Negative thermal expansion and linear compressibility in 1<i>H</i>-imidazol-3-ium 2-hydroxybenzoate with a helical network of hydrogen bonds

1<i>H</i>-Imidazol-3-ium-2-hydroxybenzoate (imidazolium salicylate or SalImi) is an organic ionic crystal with a helical hydrogen-bonded network. Crystallographic structure and quantum theory of atoms in molecule calculations are used to analyse the helical structure parameters. The negative area and negative linear temperature expansion occur in the range from 300 to 180 K and 180 to 100 K, respectively. Hydrogen bonding coupling below 180 K is associated with the temperature change in crystal behaviour. The crystal displays negative linear compressibility in the <i>a</i>-direction. The negative temperature expansion and negative linear compressibility are described by the "helical" mechanism. X-ray diffraction and vibrational spectroscopy were used for the structural and macroscopic analysis. The paper presents an analysis of the crystalline structure in the temperature range from 100 to 300 K and pressures from atmospheric to 2.27 GPa. In addition, the vibrational structure was analysed using Raman and infrared spectroscopy in the temperature range from 5 to 300 K and pressures from atmospheric to 3.56 GPa.