Tunable thermal properties in yttrium silicates switched by anharmonicity of low-frequency phonons

Coordinated modulation of thermal resistance and thermal expansion compatibility has become a chief concern in designing reliable environmental/thermal barrier coating candidates for Si-based ceramics. This study reports phonon behaviors and thermal properties of prototypical silicates X-2-Y2SiO5 and gamma-Y2Si2O7. The nature of an-harmonicity for low-frequency phonons is analyzed, and the finding is discussed with respect to specific crystal structures and interatomic force constants. Low lattice thermal conductivities (KJ basically derive from the giant phonon anharmonicity enhanced by interactions between low-frequency optic and acoustic phonons. gamma-Y2Si2O7 is predicted with considerably lower thermal expansion coefficient (av) than Y2SiO5. The mechanism is disclosed as an effect of the extensive number of low-frequency phonons with negative Griineisen constants (contributing to negative thermal expansion) in gamma-Y2Si2O7, together with its higher elastic stiffness against internal thermal pressure at elevated temperature. A guideline for simultaneously approaching low CL and low av is proposed for searching promising E/TBC candidates.