Crystal structure, thermal, dielectric and vibrational properties of a novel polar crystal: 4-Aminopyridinium-hydrogen maleate-maleic acid, [4-NH2C5H4NH][C4H3O4][C4H4O4]

[4-NH2C5H4NH][C4H3O4][C4H4O4] (APM) complex with rare stoichiometric ratio 1:2 has been synthesized and characterized by a single-crystal X-ray diffraction and powder X-ray diffraction in a wide temperature range (from 100 K up to 338 K), calorimetry and dielectric spectroscopy. The APM undergoes structural phase transition at 307/312 K (cooling/heating) of first order type classified as an order–disorder. The crystal structure of APM below 307 K consists of ordered 4-aminopyridinium cations, hydrogen maleate anions (HM) and neutral maleic acid molecules (MM) that interact via hydrogen bonds. The room temperature phase II and the high temperature phase one I is polar, space group P21, which is confirmed by the pyroelectric effect. The I → II phase transition is accompanied by a distinct electric permittivity anomaly (εmax ≈ 55 at 150 kHz) that is characteristic of ferroelectric crystals. The infrared spectra of polycrystalline APM are presented and compared with other closely related analogs.