The bond lengths and bond angles of orthorhombic black phosphorus have been determined as a function of hydrostatic pressures to 26.6(5) kbar using time-of-flight neutron powder diffraction. We show that the markedly anisotropic compression reported previously results from a large pressure-induced shortening of the van der Waals bonds separating layers of atoms combined with a shear motion within the layers. Covalently bonded chains of atoms along the a direction remain very rigid. The average effective linear compressibility for van der Waals bonds is 1.48(9) ×10−3 kbar−1 while the average effective linear compressibility for covalent bonds is an order of magnitude smaller, 2.6(8) ×10−4 kbar−1.

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