Synthesis, Crystal Structures, Optical Properties, and Phase Transitions of the Layered Guanidinium-Based Hybrid Perovskites [C(NH2)3]2MI4; M = Sn, Pb

The title compounds were obtained by the treatment of MI2 in conc. HI with stoichiometric amounts of [C(NH2)(3)](2)CO3. They show several phase transitions. The Pb/Sn HT phases {[C(NH2)(3)](2)PbI4-I, Pnma, Z = 4, a = 13.04 angstrom, b = 13.62 angstrom, c = 9.36 angstrom} and {[C(NH2)(3)](2)SnI4-I: a = 13.04 angstrom, b = 13.57 angstrom, c = 9.45 angstrom}, as well as [C(NH2)(3)](2)PbI4-II (Pnnm, Z = 8, a = 27.03 angstrom, b = 9.31 angstrom, c = 12.92 angstrom) and [C(NH2)(3)](2)SnI4-II (a = 26.89 angstrom, b = 9.30 angstrom, c = 12.94 angstrom) and also [C(NH2)(3)](2)PbI4-III (P2(1)/n, Z = 8, a = 9.21 angstrom, b = 26.88 angstrom, c = 12.68 angstrom, beta = 90.49 degrees) and [C(NH2)(3)](2)SnI4-III (a = 9.28 angstrom, b = 26.95 angstrom, c = 12.81 angstrom, beta = 90.76 degrees) are isotypic with each other. For [C(NH2)(3)](2)SnI4 the LT phase realizes a triclinic variant (P (1) over bar, Z = 4, a = 9.27 angstrom, b = 12.67 angstrom, c = 14.44 angstrom, alpha = 66.98 degrees, beta = 85.88 degrees, gamma = 88.26 degrees). The structures are (110)-oriented perovskites A(2)MX(4) with corrugated layers of edge-sharing MI6 octahedra. HT-and MT-forms show a 2+4 pattern of M-I distances. [C(NH2)(3)](2)SnI4-IV represents a new structure type. The different modifications are distinguished by the ordering pattern of the cations and the tilt of the MI6 octahedra. The structural characterizations are amended by vibrational and optical spectroscopy as well as thermal analysis. The band gap of the Sn compound (2.06 eV) makes it a possible candidate as an absorber for the tandem solar cell. For the Pb compound an excitonic absorption peak is observed right below the band gap (2.49 eV) indicating stable excitons at room temperature.