  The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0
$\leq x \leq$ 0.10) was investigated with high resolution powder x-ray
scattering. In agreement with previous studies, the parent compound SmFeAsO
exhibits a tetragonal-to-orthorhombic structural distortion at
T$\rm{_{S}}$~=~130~K which is rapidly suppressed by $x \simeq$ 0.10 deep within
the superconducting dome. The change in unit cell symmetry is followed by a
previously unreported magnetoelastic distortion at 120~K. The temperature
dependence of the thermal expansion coefficient $\alpha_{V}$ reveals a rich
phase diagram for SmFeAsO: (i) a global minimum at 125 K corresponds to the
opening of a spin-density wave instability as measured by pump-probe
femtosecond spectroscopy whilst (ii) a global maximum at 110 K corresponds to
magnetic ordering of the Sm and Fe sublattices as measured by magnetic x-ray
scattering. At much lower temperatures than T$\rm{_{N}}$, SmFeAsO exhibits a
significant negative thermal expansion on the order of -40~ppm~$\cdot$~K$^{-1}$
in contrast to the behavior of other rare-earth oxypnictides such as PrFeAsO
and the actinide oxypnictide NpFeAsO where the onset of $\alpha <$ 0 only
appears in the vicinity of magnetic ordering. Correlating this feature with the
temperature and doping dependence of the resistivity and the unit cell
parameters, we interpret the negative thermal expansion as being indicative of
the possible condensation of itinerant electrons accompanying the opening of a
SDW gap, consistent with transport measurements.
