FERROELASTIC STRUCTURES OF n- PENTYL-, n- HEXYL- AND n-NONYLAMMONIUM DIHYDROGEN PHOSPHATE CRYSTALS

Jan Fábry¹, Václav Petricek¹, Ivana Císarová², Jan Kroupa¹

¹Institute of Physics, Czech. Acad. Sci., Na Slovance 2, 180 40 Praha 6, Czech Republic
²Department of Inorganic Chemistry of Faculty of Science, Charles University, Hlavova 8/2030, 128 40 Praha 2, Czech Republic

Keywords: ferroelasticity, hydrogen bonding, twinning

This study deals with the structure determinations of C₅H₁₁NH₃⁺·H₂PO₄^- (C5ADP), C₆H₁₃NH₃⁺·H₂PO₄^- (C6ADP) and C₉H₁₉NH₃⁺·H₂PO₄^- (C9ADP). The samples used in this study were not subjected to a phase transition after they had been crystallized. All these structures exhibit a reproducible ferroelastic switching. The hypothetical prototypic phase for all the compounds is orthorhombic with the space group No. 60 P 2/b 2₁/n 2₁/a. All the samples contained a fraction of the other domain (not more than 15%). The overlapping of reflections of the hk0 type was taken into account by the refinement of a scale factor for these reflections apart. The structures are monoclinic (P1 2₁/n 1), quite similar to each other and to the previously determined structures of C₇H₁₅NH₃⁺·H₂PO₄^- (C7ADP) and C₈H₁₇NH₃⁺·H₂PO₄^- (C8ADP) [1]. Despite the similarity, there is a difference between C5ADP and the rest of the compounds. The molecules in C5ADP are shifted by 0.25 in the x-fractional coordinate in contrast to the other known compounds of the family. The packing of n-alkylammonium milecules (n = 5 - 9) in these compounds depends on the parity of the number of C-atoms in a n-alkylammonium chain. A regularity of packing can be derived. 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 - under the action of them. Each of these two different hydrogens is involved in an asymmetric hydrogen bond between an oxygen pair. Under the action of a lost symmetry operation each of these hydrogens 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 oxygens. The compound is therefore paradoxial: Exerted pressure calls for a "chemical change" - the hydrogen bonds are reestablished during the ferroelastic switching. The result is, however, the same compound but in the different orientation.

Acknowledgements: This study was supported by the grants 203/96/0111 and 202/96/0085 of the Grant Agency of the Czech Republic.

[1] Fábry, J., Petricek, V., Císarová, I. & Kroupa, J. (1997). Acta Cryst. B53, 272 - 279.