Samples of Y1‐xCaxBa2Cu4O8 are prepared by means of a new high pressure technique employing the oxygen‐HIP. We find that the transition temperature increases with Ca content, and that Tc=90K is realized at x=0.1. These data are explained by the hole concentration, i.e. average charge per [Cu‐O] site.
KEY‐WORDS: YBa2Cu4O8, Ca‐substitution, Transition temperature, Hole concentration