Theoretical investigation of phonon contributions to thermal expansion coefficients for rare earth monosilicates RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu)

Abstract

Owing to the superior chemical and thermal compatibility with silicon-based ceramics, rare earth (RE) monosilicates RE2SiO5 are promising as environmental barrier coating materials for the applications in high-temperatures aero-engines. Herein, thermal expansion and phonon characteristics of RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu) were investigated by first-principles calculations. The thermal expansion coefficients were predicted as a function of temperature and ranged from 8.87 to 7.72 × 10−6 /K at 1600 K for the studied RE2SiO5. The isolated RE and O5 atoms determine the thermal expansion behaviours of RE2SiO5 especially at low phonon frequencies; while the [SiO4] tetrahedrons contribute positively, but less significantly to the thermal expansion. The balance between negative contribution from distortion of RE polyhedrons and positive contribution from stretching of REO bonds is identified as the key to tune the thermal expansion coefficient of RE silicates.