MEASUREMENT OF ANISOTROPIES OF THE KINETIC-ENERGY AND ANHARMONICITY IN PYROLYTIC-GRAPHITE BY NEUTRON COMPTON-SCATTERING

Neutron Compton scattering (NCS) measurements of the anisotropy of the momentum distribution and the mean Laplacian of the interatomic poten tial [del(2)V] have been performed using electron volt neutrons, with wave vector transfers between 24 Angstrom(-1) and 98 Angstrom(-1). The measured momentum distribution of the atoms displays significantly mo re anisotropy than a calculation using a model density of states. We h ave observed anisotropies in [del(2)V] for the first time. The results suggest that the atomic potential is harmonic within the graphite pla nes, but anharmonic for vibrations perpendicular to the planes.