Thermal properties of boron and borides

The influence of point defects on the thermal conductivity of polycrystalline β-B has been measured from 1 to 1000 K. Above 300 K, samples containing 2 at. % Hf and Zr have thermal conductivities close to that of amorphous boron, indicating very strong phonon scattering. A thermal conductivity of equal magnitude has also been measured near and below room temperature for nearly stoichiometric single crystals of the theoretical composition ${\mathrm{YB}}_{68}$. On the basis of a comparison with earlier measurements to temperatures as low as 0.1 K, it is concluded that the thermal conductivity of crystalline ${\mathrm{YB}}_{68}$ is indeed very similar, if not identical, to that expected for amorphous boron over the entire temperature range of measurement (0.1–300 K). Measurements of the specific heat of nearly stoichiometric ${\mathrm{YB}}_{68}$ between 1.5 and 30 K also reveal a linear-specific-heat anomaly of the same magnitude as is characteristic for amorphous solids, in fair agreement with earlier measurements by Bilir et al. It is concluded that the lattice vibrations of crystalline ${\mathrm{YB}}_{68}$ are glasslike.