The orthorhombic γ-black phase of CsPbI 3 is well-known to be unstable at room temperature and strategies are needed to counteract its transformation tendency. In this paper we compare γ-black CsPbI 3 thin films (∼ 80 nm) formed via two different routes: a fast quenching of the cubic α-phase from 325 C (HT-γ) or spontaneously cooling the layer from 80 C (LT-γ). The successful application of the second procedure is allowed by the use of a mother solution containing Europium with an Eu/Pb ratio as small as 5%. This has been indeed used to form both LT-γ and HT-γ thin films. The phase transition during the heating and cooling pathways is followed in situ by spectroscopic ellipsometry and x-ray diffraction analyses. We demonstrate that both γ-black phases exhibit the same absorption features and critical points as depicted in very details by the dielectric functions. Minor differences can be found in the intensity of …