Structural investigation of the phase transitions of Tribromo ammonium mercurate (II) monohydrate, NH4HgBr3•H2O

Single crystal diffraction, Infrared spectroscopy and differential scanning calorimetry DSC techniques have been used to investigate the different phases of NH(4)HgBr(3)center dot H(2)O, Tribromo ammonium mercurate (II) monohydrate, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 198 and 340 K, by DSC experiment. X-ray diffraction measurements confirm the presence of the first anomaly. At room temperature NH(4)HgBr(3)center dot H(2)O, crystallizes in the orthorhombic space group Cmc2(1), with the lattice constants a = 4.475(1) angstrom, b = 17.226(2) angstrom, c = 10.240(2) angstrom and Z = 4. Below 200 K the structure is monoclinic P2(1) with: a = 4.379(4) angstrom, b = 17.220(10) angstrom, c = 10.103(2) angstrom, beta = 90.023(9)degrees and Z = 2 (T = 120 K). The mercury atom is surrounded by four bromine atoms in an irregular tetrahedron. The tetrahedra are linked at two corners, resulting in infinite (HgBr(3))(n)(n-) chains along the "a" axis. The ammonium groups are located between the chains ensuring the stability of the structure by hydrogen bonding contacts: N-H center dot center dot center dot Br, O-H center dot center dot center dot Br, N-H center dot center dot center dot O. The structural phase transformation was attributed to an orientational disorder of ammonium groups. (C) 2007 Elsevier B.V. All rights reserved.