Atomic dynamics of diamond at a “negative” pressure

Changes in the structure and atomic dynamics of diamond irradiated by fast neutrons have been studied. It has been shown that a sample irradiated to a fluence of $10^{21}$ cm$^{-2}$ holds the diamond crystal structure and change in the cubic cell parameter corresponds to a $5\%$ increase in the atomic volume, which can be treated as the application of a “negative” pressure $P < -25$ GPa to the sample. It has been found experimentally that a peak in the phonon spectrum corresponding to optical modes of a crystal lattice is noticeably shifted toward low energies. The Grüneisen parameter for optical modes of diamond at negative pressure has been estimated. It has been established that the Grüneisen parameter of diamond in the pressure range of $-25$ GPa$<P<35$ GPa does not change and is close to unity.