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-octylammonium dihydrogenarsenate, (C8H20-N)[AsO2OH2], are both ferroelastic at room 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. Each sample contained two domains and each structure was refined as a twin. There are strong hydrogen bonds between dihydrogenarsenates and moderate hydrogen bonds between dihydrogenarsenates and n-alkylammonium groups. The hydrogen-bond distances correspond well to those observed in the dihydrogenphosphates. All the atoms except two H atoms exist in pairs linked by the lost 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 hydrogen bond between an oxygen pair. These oxygens are supposed to change their roles as hydrogen-bond donors and acceptors during the ferroelastic switching. The phase-transition sequences are affected by interactions between the neighbouring organic chains in the structure.
Fábry
J
J
Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 180 40 Praha 8, Czech Republic. fabry@fzu.cz
Kroupa
J
J
Císarová
I
I