Family tree of perovskite-related superconductors
  A simple empirical relation has been found to exist between optimum Tc and the formal mean cation charge <qc> of perovskite-related superconductors, covering both conventional superconductors and superconducting cuprates. Tc is shown to increase exponentially with decreasing <qc>. It is suggested that a Ba-based cuprate with <qc> about 2 reach a Tc around 200 K. The strong correlation may be thought of as an indication for a common mechanism of superconductivity of the whole family of compounds. In addition, the structural assignment for the 'cubic' high-Tc phase reported by Volkov (Tc = 117 K) to the orthorhombic BaCuO2.5 prototype is proposed. 
Hans Hermann Otto