Effect of isotopic constitution of diamond on its elastic constants: C 13 diamond, the hardest known material

Elastic moduli of C-13 diamond, deduced from the Brillouin shifts experienced by inelastically scattered monochromatic radiation, are about 0.5 percent higher than the corresponding values for natural diamond. This suggests C-13 diamond is harder than natural diamond. A simple model incorporating the zero-point motion and the anharmonicity of the interatomic forces is proposed; it explains quantitatively the increase in the elastic moduli, the decrease in the lattice parameter, and a correction to the Raman frequency beyond its M exp -1/2 dependence (where M is the average atomic mass) observed for C-13 diamond with respect to their values for natural diamond. (Author (revised))