Phase transitions in n-alkylammonium dihydrogenphosphates and -arsenates and ferroelastic n-hexyl- and n-octylammonium dihydrogenarsenate

n-Hexylammonium dihydrogenarsenate, (C6H16N)[AsO2(OH)(2)], and n-octylammon ium dihydrogenarsenate, (C8H20N)[AsO2(OH)(2)], are both ferroelastic at roo m temperature. The samples used in this study were not subjected to a phase transition after they had been crystallized. The structures are monoclinic (P2(1)/n) and isostructural with the corresponding dihydrogenphosphates. E ach sample contained two domains and each structure was refined as a twin. There are strong hydrogen bonds between dihydrogenarsenates and moderate hy drogen bonds between dihydrogenarsenates and n-alkylammonium groups. The hy drogen-bond distances correspond well to those observed in the dihydrogenph osphates. All the atoms except two H atoms exist in pairs linked by the los t symmetry operations derived from the prototypic space group P2/b2(1)/n2(1 )/a. Each of these two different H atoms is involved in an asymmetric hydro gen bond between an oxygen pair. These oxygens are supposed to change their roles as hydrogen-bond donors and acceptors during the ferroelastic switch ing. The phase-transition sequences are affected by interactions between th e neighbouring organic chains in the structure.