the gas flow tend to lose less P, possibly because they are exposed to a finite P partial pressure due to P evaporating from the samples further upstream. We observe a decrease in the apparent heat capacity of an as-deposited CuP 2.5 film at around 300 • C by nanocalorimetry (Fig. ). This indicates an exothermic signal, which may be related to the transition from the amorphous to the (more stable) polycrystalline state. The heat capacity baseline needed to calculate the heat of crystallization is not straighforward to define. If we assume the baseline shown in Fig. (c), we estimate the crystallization energy of CuP 2 as 13 meV/atom. Although the uncertainty on this value is substantial, some qualitative conclusions can still be drawn. The calculated formation enthalpy of CuP 2 from the elements in their standard state is 112 meV/atom. Since this value is much larger than the estimated crystallization energy, most of the formation energy has probably already been released during formation of the amorphous compound. The thermal energy of a solid at 400 • C is approximately 3kT = 174 meV/atom using the Dulong-Petit law. Thus, the extra stabilization achieved by crystallizing CuP 2 is only a small fraction of the thermal energy available at that temperature.