Effect of high pressure on the supramolecular structures of guanidinium based ferroelectrics

Abstract

Ferroelectric crystals of guanidinium tetrafluoroborate, C(NH₂)₃BF₄, and guanidinium perchlorate, C(NH₂)₃ClO₄, have been grown in situ in a diamond anvil cell, and their structures have been determined by single-crystal X-ray diffraction at different pressures. In the pressure range to about 2 GPa both crystals retain their architecture built of honeycomb hydrogen-bonded sheets. However, in guanidinium tetrafluoroborate this structural motif becomes distorted at elevated pressures, as a result of the first-order phase transition revealed at 0.7 GPa. The transition involves the symmetry lowering from R3m to R3 due to the rotations of the ions around their three-fold symmetry axes by about 7°. On the contrary, the structure of guanidinium perchlorate is stable in the whole pressure range studied. Moreover, along the direction perpendicular to the H-bonded sheets this crystal is stiffer than guanidinium tetrafluoroborate. The distinct response to pressure of these isosymmetric materials was attributed to the size of the tetrahedral anions. The structures determined as a function of pressure have been used for modelling the pressure dependence of spontaneous polarization.