CRYSTAL STRUCTURE OF [NH2(CH3)2]2 CoCl4 AT ELEVATED TEMPERATURES

Vratislav Langer and Amir H. Mahmoudkhani

Department of Inorganic Chemistry, Göteborg University and Chalmers University of Technology, S-412 96 Göteborg, SWEDEN.

Keywords: structure determination, phase transition, hydrogen bonding.

The crystals of dimethylammonium(DMA) tetrachlorometallates, [NH2(CH3)2]2 MCl4 (M = Zn, Co, Cu, Mn, Cd) have been widely investigated due to the presence of different phase transitions including ferroelectric, ferroelastic and incommensurate ones. It has been shown that crystals of DMA-CoCl4 possess a ferroelectric phase transition at 238 K which has been characterized by an X-ray diffraction study. Structure determinations at 220 and 295 K, show that hydrogen-bond network, namely N-H….Cl, undergoes considerable change in course of this transition [1]. Further studies such as optical and spectroscopic measurements, dielectric and dilatometric investigations, have been explained by the existence of phase transitions at 313, 353, 380, and 419 K [2-6]. It has been suggested that these transitions are connected mainly with rearrangements of the organic sub-lattice of the crystal and specially with the modifications of hydrogen bond network [5, 6]. Moreover, the considerable dependence of physical properties of DMA-CoCl4 on the preceding history of the sample, reveals the influence of extrinsic water in the proton transport. Indeed crystals of DMA-MCl4 are hygroscopic [2, 4-6].

This has promoted us to investigate the structure of DMA-CoCl4 at elevated temperatures and clarify the possible changes in crystal structure during phase transitions. However, we have not found any significant changes in the structure up to 370 K. Possible explanation for anomalous physical properties together with structural data will be presented in more detail.

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