Direct spectroscopic evidence of increased stiffness in a series of similarly sized nanopolycrystalline diamond-lonsdaleite grains in Popigai impact diamonds

Abstract

The extreme vibrational properties of unique Popigai impact diamonds were studied using second-order micro-Raman scattering with ultraviolet excitation, allowing the study of the phonon density of states over the entire Brillouin zone. Nanopolycrystalline samples with an average size of 200 μm, representing a composite of cubic diamond and hexagonal lonsdaleite phases, were separated and processed from complex graphite-diamond mixed morphology samples collected from the Popigai Astrobleme, formed by an asteroid impact 35 million years ago in northeastern Russia. It was found that the optical edge phonons of the Brillouin zone, which are the main contributors to the second-order Raman spectra, are very sensitive to the local microscopic environment and the presence of the hexagonal phase. The frequency shift of the Raman spectral maximum at 2450 cm-¹, corresponding to the phonon density of states of the longitudinal optical modes, was analyzed in a series of nearly uniformly sized nanopolycrystalline grains (∼100 nm) as a function of lonsdaleite content. An anomalous steady increase in the spectral maximum frequency was observed up to ∼25 % hexagonal phase content, which directly indicates an increase in the stiffness of the carbon-carbon bond and thus an increase in the stiffness of the two-phase lattice.