Study of thermochromic material and its properties for visual indicators

Abstract This work deals with the preparation and characterization of inorganic hybrid phosphite salt (enH 2 )[Ni(H 2 O) 6 ](HPO3) 2 , where “en” stands for ethylenediamine (C 2 N 2 H 8 ). This material belongs to a broad family of chromogenic compounds, which may have wide applications as pigments for visual indicators due to their color change in response to external stimuli (e.g. temperature, pressure, radiation, etc.). Such pigment can be used to develop new smart textiles, smart windows, safety sensors for innovative packaging, etc. However, before that it is necessary to understand the origin of color transition and the impact of the transition on its environment. Therefore, the (enH 2 )[Ni(H 2 O) 6 ](HPO 3 ) 2 was studied by various thermoanalytical (OTM, TMA, DTA, TG, EGA), structural (HT XRPD, XPS), and optical (Raman spectroscopy) methods. Several color transitions were identified in the range from 67 to 215 °C, accompanied either by decomposition, recrystallization, or both. The first color change (from a translucent blue to an opaque light green) occurred at 67 °C due to a release of crystalline H 2 O. The final color change (from a green to a dark blue) was at ca. 215 °C due to the formation of the new phases (Ni x P and Ni 2 P). HT XRPD, Raman spectroscopy, and XPS observed the evolution of the crystallinity state and chemical bonding. Finally, the kinetic parameters (activation energy and reaction rate) of the first decomposition effect were determined via the Kissinger method.