Boron-Purity-Dependent Raman Spectra of CaB6

Abstract

We report significant differences in the Raman spectra of two different kinds of CaB6 single crystals grown from boron with a purity of 99.9 % (3N) or 99.9999 % (6N). Our Raman spectra of CaB6 (3N) show peaks around 781 (T2g), 1141 (Eg), and 1283 cm.1 (A1g), and they are very similar to previous Raman spectra of CaB6. The Eg mode shows a characteristic double-peak feature due to an additional weak broad peak centered around 1158 cm.1. However, the Raman spectra of CaB6 (6N) show sharp peaks around 771 (T2g), 1137 (Eg), and 1266 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 %-pure (3N) boron is C. Thus, we doped CaB6 (6N) with C, making Ca(B0.995C0.005)6, and looked for differences in the Raman spectra. The Raman spectra of Ca(B0.995C0.005)6 are nearly identical to those of CaB6 (6N), indicating that the differences between the Raman spectra of CaB6 (3N) and CaB6 (6N) are not due to a C impurity. The Raman results show that the presence of impurities, not the amount of them, is enough to trigger local symmetry breaking in CaB6. The broadening of T2g, the additional Eg2 mode and the asymmetry of A1g in CaB6 (3N) can be understood in terms of the symmetry of the arrangements of the boron octahedra lowered by local symmetry breaking.