Isotopic dependence of the lattice constant of diamond

We report measurements of lattice constants of single-crystal synthetic diamonds that cover the range of isotopic compositions from almost-pure ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ to almost-pure ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$. These provide a demonstration of an effect of isotopic composition on the lattice constant of this material. The results are consistent with a linear dependence ${\mathit{a}}_0$(C,25 °C,x)=3.567 15-0.000 53x Å, where x is the atom fraction of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$. This corresponds to a fractional change between the end compositions of Δ${\mathit{a}}_0$/${\mathit{a}}_0$=-1.5×${10}^{\mathrm{-}4}$, which is less than the smallest value in a range of theoretical estimates that have been published previously. The synthetic diamonds yielded (400) Bragg peaks with widths in the range 12–17 arc sec that are not perceptibly dependent on the isotopic composition.