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.

The lattice parameters, bond lengths and bond angles of Si2N2O have been determined at pressures up to 23 kbar by time‐of‐flight neutron diffraction. The average compressibility coefficients of the orthorhombic structure are Ka = 21 (2), Kb = 34 (2), Kc = 25 (2) and Kν = 79 (4) x 10−5 kbar−1, though the anisotropy is more pronounced at lower pressures with Ka (at zero pressure) estimated to be 10 (8) × 10–5 kbar−1. The major contribution to the compression is a cooperative rotation of adjacent SiN3O tetrahedra brought about by a decrease in the Si–O–Si bond angle. The shortening of one Si–N bond within each tetrahedron further enhances the compression, especially along the b direction.

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High-pressure time-of-flight neutron diffraction studies have been carried out on orthorhombic Ge2N20 to 25.4 × 108 Pa and by X-ray diffraction to 49 × 108 Pa.