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

Authors:

• Hanna Małuszyńska,
• Alain Cousson,
• Piotr Czarnecki

Abstract

The crystal and molecular structure of pyridinium perrhenate [H ₅C₅NH]⁺[ReO₄]^-(hereafter referred to as PyReO₄) was determined by single-crystal neutron diffraction at 350, 300 and 100K. 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 → Cmc2₁ → 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 (350K), ferroelectric (300K) with the Curie point at 336K and antiferroelectric (100K) crystal phases. The phase transition at 336K can be classified as an order-disorder ferroelectric with a small displacive component. © 2010 IOP Publishing Ltd.