Revision of Ferroelastic Structures of n-Heptyl- and n-Octylammonium Dihydrogen Phosphate Crystals

This study deals with the structure determination of C7H₁₅NH₃⁺.H₂PO₄^- (C7ADP) and C₈H₁₇NH₃⁺.H₂PO₄^- (C8ADP). The samples used in this study were not subjected to a phase transition after they had been crystallized. Unlike a previous structure determination, weak reflections, now with indices h = 2n + 1, were included. This means that both structures are described in unit cells with the lattice parameters a twice as long as given previously. Both structures are quite similar; two double layers of dihydrogen phosphates, which are interconnected by hydrogen bonds (2.52-2.62 Å), pass through each unit cell. Alkylammonium groups interact with these dihydrogen phosphates via longer hydrogen bonds (>2.75 Å), while the rest of the aliphatic chains interact via van der Waals contacts. All H atoms were localized and no disorder of the H atoms was detected. Both structures described in the space group P12₁/n1 exhibit a reproducible ferroelastic switching. The hypothetical prototypic phase is orthorhombic with the space group number 60 P2/b2₁/n2₁/a. All atoms except two hydrogen species exist in pairs linked by the lost symmetry operations derived from the prototypic space group and are brought close to each other - up to 0.25 Å - under the action of them. Each of these two different H atoms is involved in an asymmetric hydrogen bond between an oxygen pair. Under the action of a lost symmetry operation each of these H atoms is displaced from one oxygen towards the other. Therefore, it is assumed that during the ferroelastic switching the jumps of these two hydrogen species take place between the pertinent hydrogen-bond acceptor and donor O atoms. Hence, these O atoms reverse their role as hydrogen-bond donors and acceptors during the ferroelastic switching.

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