ISOTOPE 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 C-12 to almost-pure C-13. 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 a0(C,25-degrees-C,x) = 3.56715-0.00053x angstrom, where x is the atom fraction of C-13. This corresponds to a fractional change between the end compositions of DELTA-a0/a0 = -1.5 x 10(-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.