Structure of phase change energy storage material Ca(NO3)2·4H2O solution

[Display omitted] •The coordination number of Ca2+ is 7–8 in liquid Ca(NO3)2·4H2O.•An NO3− links two Ca2+ in monodentate to form a long chain or a short ring structure.•Edge-sharing mono-oxygen bridge bonds between NO3− and two Ca2+.•An underlying mechanism of deep supercooling of liquid Ca(NO3)2·4H2O is revealed. Calcium nitrate tetrahydrate, Ca(NO3)2·4H2O, has the potential prospects as a room temperature phase change material due to appropriate melting point and high enthalpy. However, the supercooling problem prevents its widespread use in an energy storage field. In this work, the microscopic structure of liquid Ca(NO3)2·4H2O at different temperatures are studied by X-ray diffraction and Raman spectroscopy and compared with that of Ca(NO3)2·4H2O crystal. The first coordination layer of a calcium ion contains 3 water molecules and 4–5 nitrate ions. A nitrate ion acts as a dioxygen bridge to link two calcium ions in a monodentate fashion and form a longer chain or a shorter ring structure in the liquid state. In the crystalline state, two oxygen atoms of a nitrate ion bind with a calcium ion in a bidentate manner. The structure and stability of [Ca2+m(NO3−)n(H2O)q] clusters are examined by DFT calculations to confirm the results obtained by X-ray diffraction and Raman spectroscopy. The key factor of severe supercooling problem would be the conversion from edge-sharing mono-oxygen bridge bonds of Ca-NO3 (Ca-O(NO)O-Ca) in liquid to corner-sharing dioxygen bridge bonds of Ca-NO3 (Ca-O(NO2)-Ca) in solid. This work provides a theoretical basis for solving the supercooling problem.