DISORDER IN N-ALKYL AMMONIUM DIHYDROGEN PHOSPHATE CRYSTALS - CNH2N-DOT-H2PO4-(1NH3+CENTER)

Two homologues of a new family of the n-alkylammonium dihydrogen phosp hate crystals, n-heptyl C7H15NH3+.H2PO4- (C7ADP) and n-octyl C8H17NH3.H2PO4- (C8ADP), have been analysed by single-crystal X-ray diffractio n. At room temperature, the crystals are ferroelastics with a dense la mellar domain structure when observed under a polarizing microscope. T he crystal structures are characterized by the arrangement of the plan ar all-trans n-alkylammonium chains and disordered PO4 tetrahedra. The organic chains are linked to the H2PO4 anions through a system of N-H ... O hydrogen bonds. The phase transitions at T-c=344 K in C7ADP and T-c=362 K in C8ADP are related to the dynamics of the consituent stru ctural groups.