  The effect of pressure on the crystalline structure and superconducting
transition temperature (Tc) of the 111-type Na1-xFeAs system using in situ high
pressure synchrotron x-ray powder diffraction and diamond anvil cell techniques
is studied. A pressure-induced tetragonal to tetragonal isostructural phase
transition was found. The systematic evolution of the FeAs4 tetrahedron as a
function of pressure based on Rietveld refinements on the powder x-ray
diffraction patterns was obtained. The non-monotonic Tc(P) behavior of
Na1-xFeAs is found to correlate with the anomalies of the distance between the
anion (As) and the iron layer as well as the bond angle between As-Fe-As for
the two tetragonal phases. This behavior provides the key structural
information in understanding the origin of the pressure dependence of Tc for
111-type iron pnictide superconductors. A pressure-induced structural phase
transition is also observed at 20 GPa.
