CRYSTAL GROWTH AND ELECTRONIC STATES OF CaB_2C_2

自从发现La掺杂CaB_6的高温铁磁性以来，人们对这一系列硼化物的电子态进行了广泛的研究。为了阐明高温铁磁性的机理，研究类似化合物CaB_2C_2的电子结构是非常重要的。在本研究的第一阶段，我们发现自发磁化发展低于600 K和450 K，但通过去除铁磁杂质的污染，CaB_2C_2的磁化率变成反磁性。用X射线和中子衍射方法分别对粉末和单晶进行了结构分析。其结构测定如下：四氢碘4/mcm（140），a= 5.367 A，c= 7.398 A，V=213.1A^3 Z=4。在硼碳网络中的四元环中，BCB的键角小于CBC的键角，因此随着硼-硼距离的减小，四元环发生畸变。这一结果从I_4/mcm结构的能带结构上表明了CaB_2C_2的半导化行为。我们还测量了CaB_2C_2的光学反射率，发现它是一种带隙很窄的半导体。

Since the discovery of high-temperature ferromagnetism in La-doped CaB_6, the electronic states of this series of borides have been studied very extensively. In order to elucidate the mechanism of the high-temperature ferromagnetism, it is very important to study the electronic structure of the analogous compounds CaB_2C_2.We have succeeded in preparing single crystals of CaB_2C_2 using self-flux method. At the first stage of this study, we found spontaneous magnetization developing below 600 K and 450 K, however, by removing contaminations of the ferromagnetic impurities, the susceptibility of CaB_2C_2 becomes diamagnetic. We performed structure analyzes on powders and single crystals by X-ray and neutron diffraction method, respectively. The structure is determined as follows ; tetragonal I4/mcm (140), a=5.367A, c=7.398A, V=213.1A^3 Z=4. In the four-membered rings in boron-carbon network, the bond angle BCB is smaller than that of CBC so the four-membered rings are distorted as the boron-boron distance becomes closer. This result indicates the Semiconducting behavior in CaB_2C_2 in consideration of the band structure of this compound with the I4/mcm structure. We also measured the optical reflectivity of CaB_2C_2 and found that this compound is a semiconductor with very narrow band gap.