Contributions of bonding heterogeneity to mechanical and thermal properties of rare earth molybdates for thermal barrier coatings

Abstract

Rare earth molybdate RE2MoO6 (RE = Dy, Ho, Er, Tm, Yb, and Lu) holds great promise as new candidate materials for thermal barrier coating applications. In this work, we systematically predict the equilibrium crystal structures, mechanical and thermal properties of RE2MoO6 by using density functional theory (DFT) calculations. Lanthanide contraction can be still recognized in RE2MoO6 and the lattice parameters as well as all bonding lengths will decrease will increasing atomic number of rare earth element. It is found that both bonding heterogeneity and polyhedral distortion are enhanced with RE element, leading to the complex variation of mechanical properties and thus strong anisotropy in both Young's and shear moduli. Moreover, all RE2MoO6 molybdates present very low thermal conductivity from room temperature to elevated temperatures and the theoretical minimum thermal conductivities range from 0.769 to 0.805 W m−1 K−1. It is further demonstrated that phonon anharmonicity plays a dominant role in reduced thermal conductivities while intrinsic properties such as mass and structure may lead to increased thermal conductivity with rare earth element. This work suggests rare earth molybdates as thermal barrier coating materials and provides some insights into developing high-performance ceramics for thermal insulation applications.