NQR, NMR and Crystal Structure Studies of [C(NH2)3]3Sb2Br9

The crystal structure of [C(NH2)3]3Sb2Br9 was determined at 143 K: monoclinic, space group C2/c, Z = 4, a = 15.695 (3), b = 9.039(2), c 18.364(3) A, β = 96.94(1)o. The structure consists of two crystallographically independent guanidinium ions and two-dimensional corrugated sheets of (Sb2Br93-)n, in which SbBr6 octahedra are connected through three bridging Br atoms each other. One of the cations situates in a cavity of the (Sb2Br93-)n layer with statistical disorder, while the other situates between the layers without disorder. Three 81Br NQR resonance lines were assignable to terminal Br atoms, while only one line was found for two inequivalent bridging Br atoms. All the 81Br NQR resonance lines were subjected to fade-out at low temperatures. The temperature dependence curve of 1H NMR T1 showed well defined two minima, which were explained by postulating the C3 reorientations of two types of cations with very different activation energies. The DTA (DSC) measurement revealed a phase transition of a first-order type at 444 K.