We have carried out diffusion Monte Carlo calculations for an A1B-1-stacked bilayer blue phosphorene to find that it undergoes a semiconductor-metal transition as the interlayer distance decreases. While the most stable bilayer structure is a semiconducting one with two monolayers coupled through a weak van der Waals interaction, the metallic bilayer at a shorter interlayer distance is found to be only metastable. This is in contrast to a recent theoretical prediction based on a random phase approximation that the metallic phase would be the most stable bilayer configuration of blue phosphorene. Our analysis of charge density distributions reveals that the metastable metallic phase is induced by interlayer chemical bonding and intralayer charge redistributions. This study enriches our understanding of interlayer binding of a blue phosphorene and contributes to the establishment of correct energetic order between its different phases, which will be essential in devising an experimental pathway for a metallic phosphorene.
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