Anomalously low thermal conductivity in a light-element cubic semiconductor: Cubic BeB2

Cubic semiconductors composed of light elements, such as diamond and boron nitride, typically exhibit exceptionally high thermal conductivity. Recently, a cubic phase of BeB2 was predicted to possess superior electronic properties, including high carrier mobility. Here, using the Boltzmann transport equation with force constants derived from first-principles calculations, we reveal that cubic BeB2 displays an anomalously low thermal conductivity, reduced by approximately 98.4% compared with diamond at 300 K. This dramatic suppression originates from its unique structural feature, namely, the Be atom resides at the center of an octahedral cage formed by boron atoms, exhibiting a rattling-like motion. The resulting low-frequency optical phonon modes anti-overlap with acoustic modes, leading to reduced group velocities and enhanced scattering of acoustic phonons. In particular, optical-mode-mediated three-phonon scattering channels, such as TA+TA→O and TA+O→O, dominate the scattering processes, substantially shortening phonon lifetimes. The combination of weakened group velocities and intensified scattering leads to severely suppressed thermal conductivity. These low-frequency optical modes correspond to vibrations of Be atoms within the boron cages, as confirmed by their large mean-squared displacements. Our findings suggest that incorporating weakly bonded interstitial impurities could provide an effective strategy for tuning the thermal conductivity of semiconductors.