Ultrasonic phase velocity and elastic modulus in isotopically enhanced manufactured diamonds

We have performed ultrasonic pulse-echo experiments to measure the longitudinal and transverse acoustic velocities in nine single-crystal manufactured diamonds. The 13C concentration in the samples ranged from nearly 0% to 99%. Small (4%–5%) but distinct decreases in both the longitudinal and transverse 〈100〉 phase velocities with increasing 13C content were observed. Using these velocities and density data, values for the cubic elastic moduli c11 and c44 were determined. Within experimental uncertainty (typically