Atomistic mechanism of the Mott transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>PbCr</mml:mi> <mml:msub> <mml:mi mathvariant="normal">O</mml:mi> <mml:mn>3</mml:mn> </mml:msub> </mml:mrow> </mml:math>

The Mott transition is primarily characterized by an isostructural insulator-metal transition in a correlated system under pressure, which is at the heart of many emergent phenomena. The associated transition mechanism, however, has been a long-standing challenge. Here we present a study of the mechanism underlying Mott transition in $\mathrm{PbCr}{\mathrm{O}}_{3}$ by exploring its bonding and electronic behaviors under pressure, which involves intricate lattice-charge interactions. We find that its Mott transition is mainly dictated by the Cr--O sublattice with largely tunable electronic and elastic properties under tensile stress, as exerted by the contracted Pb--O sublattice due to the presence of bond-length mismatch. Pressure can effectively balance tensile stress in the Cr--O sublattice for shortening its bond to a critical length, at which the $\mathrm{Cr}\phantom{\rule{4pt}{0ex}}3d\text{--O}\phantom{\rule{4pt}{0ex}}2p$ covalent hybridization occurs to form strong Cr--O bonds with a profoundly enhanced stiffness. As a response, extra bond shrinking spontaneously takes place to drive $3d$-electron delocalization and lattice collapse for the Mott transition. In addition, the electronic stability and structural symmetry of this material are also revealed to be closely related to its valence-electron density. These discoveries provide powerful insights into the atomistic mechanism of the Mott transition, which could also be applicable for understanding structural transitions in many other perovskite compounds.