The title compounds were obtained by the treatment of MI 2 in conc. HI with stoichiometric amounts of [C(NH 2 ) 3 ] 2 CO 3 . They show several phase transitions. The Pb/Sn HT phases {[C(NH 2 ) 3 ] 2 PbI 4 -I, Pnma, Z = 4, a = 13.04 Å, b = 13.62 Å, c = 9.36 Å} and {[C(NH 2 ) 3 ] 2 SnI 4 -I: a = 13.04 Å, b = 13.57 Å, c = 9.45 Å}, as well as [C(NH 2 ) 3 ] 2 PbI 4 -II (Pnnm, Z = 8, a = 27.03 Å, b = 9.31 Å, c = 12.92 Å) and [C(NH 2 ) 3 ] 2 SnI 4 -II (a = 26.89 Å, b = 9.30 Å, c = 12.94 Å) and also [C(NH 2 ) 3 ] 2 PbI 4 -III (P2 1 /n, Z = 8, a = 9.21 Å, b = 26.88 Å, c = 12.68 Å, = 90.49°) and [C(NH 2 ) 3 ] 2 SnI 4 -III (a = 9.28 Å, b = 26.95 Å, c = 12.81 Å, = 90.76°) are isotypic with each other. For [C(NH 2 ) 3 ] 2 SnI 4 the LT phase realizes a triclinic variant (P1, Z = 4, a = 9.27 Å, b = 12.67 Å, c = 14.44 Å, α = 66.98°, = 85.88°, γ = 88.26°). The [a] 1125 band gaps, the presented guanidinium-based hybrid perovskites are not suitable as dyes in (single junction) solid-state mesoscopic solar cells. Nevertheless, [C(NH 2 ) 3 ] 2 SnI 4 may be suitable as a top cell in tandem configurations.

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