In this experiment we will determine the detailed structure of the 6 K superconductor LiFeP and phases close in composition which are iron rich: Li1-xFe1+xP. This will enable the range of superconducting compositions to be determined and will allow correlation of the structures of LiFeP and other iron phosphides with their electronic and superconducting properties. We will also measure the evolution of the crystal structures of the members of the solid solution between LiFeP and LiFeAs in order to enable correlation of structures and properties.