Correlation between d-orbital bandwidth and local coordination environment in RE2SiO5 compounds with implications in minimizing the coefficient of thermal expansion anisotropy (RE = Sc, Y, La)

Abstract

Y2SiO5 is one of the promising environmental barrier coating (EBC) materials that protect the gas turbine engine components from unfavorable reactions at higher temperatures. The Y2SiO5 compound forms in the monoclinic crystal structure (C2/c space group), and one of the drawbacks is its appreciable coefficient of thermal expansion (CTE) anisotropy, which adversely affects its lifetime as the EBC material. The objective of this work is to uncover previously unknown correlation between the electronic structure and crystal structure of RE2SiO5 compounds in the equilibrium and hypothetical C2/c structures (where RE = Sc, Y, or La). Our density functional theory calculations reveal a trend in the RE-cation d-orbital bandwidth as a function of the RE electronic configuration, local RE–O coordination environment, and unit cell volume. We predict that the Y-4d orbital bandwidth can become narrower when Y2SiO5 forms in an open structure with a reduced Y–O effective coordination number. We conjecture that a narrow Y-4d orbital bandwidth may give rise to smaller CTE anisotropy compared to Y2SiO5 in its equilibrium structure. The outcome of this work has potential implications in the rational design of Y2SiO5-based EBCs for use under extreme temperature environments.