• Maluszyńska, Hanna
• Czarnecki, Piotr
• Scherf, Christian
• Cousson, Alain

Abstract

<p>The crystal and molecular structure of pyridinium periodate [H₅C₅NH]⁺[IO₄]^- (hereafter referred to as PyIO₄) was re-determined by single-crystal neutron diffraction at 350, 300 and 100 K. The structures of the high-temperature paraelectric and the ferroelectric intermediate phase confirmed the x-ray results. The low-temperature ferroelectric phase at 100 K, for which x-ray results did not agree with dielectric measurements, was uniquely determined by neutron diffraction. The sequence of space groups and continuous phase transitions is Cmcm(I) 321 K → Cmc2₁(II) 210 K → C112₁(III). The structure of the low-temperature phase III is consistent with all physical measurements. The continuous phase transition at T₂ = 210 K is not just accompanied by symmetry lowering but also by reversible twinning by pseudo-merohedry. The dipole moment of the ions of the low ferroelectric phase was calculated using the Gaussian 98 program. The theoretical value agrees well with the experimental spontaneous polarization measurements from the pyroelectric effect. The PyIO₄ belongs to the order-disorder class of ferroelectrics.</p>

Topics

• impedance spectroscopy
• phase
• phase transition
• neutron diffraction
• space group
• molecular structure