Studies of structure and phase transition in [C(NH2)3]HgBr3 and [C(NH2)3]HgI3 by means of halogen NQR, 1H NMR, and single crystal X-ray diffraction

The crystal structure of [C(NH2)(3)]HgBr3 was determined at room temperature: monoclinic, space group C2/c, Z = 4, a = 775.0(2), b = 1564.6(2), c = 772.7(2) pm, beta = 109.12(2)degrees In the crystal, almost planar HgBr3- ions are connected via Hg ... Br bonds, resulting in single chains of trigonal bipyramidal HgBr5 units which run along the c direction. [C(NH2)(3)]HgI3 was found to be isomorphous with the bromide at room temperature. The temperature dependence of the halogen NQR frequencies (77 < T/K < ca. 380) and the DTA measurements evidenced no phase transition for the bromide, but a second-order phase transition at (251 +/- 1) K (T-c1) and a first-order one at (210 +/- 1) K (T-c2) for the iodide. The transitions at T-c2 are accompanied with strong supercooling and significant superheating. The room temperature phase (RTP) and the intermediate temperature phase (ITP) of the iodide are characterized by two (127)(m=1/2<->3/2) NQR lines which are assigned to the terminal and the bridging I atoms, respectively. There exist three lines in the lowest temperature phase (LTP), indicating that the resonance line of the bridging atom splits into two. The signal intensities of the I-127((m=1/2<->3/2)) NQR lines in the LTP decrease with decreasing temperature resulting in no detection below ca. 100 K. The I-127((m=1/2<->3/2)) NQR frequency vs. temperature curves are continuous at T-c1, but they are unusual in the LTP. The T-1 vs. T curves of H-1 NMR for the bromide and iodide are explainable by the reorientational motions of the cations about their pseudo three-fold axes. The estimated activation energies of the motions are 35.0 kJ/mol for the bromide, and 24.1, 30.1, and 23.0 kJ/mol for the RTP, ITP, and LTP of the iodide, respectively.