The perovskite-derived crystal structure of (NH 3 (CH 2 ) 7 NH 3 ) 2 Sn 3 I 10 exhibits three-dimensionally connected Sn-I coordination polyhedra, in contrast to the layered Ruddlesden-Popper derivatives that typically form from large diammonium cations. This structural model agrees with the model initially published in a thesis. (NH 3 (CH 2 ) 7 NH 3 ) 2 Sn 3 I 10 crystallizes in the monoclinic space group C2/c symmetry as revealed by SCXRD data (Details in Tables 1 Given the strong twinning of the crystal that is not captured in the analysis, the solved structure is used for further refinement against high-resolution synchrotron powder X-ray diffraction data. Bulk powder synthesis facilitates this analysis and subsequent measurement of optical and electronic properties. In the powder diffraction data refinement, the C, N, and H positions and isotropic atomic displacement parameters (ADP) are not refined from the coordinates determined by SCXRD. However, the lattice parameters, Sn atom, and I atom positions are refined. The isotropic atomic displacement parameters (ADP) of Sn and I are refined but linked to the same value. The fit quality and comparison to the refined pattern calculated from the SCXRD-determined structure are illustrated in Figure (R wp = 5.68%). The atomic position for I(3) displays split-site occupancy but is close to the special coordinate, (1/2, 1/2, 1/2). The alternative hypothesis without split-site occupancy disorder of I(3) was also tested against the PXRD data (R wp = 5.77%), which also resulted in a larger isotropic thermal displacement parameter (7.2(3) Å2 vs 3.1(4) Å2 with split site disorder). While the split-site model generates a slightly worse fit, the singlesite model without disorder is more amenable for further analysis using first-principles calculations.