Temperature‐induced reversible structural phase transition and X‐ray diffuse scattering in 2‐amino‐3‐nitropyridinium hydrogen sulfate

The novel polar material 2‐amino‐3‐nitropyridinium hydrogen sulfate, C5H6N3O2(HSO4) (abbreviated as 2A3NP‐HS), was obtained and structurally characterized by means of single‐crystal X‐ray diffraction. At room temperature, 2A3NP‐HS crystallizes as a non‐centrosymmetric disordered phase (I) in the orthorhombic Pna21 space group. On cooling below 298 K, 2A3NP‐HS undergoes a reversible phase transition to phase (II) with the monoclinic non‐centrosymmetric P21 space group. This transition might be classified as an `order–disorder' type. The structural details in both phases are analysed. Additionally, for phase (I), in the 304–365 K temperature range, diffuse scattering was found to be present in the form of elongated streaks parallel to the a* direction. This can be unravelled when implementing a short‐range order affecting anionic cationic ribbons occurring in the structure, with correlations acting both in the a‐direction and in the bc‐plane. The results of Monte Carlo simulations, adapting a two‐dimensional Ising‐type model, reveal the formation of domains, which are b‐elongated and thin along a. Locally, the stacking of the ribbons in the domains reflects the ordered arrangement observed in the low‐temperature monoclinic phase (II). A novel polar material, C5H6N3O2(HSO4), was synthesized, and its structure was determined in two phases. The origin of the X‐ray diffuse scattering in a high‐temperature disordered phase is explained as a precursor effect of the long‐range ordered low‐temperature phase.