A series of thermochromic [RxNH4-x]2NiCl4 compounds, first reported in 1978, are brown-yellow in color at room temperature and transform abruptly to a blue phase at higher temperature. Based on color, it has been surmised that the Ni(II) center is in octahedral coordination in the yellow-brown phase and tetrahedral coordination in the blue phase. However, these compounds are prepared from a melt of 2:1 molar ratios of the organic cation and NiCl2 so that single crystals have not been available for structural characterization. In our research, salts of the dimethyl-, diethyl-, and N-methylbenzylammonium cations are prepared and analyzed by variable temperature powder X-ray diffraction, differential scanning calorimetry, and UV-vis spectroscopy to develop greater insight into the structures of the thermochromic phases and the dynamics of the thermochromic transition.

Reinvestigation of the structures and physical properties of some thermochromic A2NiCl4 salts

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