Raman scattering study of calcium hexaboride

We report significant difference in the Raman spectra of two different kinds of CaB6 single crystals grown from boron purity 99.9% (3N) or 99.9999% (6N), respectively. Our Raman spectra of CaB6 (3N), which are similar to those of previous measurement [N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii, Phys. Rev. B 68 (2003) 224305], show peaks at 781.3 cm−1 (T2g), 1140.1 cm−1 (Eg), and 1283.5 cm−1 (A1g). The Eg mode shows a characteristic double-peak feature due to an additional weak broad peak centered at 1156.0 cm−1. However, the Raman spectra of CaB6 (6N) show sharp peaks at 772.5 cm−1 (T2g), 1137.9 cm−1 (Eg), and 1266.6 cm−1 (A1g). The peak frequencies are down shifted as much as 17 cm−1. In addition, no additional peak feature is observed for the Eg mode so that the mode is symmetric in the case of CaB6 (6N). The X-ray powder diffraction patterns for both CaB6 (3N) and CaB6 (6N) show that the lattice parameters are essentially the same. The majority of the impurity in the 99.9% (3N) boron is assessed to be C. Thus we prepared Ca(B0.995C0.005)6, CaB6 (6N) doped with C, and looked for the difference in the Raman spectra. The Raman spectra of Ca(B0.995C0.005)6 are nearly identical to those of CaB6 (6N), indicating that the difference in the Raman spectra of CaB6 (3N) and CaB6 (6N) is not due to C impurity. However, presence of impurity, even if small amount, seems to be enough to trigger local-structure changes to lower symmetry inducing the difference in Raman spectra of CaB6 (3N) and CaB6 (6N).