REVISION OF FERROELASTIC STRUCTURES OF N-HEPTYLAMMONIUM 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 w ere not subjected to a phase transition after they had been crystalliz ed. Unlike a previous structure determination, weak reflections, now w ith indices h = 2n + 1, were included. This means that both structures are described in unit cells with the lattice parameters a twice as lo ng as given previously. Both structures are quite similar; two double layers of dihydrogen phosphates, which are interconnected by hydrogen bonds (2.52-2.62 Angstrom), pass through each unit cell. Alkylammonium groups interact with these dihydrogen phosphates via longer hydrogen bonds (>2.75 Angstrom), while the rest of the aliphatic chains interac t via van der Waals contacts. All H atoms were localized and no disord er of the H atoms was detected. Both structures described in the space group P12(1)/nl exhibit a reproducible ferroelastic switching. The hy pothetical prototypic phase is orthorhombic with the space group numbe r 60 P2/b2(1)/n2(1)/a. All atoms except two hydrogen species exist in pairs linked by the lost symmetry operations derived from the prototyp ic space group and are brought close to each other - up to 0.25 Angstr om - under the action of them. Each of these two different hydrogens i s involved in an asymmetric hydrogen bond between an oxygen pair. Unde r the action of a lost symmetry operation each of these hydrogens is d isplaced from one oxygen towards the other. Therefore, it is assumed t hat during the ferroelastic switching the jumps of these two hydrogen species take place between the pertinent hydrogen-bond acceptor and do nor oxygens. Hence, these oxygens reverse their role as hydrogen-bond donors and accepters during the ferroelastic switching.