Charge order in the mixed valence oxide Fe_2BO_4

用同步X射线衍射法、中子衍射法、ELE衍射法和穆斯堡尔谱研究了混合价氧化物Fe_2BO_4的晶体结构和电荷级数。我们的研究结果总结如下：1)样品的制备：制备了多晶、含B同位素的多晶和单晶。2)穆斯堡尔谱对样品的电荷顺序和磁结构进行了评估：利用四极效应对已报道的电荷顺序模式进行了检验，得出了模式或结构参数可能是错误的结论。我们还通过施加外加磁场a温度来检查磁结构，发现磁结构是寄生结构(斜面或螺旋面)。3)多晶衍射测量：在同步加速器X射线衍射中，我们不能沿着a轴观察到与单位晶胞的点相对应的超结构反射。然而，我们发现晶格参数在转变温度附近有反常的温度依赖关系。另一方面，在中子衍射中，我们发现了一些微弱的超结构反射。假设一个空间群，我们可以看到Fe-O在每个Fe位置上的键价不同，这可能表明一个电荷顺序。我们还发现B参数沿a轴方向拉长。4)单晶电子显微镜：我们在室温下发现了清晰的超结构反射，对应于TH细胞沿a轴的倍增。随着温度的升高，反射变得不成公度，并最终消失。我们打算进一步研究确定空间群和电荷有序模式，以及磁结构。

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.