The neutron diffraction study, calorimetry and spontaneous polarization of pyridinium perrhenate at 350, 300 and 100 K.

The crystal and molecular structure of pyridinium perrhenate [H(5)C(5)NH](+) [ReO(4)](-) (hereafter referred to as PyReO(4)) was determined by single-crystal neutron diffraction at 350, 300 and 100 K. The neutron study confirmed the x-ray diffraction results in all three phases. The three temperature-dependent polymorphs are orthorhombic, with the following sequence of phases: Cmcm − → Cmc2(1) − → Pbca, with the a lattice parameter doubled. In the two high temperature phases the pyridinium cations display a rotational disorder while the perrhenate anions are well ordered. The low temperature phase is fully ordered. The neutron results allow for a very precise description of the distribution of the nitrogen atoms in the disordered pyridinium cation, which enables us to analyse the calorimetric and spontaneous polarization measurements. The results from the DSC and pyroelectric measurements point to a paraelectric (350 K), ferroelectric (300 K) with the Curie point at 336 K and antiferroelectric (100 K) crystal phases. The phase transition at 336 K can be classified as an order-disorder ferroelectric with a small displacive component.