Lattice response to the radiation damage of molecular crystals: radiation-induced versus thermal expansivity

The interaction of intense synchrotron radiation with molecular crystals frequently modifies the crystal structure by breaking bonds, producing fragments and, hence, inducing disorder. Here, a second-rank tensor of radiation-induced lattice strain is proposed to characterize the structural susceptibility to radiation. Quantitative estimates are derived using a linear response approximation from experimental data collected on three materials Hg(NO₃)₂(PPh₃)₂, Hg(CN)₂(PPh₃)₂ and BiPh₃ [PPh₃ = triphenylphosphine, P(C₆H₅)₃; Ph = phenyl, C₆H₅], and are compared with the corresponding thermal expansivities. The associated eigenvalues and eigenvectors show that the two tensors are not the same and therefore probe truly different structural responses. The tensor of radiative expansion serves as a measure of the susceptibility of crystal structures to radiation damage.