Structure at 200 and 298 K and X-ray Investigations of the Phase Transition at 242 K of [NH_2(CH_3)_2]_3Sb_2Cl_9 (DMACA)

摘要

[NH2(CH3)2]3Sb2Cl9 (dimethylammonium nonachloro-diantimonate, DMACA) has, at 200K, a monoclinic Pc space group, with a = 9.470(3), b = 9.034(3), c= 14.080 (4) A, β= 95.81 (3)°. V = 1198.4(4) A3, Z = 2 [R = 0.024, wR = 0.025 for 4613 independent reflections with F > 4σ(F)]. At 298 K DMACA has P21/c space group with a = 9.686(3), b = 9.037 (3), c= 14.066 (4) A, β = 95.57(3)°, V = 1225.3 (5) A3, Z = 2 [R = 0.034, wR = 0.035 for 2736 reflections with F > 4σ(F)]. The anionic sublattice of DMACA consists of polyanionic (Sb2Cl3-9)n layers. In the low-temperature phase there are three crystallographically non-equivalent dimethylammonium cations in the crystal structure. One of the cations is located inside the polyanionic layers, two others - one ordered and one disordered - between the polyanionic layers. In the room-temperature phase there are two non-equivalent cations - both disordered - in the crystal structure. Temperature dependencies of lattice parameters between 200 and 300 K were determined. The occurrence of a second-order phase transition at T = 242K was confirmed. The dependence of lengths of Sb-Cl contacts on the presence and strength of N-H...Cl hydrogen bonds was discussed. It was found that lengths of Sb-Cl bonds may differ from each other by as much as 0.3 A, because of the presence of N-H...Cl hydrogen bonds. These differences were attributed to distortion of the lone-electron pair on antimony(III).