Acta Crystallographica Section B

Structural Science

Volume 52, Part 2 (April 1996)

research papers

Acta Cryst. (1996). B52, 287-295 [ doi:10.1107/S0108768195010615 ]

Structure at 200 and 298 K and X-ray investigations of the phase transition at 242 K of [NH₂(CH₃)₂]₃Sb₂Cl₉ (DMACA)

J. Zaleski and A. Pietraszko

Abstract: [NH₂(CH₃)₂]₃Sb₂Cl₉ (dimethylammonium nonachlorodiantimonate, DMACA) has, at 200 K, a monoclinic Pc space group, with a = 9.470 (3), b = 9.034 (3), c = 14.080 (4) Å, = 95.81 (3)°, V = 1198.4 (4) Å³, Z = 2 [R = 0.024, wR = 0.025 for 4613 independent reflections with F > 4(F)]. At 298 K DMACA has P2₁/c space group with a = 9.686 (3), b = 9.037 (3), c = 14.066 (4) Å, = 95.57 (3)°, V = 1225.3 (5) Å³, Z = 2 [R = 0.034, wR = 0.035 for 2736 reflections with F > 4(F)]. The anionic sublattice of DMACA consists of polyanionic (Sb₂Cl₉^3-), 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 = 242 K was confirmed. The dependence of lengths of Sb-Cl contacts on the presence and strength of N-HCI hydrogen bonds was discussed. It was found that lengths of Sb-Cl bonds may differ from each other by as much as 0.3 Å, because of the presence of N-HCl hydrogen bonds. These differences were attributed to distortion of the lone-electron pair on antimony(Ill).