Charge order in the mixed valence oxide Fe_2BO_4

采用同步X射线衍射、中子衍射、电子衍射和穆斯堡尔谱等方法研究了混合价态氧化物Fe_2BO_4的晶体结构和电荷顺序。我们的研究结果总结如下：1)试样制备：制备多晶、含“B”同位素多晶和单晶。2)用穆斯堡尔谱法评价电荷顺序和磁性结构：利用四极子效应对报道的电荷顺序模式进行了检验，认为可能是模式错误或结构参数错误。我们还通过施加外部磁场温度来检测磁性结构，发现磁性结构为寄生结构（倾斜或螺旋）。3)利用多晶进行衍射测量：在同步加速器X射线衍射中，我们无法确定与单元格的点沿a轴方向对应的上层结构反射。然而，我们发现晶格参数在转变温度周围的异常温度依赖性。另一方面，在中子衍射中，我们发现了一些弱的上层结构反射。假设一个暂定的空间群，我们可以看到Fe- o在每个Fe位点的键价不同，这可能表明了电荷顺序。我们还发现B参数沿着a轴被拉长。4)单晶透射电镜：在室温下，我们发现了清晰的上层结构反射，对应于细胞沿a轴的加倍。随着温度的升高，反射变得不相称，最终消失。我们打算进一步研究，以确定空间群和电荷顺序模式，以及磁性结构。

We have investigated the crystal structure and the charge order of mixed valence oxide Fe_2BO_4 by synchrotron X ray diffraction, neutron diffraction, ele diffraction, and Mossbauer spectroscopy. Our results are summarized as follows ;1)Preparation of the specimens : Polycrystalline, polycrystalline containing "B isotope, and single crystal were prepared.2)Evaluation of the charge order and magnetic structure by Mossbauer spectroscopy : By the quadrupole effect, we examined the reported charge order patten concluded that the pattern or the structure parameters might be wrong. We also examined the magnetic structure by applying external magnetic field a temperatures, and found that the magnetic structure is parasitic one (canted or spiral).3) Diffraction measurements using polycrystalline : In the synchrotron X ray diffraction, we could not fined superstructure reflections corresponding to the dot of the unit cell along the a-axis. We found, however, an anomalous temperature dependence of the lattice parameters around the transition temperature. C other hand in neutron diffraction, we found some weak superstructure reflections. Assuming a tentative space group, we could see a difference in bond valenci of Fe-O in each Fe site, which may be indicative of a charge order. We also found that the B parameters are elongated along the a-axis.4)Transmission electron microscope using single crystal : We found at room temperature clear superstructure reflections corresponding to the doubling of th cell along the a-axis. With the increase of temperature, the reflections become incommensurate, and finally disappear.We are intending to investigate further to determine the space group and the charge order pattern, together with the magnetic structure.