Ferroelastic n-heptyl­ammonium ­dihydrogenarsenate

n-Heptyl­ammonium di­hydrogenarsenate, (C₇H₁₈N)[As(O)₂(OH)₂] (C7ADA), is ferroelastic at room temperature and isostructural with n-heptyl­ammonium di­hydrogenphosphate (C7ADP). In contrast to the other known n-alkyl­ammonium di­hydrogenphosphates (CnADP) and di­hydrogenarsenates (CnADA), two independent anions in the present structure are substantially disordered (∼88 and ∼12%, respectively). There are strong hydrogen bonds between the di­hydrogenarsenates themselves, and moderate hydrogen bonds between the di­hydrogenarsenates and the n-alkyl­ammonium groups. The hydrogen-bond distances correspond well to those observed in the di­hydrogenphosphates. There are two H atoms in the structure which are involved in asymmetric hydrogen bonds between respective oxy­gen pairs. These H atoms jump from the donor to the acceptor O atoms during ferroelastic switching.