Bond switching from two- to three-dimensional polymers ofC60at high pressure

In situ high pressure x-ray diffraction experiments revealed that a transformation from the two-dimensional (2D) tetragonal ${\mathrm{C}}_{60}$ polymer to a three-dimensional (3D) polymer takes place via a highly anisotropic deformation of ${\mathrm{C}}_{60}$ molecules along the c axis, as an irreversible first-order transformation above 20 GPa. In the 3D polymer phase, the $2+2$ bonds remain in the 2D plane, while neighboring layers are connected by the $3+3$ bonds. The bulk modulus of the 3D polymer was 407 GPa, being slightly smaller than that of diamond.