  Searching for a universal trend by the same tuning method in different
high-temperature superconductors with a similar crystal structure is a common
strategy to find clues for a better understanding of the superconducting
mechanism in a unified way. It is known that the hole-doped bismuth-oxide
Ba1-xKxBiO3 possesses a similar perovskite structure to that of the hole-doped
copper-oxide (cuprate) superconductors but also holds a comparatively high
superconducting transition temperature. In this study, we report the first
observation of the pressure-induced quantum phase transition (QPT) from
superconducting to insulating states in a series of Ba1-xKxBiO3 single-crystal
samples. A similar QPT has also been observed recently in the compressed
cuprate superconductors1. Significantly, we found that the QPT observed in
Ba1-xKxBiO3 is intriguingly associated with the valence change of the Bi ions
in the material. These results lead us to propose that the pressure-induced
valence change from Bi3+ to Bi5+ destroys the hole-doping effect on stabilizing
the conductivity and corresponding superconductivity. By comparing the
high-pressure behaviors observed in these two kinds of oxides, we identified
another prominent feature shared by them - the more the hole-doping
concentration, the higher the critical pressure required for driving the QPT.
