Boron-purity-dependent Raman spectra of CaB 6

Abstract

We report significant differences in the Raman spectra of two different kinds of CaB 6 single crystals grown from boron with a purity of 99.9 % (3N) or 99.9999 % (6N). Our Raman spectra of CaB 6 (3N) show peaks around 781 (T 2g), 1141 (E g), and 1283 cm -1 (A 1g), and they are very similar to previous Raman spectra of CaB 6. The E g mode shows a characteristic double-peak feature due to an additional weak broad peak centered around 1158 cm -1. However, the Raman spectra of CaB 6 (6N) show sharp peaks around 771 (T 2g), 1137 (E g), and 1266 cm -1 (A 1g). The peak frequencies are down shifted as much as ∼17 cm -1. In addition, no additional peak feature is observed for the E g mode so that the mode is symmetric in the case of CaB 6 (6N). The X-ray powder diffraction patterns for both CaB 6 (3N) and CaB 6 (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 CaB 6 (6N) with C, making Ca(B 0.995C 0.005) 6, and looked for differences in the Raman spectra. The Raman spectra of Ca(B 0.995C 0.005) 6 are nearly identical to those of CaB 6 (6N), indicating that the differences between the Raman spectra of CaB 6 (3N) and CaB 6 (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 CaB 6. The broadening of T 2g, the additional E g2 mode and the asymmetry of A 1g in CaB 6 (3N) can be understood in terms of the symmetry of the arrangements of the boron octahedra lowered by local symmetry breaking.