Physics Department, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland.
J Phys Condens Matter. 2010 Jun 16;22(23):235901. doi: 10.1088/0953-8984/22/23/235901. Epub 2010 May 21.
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.
高氯酸根合五甲基吡啶𬭩盐的晶体和分子结构研究
通过单晶中子衍射在 350、300 和 100 K 下测定了高氯酸根合五甲基吡啶𬭩盐 H(5)C(5)NH ReO(4)（以下简称 PyReO(4)) 的晶体和分子结构。中子研究在所有三个相中都证实了 X 射线衍射结果。三种温度依赖的多晶型物为正交晶系，具有以下相序：Cmcm → Cmc2(1) → Pbca，其中 a 晶格参数加倍。在两个高温相中，吡啶𬭩阳离子显示出旋转无序，而过氯酸根阴离子则有序排列。低温相完全有序。中子结果允许对无序吡啶𬭩阳离子中氮原子的分布进行非常精确的描述，这使我们能够分析量热和自发极化测量。DSC 和热释电测量的结果表明存在顺电（350 K）、铁电（300 K），居里点为 336 K 和反铁电（100 K）晶体相。在 336 K 的相变可以归类为具有小位移分量的有序-无序铁电相变。
