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Sr2MnGaO5 brownmillerite has an interesting structure. In this compound, MnO6 octahedra and GaO4 tetrahedra are ordered along c-axis. The MnO6 octahedra are deformed due to Jahn-Teller effect with four short equatorial Mn-O distances and two long apical one. Oxygen vacancies in the GaO layer provide distorted tetrahedral coordination for the Ga cation. The oxide and/or fluoride ions are introduced into the vacancies in GaO layer. The synthesis of Sr2MnGaO4.78F1.22 using XeF2 as a fluorinating agent is reported, while the small amount SrF2 impurity is contained[1]. On the other hand, low temperature fluorination using polyvinylidenefluoride (PVDF) is powerful method for the synthesis of oxyfluoides from brownmillerite[2]. In this study, we synthesize the Sr2MnGaO5-dFx (x=0.5, 1.0, 1.5, 2.0) using PVDF and investigate the variation of crystal structure with x. Sr2MnGaO5 brownmillerites were prepared sold state reaction method. Starting materials were SrCO3, Mn2O3 and Ga2O3. The mixture was heated twice in Ar at 13000C for 72h. For the preparation of Sr2MnGaO5-dFx (x=0.5, 1.0, 1.5, 2.0), stoichiometric amounts of Sr2MnGaO5 brownmillerite and PVDF were ground and the mixture were heated in N2 at 4000C for 8h. And then, the as prepared materials were subsequently heated in O2 at 4000C for 8h. The Sr2MnGaO5-dFx were obtained without impurities. As shown in Fig., the orthorhombic distortion reduced with the increase in x. The X-ray diffraction data of Sr2MnGaO5-dF1.0 could be refined on the structure model belonging the space group Icmm. the bond valence sum of Ga3+ calculated from Ga-O bond length indicates that the fluoride ions are introduced into GaO layer preferentially. The measurements of magnetic properties for Sr2MnGaO5-dFx are in progress.
abstracts
Ferroelectricity, piezoelectricity, pyroelectricity, and second-order nonlinear optical behavior are technologically important. Because these properties are attributable to the noncentrosymmetric (NCS) structure[1], the search for materials exhibiting such characteristics must begin with a search of NCS materials. Among them, LiNbO3-type (LN-type) compounds with a chemical formula of ABX3 exhibit NSC structures with hexagonal polar space group R3c whose BX6 octahedra three-dimensionally share their corners the same as perovskite-type compounds[2]. In LN-type compounds, the A cation - B cation repulsion directs the spontaneous polarization along c-axis(Fig. 1). Therefore, we might find attractive functional properties by the selection of constituent ions based on their having a naturally occurring polar LN-type structure attributable to the cation-cation repulsion. With the ideas mentioned above, we have investigated the high-pressure synthesis and characterization of novel LN-type oxides such as ZnSnO3, PbNiO3, CdPbO3 as well as known MnMO3 (M = Ti, Sn). Recently, we have successfully synthesized a polar LN-type titanate ZnTiO3 (LN-ZTO) under high pressure and high temperature [3]. The first principles calculation indicates that LN-ZTO is a meta-stable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement reveals that LN-ZTO exhibits greater intra-distortion of the TiO6 in LN-ZTO than that of the SnO6 in LN-type ZnSnO3 (LN-ZSO). The estimated spontaneous polarization are greater than those of LN-ZSO, which is attributed to the great displacement of Ti along c-axis and the greater Born effective charge of Ti (+6.1) than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-ZTO is greater than that of LiNbO3, indicating that LN-ZTO, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-ZTO is 24 times greater than that of LN-ZSO. The findings indicate that the intra-octahedral distortion, spontaneous polarization, and SHG response are caused by the stabilization of the polar LN-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d0 ions such as Ti4+. We also discuss the relationship between the intra-distortion of BO6 and polarity in several LN-type oxides.
