Ultrasonic velocity measurements of isotopically enriched diamonds

We have performed ultrasonic pulse-echo experiments to measure the longitudinal and transverse acoustic velocities in nine single-crystal manufactured diamonds. Small (4-5%) but distinct decreases in both the longitudinal and transverse [100] velocities were observed as the ¹³C content varied from 0% to 99%. The measurements indicated that within our uncertainty (typically <0.5%), c₁₁ was constant with ¹³C content. However, a 2% decrease in C₄₄ was observed. Velocities in [111]-oriented samples remained nearly constant with isotope content. The implications of this result are discussed in terms of the effective elastic constants 1/3(c₁₁ +2c₁₂+4c₄₄) and 1/3(c₁₁-2c₁₂ +4c₄₄) cubic constants c₁₁, c₁₂, and c₄₄