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

Elastic moduli of ¹³C diamond, deduced from the Brillouin shifts experienced by inelastically scattered monochromatic radiation, are ∼0.5% higher than the corresponding values for natural diamond. This suggests ¹³C 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^-1/2 dependence (where M is the average atomic mass) observed for ¹³C diamond with respect to their values for natural diamond.