The room temperature x-ray diffraction (XRD) patterns of the compounds of the (CnH2n+1NH3)2PbI4 and (CnH2n+1NH3)2PbBr4 systems (n= 6, 8, 10, 12, 14 and 16) are reported in Figures and, respectively. The patterns of both series are dominated by the (00l) reflections of the long axis typical of the low-dimensional perovskites and related to their platelike morphology. The crystal structures of the (CnH2n+1NH3)2PbI4 (n= 6, 8, 10, 12, 14 and 16) compositions are known in the current literature and match with the present diffraction patterns of Figure . The patterns of the analogous bromide series have been treated through Le Bail method starting from the crystal structures of the I-containing analogues, providing good fits of the data. From the patterns of Figures and it is evident a progressive shift of the main peaks ((00l) reflections) to lower angles by increasing the length of the organic ligand, indicating the progressive expansion of the unit cell. The trend of the c-axis dimension vs. the The expansion of c lattice parameter for both series of 2D perovskites is roughly linear with an increase of the long axis with the number of carbon atoms of about 5 Å in the range 6≤n≤10, while a jump of ~7 Å is found in both series when passing from 10 to 12, and then the difference returns to be around 5. sheets, forming an alternated inorganic-organic layered structure. On the other hand, according to the sequence of phase transition found in these 2D perovskites, when moving to n = 12, the stable phase at RT, still orthorhombic (S.G. Pbca), is Phase III. In this novel phase, two main modifications occur: in the inorganic framework, in terms of bond angles (see later in the text), and in the tilt angle of the ammonium group which may account for the observed increase of the long axis of about ~7 Å. As a matter of fact, the transition from Phase II to Phase III leads to an increase, for example in (C10H21NH3)2PbI4, of about 1.5 Å. Phase III is then the stable RT arrangement for the n = 14 and 16 members of the (CnH2n+1NH3)2PbI4 series, thus further keeping the change in the c-axis around 5 Å. Interestingly, such behavior is also found for the (CnH2n+1NH3)2PbBr4 series where the sequence of phase transitions is analogous to that of the iodide-counterpart even though slightly shifted to lower temperatures, as determined by Differential Scanning Calorimetry (see Figures and of the Supporting Information, SI). This result will be discussed later when presenting the results of the optical properties of the investigated samples.