Since the film is polycrystalline, there are selection rules for Raman-active phonon modes and the Raman spectrum will not directly reflect the phonon DOS. Specifically, all atoms in CuP 2 are at 4e Wyckoff positions of the P2 1 /c space group, so only the A g and B g modes are Raman-active according to the character tables. With a 12-atom unit cell, the total number of Raman-active modes is (12 -1) × 2 = 22. Eight peaks can be identified the experimental spectrum (Fig. ). The Cu-Cu rattling mode identified by Qi et al. as an important scatterer of heat-transporting phonons is either symmetry-forbidden or too low in intensity to be distinguished by Raman spectroscopy.