|Budget Amount *help
¥3,600,000 (Direct Cost : ¥3,600,000)
Fiscal Year 2002 : ¥1,000,000 (Direct Cost : ¥1,000,000)
Fiscal Year 2001 : ¥2,600,000 (Direct Cost : ¥2,600,000)
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.