Role of phonons on phase stabilization of RE2Si2O7 over wide temperature range (RE = Yb, Gd)

Thermodynamic phase stabilities of alpha, beta, gamma and delta polymorphs of RE2Si2O7 (RE = Yb, Gd), promising candidates for environmental barrier coating systems of gas turbine engines in next generations, were investigated using harmonic and quasi-harmonic lattice dynamics in conjunction with ab initio calculations. By quantifying their Gibbs free energies with dynamics of atoms taken into account, we showed the phase stability of beta-Yb2Si2O7 up to melting temperature under an ideal condition, and also under thermal stresses, ensuring the reliability of beta-Yb2Si2O7 in environmental barrier coating systems. It was also found that the difference of anharmonic phonons in these polymorphs, which was measured as mode Griineisen parameters, significantly changed their phonon free energies and invoked the formation of delta-Gd2Si2O7 at high temperature. These results enable a quantitative comprehension of the phase transformation of RE2Si2O7 and future discussions of other factors which may affect the phase stability such as lattice defects.