  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.
