  The crystal structure of LnFeAsO$_{1-y}$ (Ln = La, Nd) has been studied by
the powder neutron diffraction technique. The superconducting phase diagram of
NdFeAsO$_{1-y}$ is established as a function of oxygen content which is
determined by Rietveld refinement. The small As-Fe bond length suggests that As
and Fe atoms are connected covalently. FeAs$_4$-tetrahedrons transform toward a
regular shape with increasing oxygen deficiency. Superconducting transition
temperatures seem to attain maximum values for regular FeAs$_4$-tetrahedrons.
