Fig. (b) shows the effect of annealing temperature and time on the final composition. As expected, increasing the annealing time at fixed temperature results in more severe P losses (compare the data from 30 s versus 5 min annealing time at 400 • C). In general, longer annealing times can be tolerated at lower annealing temperatures. For example, annealing at 350 • C for 15 min yields about as many P-poor samples as the case of annealing at 400 • C for 5 min. Note that the spread of final P/Cu ratios sometimes obtained for films of otherwise similar initial P/Cu ratios and thicknesses is mainly caused by the different positions of the samples inside the furnace. The samples located further downstream with respect to FIG. . Qualitative convex hull of Cu-P system at two different temperatures. Among competing phases, only Cu3P and the elements are considered. At 0 K, the Gibbs free energy only consists of enthalpy H, so the convex hull is drawn following DFT enthalpy calculations as available on the Materials Project database. CuP2 is found to be a stable phase (on the convex hull) and Cu3P is slightly metastable (above the convex hull). At 700 K, our experiments indicate that CuP2 is destabilized. Part of the reason may be a large entropic term T S for elemental P, which is in gaseous form at this temperature. When higher-energy amorphous precursors (diamond data points) are rapidly heated to 700 K, decomposition into Cu3P and gaseous P is thermodynamically favored. However, CuP2 formation is kinetically facilitated when the initial composition of the precursors is sufficiently close to the CuP2 stoichiometry (red diamond).