Structural analysis of the phase transitions of (NH4)4HgBr6

Single crystal diffraction and differential scanning calorimetry DSC techniques have been used to investigate the different phases of (NH4)(4)HgBr6, tetrammonium mercury hexabromide, from room temperature to 120 K. Two anomalies in thermal behaviour were detected for this compound at 190 and 268 K, by DSC experiment. X-ray diffraction measurements confirm these anomalies. At room temperature the structure is tetragonal P4/mnc (No. 128) with lattice parameters a = b = 9.25560(8) angstrom; c = 8.8657(11) angstrom; V = 759.49(9) angstrom(3) and Z = 2. At T = 250 K the structure is orthorhombic Pnnm with lattice parameters a = 8.8436(8) angstrom; b = 9.2191(8) angstrom; c = 9.2232(7) angstrom; V = 751.97(11) angstrom(3) and Z = 2. Below approximately 200 K the structure is monoclinic P2(1)/n (No. 14) with: a = 8.8080(9) angstrom; b = 9.1608(8) angstrom; c = 9.1498(8) angstrom; beta = 90.230(7)degrees; V = 738.28(12) angstrom(3) and Z = 2 (T = 120 K). The structure of (NH4)(4)HgBr6 consists of isolated HgBr6-octahedra in the whole temperature range which are slightly compressed in c-direction. The ammonium groups are located between the octahedra ensuring the stability of the structure by hydrogen bonding contacts: N-H center dot center dot center dot Br. The structural phase transformations are described by a rotation of the [HgBr6](2-) octahedra around the c-axis, and this behaviour is attributed to an orientational disorder of ammonium groups.