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Supramolecular ferroelectric cocrystal of phenazine (Phz) with chloranilic acid (H2ca), which exhibits three successive phase transitions, have been characterized by the interplay between their structural transformations and solid-state acid–base (proton transfer) reactions (Figure) [1]. This material undergoes a ferroelectric phase (FE-I phase) transition of displacive-type at 253 K followed by successive phase transitions to the lattice modulated phases with incommensurate periodicities and with commensurate 2-fold periodicity (FE-II phase) at lower temperature [2]. To elucidate the origin of the ferroelectricity in the FE-I phase, it is crucial to study the crystal structure using single crystals. The synchrotron x-ray diffraction experiment was carried out on the imaging-plate diffractometer at BL-8A of Photon Factory in KEK. Superstructure reflections with the modulation wave vector q=(1/2 1/2 1/2) were clearly observed below 103 K. Considering the preserved 2/m Laue symmetry, the lattice can be transformed to a C-centered monoclinic lattice, which is related by (-2a, -2b, a + c) or (2a, -2b, -a - c) with the FE-I structure. Although the lattice distortion and the intensities of the superlattice reflections are consistent with the 2/m Laue symmetry, the space group C1 is deduced from the polar nature and a subgroup symmetry of the FE-I structure. Moreover, we performed single-crystal neutron diffraction experiments at SENJU of MLF/J-PARC in order to determine the displacement of the hydrogen atom. The crystal structure analysis at 10 K was carried out using the reflections measured in a half-sphere of reciprocal space at d > 0.4. The structure analysis was performed on the basis of the space group C1, where four Phz and four H2ca become crystallographically inequivalent. Finally, all the structural parameters including all hydrogen atoms were successfully refined. In the FE-II phase, the neutral and ionic molecules alternately align along the π-molecular stack.
Keywords: ferroelectric phase transition.
abstracts
Rare earth intermetallic compound Sm2Fe17N3 exhibits notalble magnetic properties such as high Curie temperature and high coercivity which are very suitable for permanent magnets [1,2]. Although microscopic magnetic structure is one of the basic information for magnetic materials, there is no report about the magnetic structure of Sm2Fe17N3 for our knowledge. This is because samarium's neutron absorption cross section is huge enough to make researchers hesitate to have neutron diffraction experiments of Sm compounds. We have carried out powder neutron diffraction measurement of Sm2Fe17N3 with a straightforward solution to the problem by taking long measurement time. Synchrotron x-ray diffraction measurements with single crystal has also been done to obtain initial crystal structure parameters for magnetic structure analysis and we have succeeded to analyze the magnetic structure of Sm2Fe17N3 at room temperature. Among four Fe sites in the unit cell, while one Fe site which is the nearest neighbor of nitrogen shows smaller magnetic moment than normal iron, two Fe sites show enhancement in their magnetic moments. This phenomenon can be understood as 'cobaltization' of Fe by the adjacent nitrogen through hybridization.
