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

Abstract

Abstract The temperature dependence of the 81Br 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 (i. 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, β = 104.10(1)°, Z= 8, R = 0.076. The crystal has two crystallographically independent cations (CH3)2NH2+ 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 HgBr4 2- anions which are interconnected via weak intermolecular Hg···Br bonds. The splitting patterns of the 81Br 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 81Br NQR lines show anomalous temperature coefficients, attributable probably to thermal motions of cations.