Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu)

Rare earth monosilicates RE2SiO5 have been considered as promising environmental barrier coating materials for silicon-based ceramics due to their low thermal conductivity and good high-temperature stability. We herein performed a systematic study of the lattice dynamics for RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu) using first-principles calculations. The loosely bound rare earth atoms provide large Grüneisen parameters and low phonon group velocities, both of which determine the low thermal conductivity. Theoretical exploration predicts an anomalous increase of lattice thermal conductivity with increment of RE atomic number and the mechanism is explained by the stronger atomic bonding and weaker phonon anharmonicity. Although incorporating heavier atoms has long been considered as an effective way to reduce lattice thermal conductivity, this work addresses the importance of bonding heterogeneity and anharmonicity rather than atomic mass variation. This theoretical study suggests an alternative approach towards the design of new thermal insulating materials.