Dominant mechanisms of thermo-mechanical properties of weberite-type RE₃TaO₇ (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates toward multifunctional thermal/environmental barrier coating applications

Weberite-type RE₃TaO₇ (RE=La, Pr, Nd, Eu, Gd, Dy) tantalates have been studied as multifunctional thermal/environmental barrier coating materials with working temperatures above 1500 K. This study proposes the dominant mechanisms of their thermo-mechanical properties. The relative ionicity of the RE-O bonds increased with increasing RE⁺³ ionic radius, which weakened the bond strength and led to decreases in Young's modulus and hardness. A model was proposed to derive the high-temperature thermal conductivity based on the relationship between the phonon thermal conductivity and temperature, and the thermal radiative conductivity was estimated. RE₃TaO₇ exhibited a lower thermal conductivity than other fluorite-related oxides because its weak bond strength could slow down the phonon speed and enhance the an-harmonic vibrations of the lattices. At low temperatures, the RE atomic weight was a better descriptor of the thermal conductivity than the RE⁺³ ionic radius, and which could aid in further regulating the thermal conductivity. The thermal expansion coefficients of the different axes were affected by both RE-O and Ta-O bonds when weak RE-O bonds dominated the linear thermal expansion. The low oxygen ion conductivity derived from the strong covalent Ta-O bonds, which increased the activation energy for oxygen hopping in lattices of RE₃TaO₇. This study elucidates the dominant mechanisms of properties from the characteristics of crystals and chemical bonds, which are significant for high-temperature applications of tantalates.