Phase Transitions and Crystal Structure of Dimethylammonium Tribromomercurate(II), (CH3)2NH2HgBr3, as Studied by 81 Br NQR and Single Crystal X-ray Diffraction*

The temperature dependence of the 81 Br NQR frequencies of the title compound has revealed the presence of three phases (I, II and III, in the order of decreasing temperature): Tcl (II -1) = (318 ± 10) K and Tc2 (III II) = (202 ± 1) K. The transitions were confirmed by thermal analysis (DTA). Both of the II-I and I-II transitions exhibited strong hysteresis (/. e., superheating and supercooling, respectively). The crystal data, determined by single crystal X-ray diffraction, are: monoclinic, space group P2/a, a = 1384.8(3), b = 934.7(4), c = 1452.0(2) pm, ß = 104.10(1)°, Z = 8, R = 0.076. The crystal has two crystallographically independent cations (CH 3 ) 2 NH 2 + and an infinite chain of anions almost in the c-direction. The anion chain is considered to consist of a HgBr2 molecule and two different HgBr 4 ~ anions which are interconnected via weak intermolecular H g B r bonds. The splitting patterns of the 8 ' B r NQR spectra indicate that rearrangements of the Hg-Br bonds are slight at the III-II transition but serious at the II-I transition. Some of the 81 Br NQR lines show anomalous temperature coefficients, attributable probably to thermal motions of cations.