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A mechanism is proposed for the sequence of phase transitions. 2.?Experimental ? 2.1. X-ray diffraction ? Single crystals of Phz-H2ca were obtained by cosublimation of phenazine and chloranilic acid (Horiuchi. Ishii et al., 2005 ?; Noohinejad et al., 2014 ?). A diffraction experiment at T = 139?K was performed on the same crystal selleck chemicals as was employed in our previous study on the commensurate FE-II phase (Noohinejad et al., 2014 ?). X-ray diffraction data have been measured at beamline F1 of Hasylab at DESY in Hamburg, Germany, employing a MAR165 CCD detector mounted on a kappa diffractometer. The temperature of the crystal was regulated by a nitrogen gas-flow cryostat. X-ray diffraction data were collected by ? scans and �� scans for various settings of the orientation of the crystal. To better evaluate strong and weak reflections, two measurements were performed with the same measurement strategies but with different exposure times of 20 and 160?s, respectively. Data processing of the measured images has been carried out with the software EVAL15 (Schreurs et al., 2010 ?) to index and extract the integrated intensities of Bragg reflections, and with SADABS (Sheldrick, 2008 ?) for absorption correction. The latter employed groups of equivalent reflections defined according to the point group , which appeared PRDX4 as the symmetry of the diffraction. Experimental details are given in Table 1 ?. Table 1 Experimental details Indexing of the diffraction images with EVAL15 resulted in an indexing with four integers on the basis of a monoclinic unit cell closely related to the unit cell of the FE-I phase at 160?K (Horiuchi, Ishii et al., 2005 ?) together with the incommensurate modulation wavevector , where = 0.4861. However, the integration routine of EVAL15 did not accept a modulation wavevector with rational components. Therefore, the integration has been performed within the supercentered setting with = and centering translation with respect to the transformed basic structure unit cell = , = , and = (Stokes Bleomycin manufacturer et al., 2011 ?). The same setting has been employed in SADABS. 2.2. Choice of the superspace group ? The low-temperature superstructure of the FE-II phase at 100?K has been described as a commensurately modulated structure with a basic structure similar to the structure at higher temperatures and the commensurate modulation wavevector . The superspace group , with has been found to describe the symmetry of this phase (Noohinejad et al., 2014 ?). Presently, the indexing with modulation wavevector and (see ��2.1) leads to the superspace group . The non-centrosymmetric superspace group is established by the lack of inversion symmetry of both the FE-I and FE-II phases (see ��1) as well as by measurements of the electrical polarization, indicating a ferroelectric state below (Horiuchi, Kumai & Tokura et al., 2005 ?).