Phonon Properties of Isotopically Modified beta-Rhombohedral Boron

We examined optical phonon and thermal transport properties of beta-rhombohedral boron single crystals with different isotopic compositions (93.2% 10B enriched, 99.0% 11B enriched, and natural isotopic composition (19.8% 10B+80.2% 11B)), which were prepared by a floating zone method. The infrared-reflectance spectra of isotopically enriched 10B, 11B, and natural B-(beta-rhombohedral B) single crystals have been measured at room temperature. For the frequency of 10B, 11B, and natural B, both modes of IR-active A2u and Eu symmetry changed in frequency according to the inverse square root of mass number of B. The force constant was found to be slightly anisotropic for the intraicosahedral vibrational modes between 650 and 1100 cm-1. Also, the force constant for the A2u mode at about 1250 cm-1 was comparable with the value based on the valence force model for boron carbide B13C2, indicating similar bond strength between pristine B and C-incorporated B. Thermal conductivity of the isotope-modified beta-rhombohedral boron crystals was examined by a steady heat flow method. Temperature range of the measurement was between 5 and 130 K. At 41 K, the thermal conductivity of 10B-enriched crystal was about 570 W/m?K. This is the largest value of beta-rhombohedral structure and 40% larger than that of natural betarhombohedral B crystal. The result of this thermal measurement indicated that the isotope enrichment made heat flow large because of decrease in the phonon scattering factor