  We investigated LaFeAsO single crystals by means of synchrotron M\"ossbauer
spectroscopy under pressures up to 7.5 GPa and down to 13 K and provide a
microscopic phase diagram. We found a continuous suppression of the magnetic
hyperfine field with increasing pressure and it completely vanishes at $\sim$
7.5 GPa which is in contrast to the behavior in polycrystalline samples where
the magnetic order vanishes at $\sim$ 20 GPa. The different behavior of the
polycrystalline samples might be due to As-vacancies. Our results are in
qualitative agreement with density functional theory calculations where a
reduction of the magnetic moment with increasing pressure was found. We found
that among different samples at ambient pressure the magnetic phase transition
temperature as well as the low-temperature magnetic hyperfine field decrease
with increasing unit cell volume.
