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.
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