Multi-stimuli responsive (l-tartrato)oxovanadium(v) complex salt with ferroelectric switching and thermistor properties

Abstract

Molecular materials that respond to external triggers and the environment by changing certain properties are currently a very active area of research and are being considered for use in devices. The responsiveness of these highly organized structures is the result of flexible structural arrangements, which in most cases are controlled by non-covalent interactions. Here, we report the exceptional structural flexibility of a complex salt with tetraethylammonium cations, (L-tartrato)oxovanadium(V) anions and water molecules of crystallization. This compound undergoes multiple crystal transformations from the undecahydrate phase to the dehydrated phase triggered by changes in humidity and temperature. The structural transformations are characterized by single-crystal X-ray diffraction, powder X-ray diffraction and infrared spectroscopy at variable temperature and humidity and by thermal analysis. The dynamics of the structural transformations occurring in this tetraethylammonium (L-tartrato)oxovanadium(V) system indicates a low energy barrier for reversible release and uptake of water molecules from air. During these transformations, which occur over a very narrow temperature and humidity range, the compound changes from the non-polar P2₁2₁2₁ structure to the polar P2₁, C2 and P2 structures. The structural transformations are accompanied by significant changes in the electrical behaviour of the material: from proton conductivity in the non-polar P2₁2₁2₁ phase to ferroelectricity in the P2₁ phase at room temperature, and thermistor behaviour in the temperature range of 313–333 K. In addition, the optical properties of the material, which exhibits pleochroism and photoresponsive behaviour, were investigated using UV-vis diffuse reflectance spectroscopy.