Ferroelastic n-pentylammonium dihydrogenarsenate

n-Pentylammonium dihydrogenarsenate, C5H14N+.H2AsO4- (C5ADA), is ferroelast ic at room temperature. The structure is monoclinic (P2(1)/n) and isostruct ural with n-pentylammonium dihydrogenphosphate (C5ADP). In contrast to C7AD A and C9ADA, as well as to C7ADP and C9ADP, two independent dihydrogenarsen ates in the present structure are not disordered. Similarly, as in other kn own members of the series (C2ADP-C10ADP and C6ADA-C9ADA), there are strong hydrogen bonds between the anions, and moderate hydrogen bonds between the anions and the cations. The hydrogen-bond distances correspond well to thos e observed in the dihydrogenphosphates. There are two H atoms in the struct ure which are involved in asymmetric hydrogen bonds between respective oxyg en pairs. These H atoms jump from the donor to the acceptor O atoms during ferroelastic switching. A phase transition was observed at about 380 and 37 1 K during heating and cooling, respectively.