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
¹³
C content varied from 0% to 99%. The measurements indicated that within our uncertainty (typically <0.5%), c
₁₁
was constant with
¹³
C content. However, a 2% decrease in C
₄₄
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(c
₁₁
+2c
₁₂
+4c
₄₄
) and 1/3(c
₁₁
-2c
₁₂
+4c
₄₄
) cubic constants c
₁₁
, c
₁₂
, and c
₄₄