In agreement with nanocalorimetry results, the originally amorphous CuP 2+x films only begin to show crystalline XRD peaks above 300 • C annealing temperature (Fig. , Supporting Information). Beyond this lower limit, it is possible to obtain polycrystalline CuP 2+x films in the P2 1 /c structure under various annealing conditions. As long as the final composition is close to the nominal CuP 2 stoichiometry, XRD patterns of films processed under different annealing conditions are rather similar (Fig. , Supporting Information). When the fi-nal P/Cu ratio is significantly less than 2, XRD peaks associated with Cu 3 P in the hexagonal P6 3 cm structure begin to appear (Fig. , Supporting Information). As an example, the XRD pattern of a CuP 2 film annealed at 450 • C for 30 s (Fig. ) contains all the peaks expected for the P2 1 /c structure, without major preferential orientation effects and without clear peaks from secondary phases above the noise level.