  High resolution synchrotron X-ray powder diffraction (SXRPD) was used to
study the temperature dependence of the oxygen deficient NdFeAsO$_{0.85}$
superconducting compound. By employing a dense temperature sampling we have
managed to reveal unnoticed structural modifications that start around
$\sim$180K, and disappear at the transition temperature. The data show minor
changes of the structural characteristics in the Nd-O charge reservoir layer
while in the superconducting Fe-As layer the FeAs$_{4}$ tetrahedron shows
gradual modifications below $\sim$180K, which suddenly disappear at T$_{\rm c}$
strongly indicating a connection with superconductivity.
