  In this work, we study thermal conductivity (k) of BC5, an ultra-hard
diamondlike semiconductor material, using first-principles computations and
analyze the effect of both isotopic disorder as well as length scale
dependence. k of isotopically pure BC5 is computed to be 169 Wm- 1K-1 (along
a-axis) at 300K; this high k is found to be due to the high frequencies and
phonon group velocities of both acoustic and optical phonons owing to the light
atomic mass of Carbon (C) and Boron (B) atoms and strong C-C and B-C bonds. We
also observe a dominance of optical phonons (~ 54%) over acoustic phonons in
heat conduction at higher temperatures (~500 K). This unusually high
contribution of optical phonons is found to be due to a unique effect in BC5
related to a weaker temperature dependence of optical phonon scattering rates
relative to acoustic phonons. The effect is explained in terms of high
frequencies of optical phonons causing decay into other high frequency phonons,
where low phonon populations cause the decay term to become insensitive to
temperature. The effect further leads to high nanoscale thermal conductivity of
77 Wm-1K-1 at 100 nm length scale due to optical phonon meanfreepaths being in
nanometer regime. These results provide avenues for application of BC5 in
nanoscale thermal management.
