Structure of pentakis(methylammonium) undecachlorodibismuthate(III), [NH3(CH3)]5Bi2Cl11, at 130 K and mechanism of the phase transitions

Pentakis(methylammonium) undecachlorodibismuthate(III), [NH 3 (CH 3 )] 5 Bi 2 Cl 11 , PMACB, at 130 K has the monoclinic structure (MLT). In the low-temperature phase, all the methylammonium cations are ordered (the C-N axes are fixed) and hydrogen bonded by weak N-H...Cl interactions, with distances between 3.04 and 3.42 A. In comparison to the orthorhombic room-temperature phase (ORT), in the MLT phase one can observe only a change in the torsion angle of bioctahedra units without any essential distortion of the BiCl 6 octahedra. The temperature evolution of the monoclinic angle showed that PMACB displays a continuous and subtle orthorhombic→monoclinic (Pca2 1 →P2 1 ) transition of ca 250±10 K. The change in volume of the unit cell around 170 K corroborated the presence of a structural anomaly. It was suggested that the 170 K anomaly is not accompanied by a change in the space group. Analysis of the disordering of the methylammonium cations between 349 and 130 K indicates the leading role played by the methylammonium cations (12) and (22) in the low-temperature transitions at 250 and 170 K. The crystal data of the low-temperature modifications at 130 K are: M r , = 968.3, monoclinic, P2 1 , a = 12.880 (4), b = 13.916 (3), c = 15.059 (1) A, γ = 90.13°, Z = 4, V = 2699 (1) A 3 , D x = 2,383 Mg m -3 , λ(Mo Kα) = 0.7107 A, μ = 140. 878 cm −1 , R = 0.031 for 5931 observed reflections [I > 3σ(I)]