Unique Nanomechanical Properties of Diamond–Lonsdaleite Biphases: Combined Experimental and Theoretical Consideration of Popigai Impact Diamonds

For
the first time, lonsdaleite-rich impact diamonds from one of
the largest Popigai impact crater (Northern Siberia) with a high concentration
of structural defects are investigated under hydrostatic compression
up to 25 GPa. It is found that, depending on the nature of a sample,
the bulk modulus for lonsdaleite experimentally obtained by X-ray
diffraction in diamond-anvil cells is systematically lower and equal
to 93.3–100.5% of the average values of the bulk moduli of
a diamond matrix. Density functional theory calculations reveal possible
coexistence of a number of diamond/lonsdaleite and twin diamond biphases.
Among the different mutual configurations, separate inclusions of
one lonsdaleite (001) plane per four diamond (111) demonstrate the
lowest energy per carbon atom, suggesting a favorable formation of
single-layer lonsdaleite (001) fragments inserted in the diamond matrix.
Calculated formation energies and experimental diamond (311) and lonsdaleite
(331) powder X-ray diffraction patterns indicate that all biphases
could be formed under high-temperature, high-pressure conditions.
Following the equation of states, the bulk modulus of the diamond
(111)/lonsdaleite (001) biphase is the largest one among all bulk
moduli, including pristine diamond and lonsdaleite.