y<SUB>1&#8722;x</SUB>Ca<SUB>x</SUB>Ba<SUB>2</SUB>Cu<SUB>2</SUB>O<SUB>7</SUB> series show maximum T<SUB>c</SUB> at an optimal hole concentration. The T<SUB>c</SUB> maximum occurs at a Cu&#8212;O distance of ca. 1.924 &#197;