  We report on measurements of the crystal structure and hole density in a
series of as-grown and annealed GaMnAs samples. The measured hole densities are
used to obtain the fraction of incorporated Mn atoms occupying interstitial and
substitutional sites. This allows us to make a direct comparison of the
measured lattice parameters with recent density functional theory (DFT)
predictions. We find that the decrease in lattice constant observed on
annealing is smaller than that predicted due to the out diffusion of
interstitial Mn during annealing. The measured lattice parameters after
annealing are still significantly larger than that of GaAs even in samples with
very low compensation. This indicates that the intrinsic lattice parameter of
GaMnAs is significantly larger than that of GaAs, in contradiction to the DFT
prediction.
